A4992KLPTR-T - ALLEGRO MICROSYSTEMS

Description

The A4992 is a flexible microstepping motor driver with

integrated phase current control and a built-in translator for

easy operation. It is a single chip solution designed to operate

bipolar stepper motors in full, half, quarter, eighth, and

sixteenth step modes, at up to 28 V. At power-on the A4992

is configured to drive most small stepper motors with simple

step and direction inputs.

The current regulator operates with fixed frequency PWM. It

uses adaptive mixed current decay to reduce audible motor

noise and increase step accuracy.

The current in each phase of the motor is controlled through a

DMOS full bridge using synchronous rectification to improve

power dissipation. Internal circuits and timers prevent cross-

conduction and shoot-through when switching between

high-side and low-side drives.

The outputs are protected from short circuits. Features

for low load current and stalled rotor detection are

included. Chip level protection includes hot thermal

warning, overtemperature shutdown, and overvoltage and

undervoltage lockout.

An optional serial interface mode, using the STEP, DIR, and

MS inputs, can be used to configure several motor control

parameters and diagnostics.

The A4992 is supplied in a 20-pin TSSOP power package with

an exposed thermal pad (package type LP). This package is

lead (Pb) free with 100% matte-tin lead frame plating.

A4992-DS, Rev. 1

Features and Benefits

• Low RDS(on) outputs, 0.5 Ω source and sink typical

• Continuous operation at high ambient temperature

• 3.5 to 50 V supply operation

• Adaptive mixed current decay

• Synchronous rectification for low power dissipation

• Internal overvoltage and undervoltage lockout

• Hot warning and overtemperature shutdown

• Crossover-current protection

• Short circuit and open load diagnostics

• Stall detect features

• Configurable through serial interface

Automotive Stepper Driver

Package: 20-pin TSSOP with exposed

thermal pad (suffix LP)

Typical Application Diagram

Not to scale

A4992

utomotive

12 V Power Net

Logic

Supply

Micro-

controller

ECU

OAP

PGND

OAM

OBP

OBM

Stepper

Motor

AGND

STEP

DIR

RESETn

DIAG

SENSA

SENSB

REF

CP1 CP2 VCP VBB

VREG

A4992

Applications:

• Automotive stepper motors

• Engine management

• Headlamp positioning

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Absolute Maximum Ratings with respect to PGND

Characteristic Symbol Notes Rating Unit

Load Supply Voltage VBB –0.3 to 50 V

Pin CP1 –0.3 to VBB V

Pin CP2, VCP –0.3 to VBB + 8 V

Pins STEP, DIR, MS –0.3 to 6 V

Pin VREG –0.3 to 8.5 V

Pin RESETn Can be pulled to VBB with 38 kΩ –0.3 to 6 V

Pin DIAG –0.3 to 6 V

Pin REF –0.3 to 6 V

Pins OAP, OAM, OBP, OBM –0.3 to VBB V

Pin SENSA, SENSB –0.3 to 1 V

Pin AGND –0.1 to 0.1 V

Ambient Operating Temperature

Range TALimited by power dissipation –40 to 150 °C

Maximum Continuous Junction

Temperature TJ(max) 150 °C

Transient Junction Temperature TtJ

Overtemperature event not exceeding 10 s, life-

time duration not exceeding 10 hours, ensured

by design and characterization.

175 °C

Storage Temperature Range Tstg –55 to 150 °C

Thermal Characteristics may require derating at maximum conditions, see application information

Characteristic Symbol Test Conditions* Value Unit

Package Thermal Resistance RθJA

Estimated, on 4-layer PCB based on JEDEC

standard 29 ºC/W

*Additional thermal information available on the Allegro website.

Selection Guide

Part Number Packing* Package

A4992KLPTR-T 4000 pieces per 13-in. reel 4.4 mm x 6.5 mm, 1.2 mm nominal height

20-pin TSSOP with exposed thermal pad

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Table of Contents

Specifications 2

Functional Block Diagram 4

Pin-out Diagram and Terminal List 5

Electrical Characteristics Table 6

Interface Timing Diagrams 9

Functional Description 11

Pin Functions 11

Driving a Stepper Motor 12

Phase Current Control 12

Step Angle and Direction Control 12

Diagnostics 13

System Diagnostics 14

Supply Voltage Monitors 14

Temperature Monitors 15

Bridge and Output Diagnostics 15

Short to Supply 15

Short to Ground 16

Shorted Load 16

Short Fault Blanking 16

Short Fault Reset and Retry 16

Open Load Detection 16

Stall Detection 16

Serial Interface 18

Configuration and Run Registers 19

Applications Information 22

Motor Movement Control 22

Phase Table and Phase Diagram 22

Using Step and Direction Control 24

Control Through the Serial Interface 25

Layout 27

Decoupling 27

Grounding 27

Current Sense Resistor 27

Package Outline Drawing 28

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Functional Block Diagram

Gate

Drive

Charge

Pump

STEP

REF

6-bit

DAC

6-bit

DAC

Oscillator

+ –

SENSB

SENSA

VBB

OAP

OAM

SENSA

RSB

RSA

Bridge A

Bridge B

OBP

OBM

SENSB

DIR

RESETn

DIAG

VLOGIC

REF

VBAT

REF

Undervoltage, Overvoltage

Hot Warning, Overtemperature

Short Detect, Open Detect

Stall Detect

DNGPDNGA

Regulator

Logic Supply

Regulator

VREG

PAD

VBB

PWM

Control

Bridge

Control

Logic

PWM

Control

Translator

Serial Interface

System

Control

and

Registers

CP2

VCP

CP1

VBB

In Serial mode:

STEP => STRn

DIR => SDI

MS => SCK

+ –

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Pin-out Diagram

Terminal List Table

Name Number Function

AGND 6 Analog reference ground

CP1 17 Charge pump capacitor

CP2 18 Charge pump capacitor

DIAG 14 Diagnostic output, active low (inverted for serial transfer

acknowledgement)

DIR 4 Direction select input (SDI in Serial mode: serial word input)

MS 7 Microstep select input (SCK in Serial mode: serial clock input)

OAP 2 Bridge A positive output

OAM 19 Bridge A negative output

OBP 8 Bridge B positive output

OBM 12 Bridge B negative output

PAD – Exposed thermal pad

PGND 9 Power ground

REF 5 Reference input voltage

RESETn 3 Chip reset, active low

SENSA 1 Current sense node – bridge A

SENSB 10 Current sense node – bridge B

STEP 15 Step input (STRn in Serial mode: active low serial data strobe

and serial access enable input)

VBB 11 Motor supply voltage

VBB 20 Motor supply voltage

VCP 16 Pump storage capacitor

VREG 13 Regulated voltage

Ref

Reg

SENS

OAP

REF

AGND

OBP

PGND

VBB

OAM

CP2

CP1

DIAG

VREG

OBM

RESETn

VCP

DIR

STEP

SENSBVBB

Charge

Pump

I/O and Control

MS 14

PAD

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

ELECTRICAL CHARACTERISTICS Valid at TJ = –40°C to 150°C, VBB = 7 to 28 V; unless otherwise specified

Characteristic Symbol Test Conditions Min. Typ. Max. Unit

Supplies

Supply Voltage Range1VBB

Functional, no unsafe states 0 – 50 V

Outputs driving 3.8 – VBBOV V

Supply Quiescent Current IBBQ

DIS = 1 – – 15 mA

RESETn < 0.5 V – 1 10 µA

Charge Pump Voltage VCP

With respect to VBB, VBB >7.5 V, DIS=1,

RESETn = 1 – 6.7 – V

Internal Regulator Voltage VREG DIS = 1, RESETn = 1, VBB >7.5 V – 7.2 – V

Internal Regulator Dropout Voltage VREGDO DIS = 1, RESETn = 1, VBB > 6 V – 100 200 mV

Motor Bridge Output

High-Side On-Resistance2RDS(on)H

VBB = 13.5 V, IOUT = –1 A, TJ = 25°C – 500 600

mΩVBB = 13.5 V, IOUT = –1 A, TJ = 150°C – 900 1100

VBB = 7 V, IOUT = –1 A, TJ = 25°C – 625 750

High-Side Body Diode Forward

Voltage VfH If = 1 A – – 1.4 V

Low-Side On-Resistance RDS(on)L

VBB =13.5 V, IOUT = 1 A, TJ = 25°C – 500 600

mΩVBB = 13.5 V, IOUT = 1 A, TJ = 150°C – 900 1100

VBB = 7 V, IOUT = 1 A, TJ = 25°C – 625 750

Low-Side Body Diode Forward

Voltage2VfL If = –1 A – – 1.4 V

Output Leakage Current2IOUT(Lkg)

DIS = 1, RESETn = 1, VOUT = VBB –120 –65 – µA

DIS = 1, RESETn = 1, VOUT = 0 V –200 –120 –

DIS = 1, RESETn = 0, VOUT = VBB – < 1.0 20 µA

DIS = 1, RESETn = 0, VOUT = 0 V –20 < 1.0 –

Current Control

Internal Oscillator Frequency fOSC 3.6 4 4.4 MHz

Blank Time3tBLANK Default blanking time – 3.5 – µs

PWM Frequency3fPWM Default frequency – 21.7 – kHz

Reference Input Voltage VREF 0.8 – 2 V

Internal Reference Voltage VREFint VREF > 2.5 V 1.1 1.2 1.3 V

Reference Input Current2IREF –3 0 3 µA

Maximum Sense Voltage VSMAX – 125 – mV

Current Trip Point Error4EITrip VREF = 2 V – – ±5 %

Continued on the next page…

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Logic Input and Output – DC Parameters

Input Low Voltage VIL – – 0.8 V

Input Low Voltage for Sleep Mode VILS RESETn input only – – 0.5 V

Input High Voltage VIH 2.0 – – V

Input Hysteresis VIHys 100 300 – mV

Input Pull-Down Resistor RPD – 50 – kΩ

Output Low Voltage VOL IOL = 2 mA – – 0.4 V

Output Leakage2IO(Lkg) 0 V < VO < 5 V –1 – 1 µA

Logic Input and Output – Dynamic Parameters (see figures 1 and 4)

Reset Pulse Width tRST 1 – 6 µs

Reset Shutdown Pulse Width tRSD 40 – – µs

Input Pulse Filter Time tPIN STEP, DIR – 80 – ns

STEP High tSTPL 1 – – µs

STEP Low tSTPH 1 – – µs

Setup Time tSU

MS, DIR; from control input change to STEP

change 200 – – ns

Hold Time tH

MS, DIR; from STEP change to control input

change 200 – – ns

Wake-Up from Reset tEN – – 1 ms

Serial Interface – Dynamic Parameters (see figures 2 and 3)

Clock High Time tSCKH Reference A 50 – – ns

Clock Low Time tSCKL Reference B 50 – – ns

Strobe Lead Time tSTLD Reference C 30 – – ns

Strobe Lag Time tSTLG Reference D 30 – – ns

Strobe High Time tSTRH Reference E 1100 – – ns

Data In Setup Time to Clock Rising tSDIS Reference F 15 – – ns

Data In Hold Time from Clock Rising tSDIH Reference G 10 – – ns

Interface Mode Switch Timing (see figures 3 through 5)

Sequence Minimum Hold Time tSSH Reference H 58 64.5 72 µs

Serial Mode Exit Time tSSEX Reference J – – 2 µs

Serial Mode Acknowledge Time tSAT Reference K 59 65.5 73 µs

Serial Mode Acknowledge Pulse tSAP Reference L; consistent fault status 921 1024 1127 µs

ELECTRICAL CHARACTERISTICS (continued) Valid at TJ = –40°C to 150°C, VBB = 7 to 28 V; unless otherwise specified

Characteristic Symbol Test Conditions Min. Typ. Max. Unit

Continued on the next page…

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Diagnostics and Protection

VBB Overvoltage Threshold VBBOV VBB rising 32 34 36 V

VBB Overvoltage Hysteresis VBBOVHys 2 – 4 V

VBB Undervoltage Threshold VBBUV VBB falling 5.2 5.5 5.8 V

VBB Undervoltage Hysteresis VBBUVHys 500 760 – mV

VBB Power-On Reset Threshold VBBPOR VBB falling – 2.8 3.0 V

VBB Power-On Reset Hysteresis VBBPORHys 50 100 – mV

VREG Undervoltage Threshold – High VREGUVH VREG falling 4.6 4.8 4.95 V

VREG Undervoltage Hysteresis – High VREGUVHHys 250 370 – mV

VREG Undervoltage Threshold – Low VREGUVL VREG falling 3.2 3.35 3.5 V

VREG Undervoltage Hysteresis – Low VREGUVLHys 100 230 – mV

High-Side Overcurrent Threshold IOCH Sampled after tSCT 1.4 2.05 2.65 A

High-Side Current Limit ILIMH Active during tSCT 3 5.5 8 A

Low-Side Overcurrent Sense Voltage VOCL Sampled after tSCT 210 250 290 mV

Overcurrent Fault Delay tSCT Default fault delay 1500 2000 2700 ns

Open Load Current Threshold Error EIOC VREF = 2 V – – ±10 %

Hot Temperature Warning Threshold TJWH Temperature increasing 125 135 145 ºC

Hot Temperature Warning Hysteresis TJWHHys – 15 – ºC

Overtemperature Shutdown TJF Temperature increasing 155 170 – ºC

Overtemperature Hysteresis TJHys Recovery = TJF – TJHys – 15 – ºC

1The term functional indicates operation is correct but parameters may not be within specification above or below the general limits (7 to 28 V). Outputs

not operational above VBBOV or below VREGUVL.

2For input and output current specifications, negative current is defined as coming out of (sourcing) the specified device pin.

3Assumes a 4 MHz clock.

4Current Trip Point Error is the difference between the actual current trip point and the target current trip point, referred to maximum full scale (100%)

current: ETrip = 100 × ( ITripActual – ITripTarget ) / IFullScale%

ELECTRICAL CHARACTERISTICS (continued) Valid at TJ = –40°C to 150°C, VBB = 7 to 28 V; unless otherwise specified

Characteristic Symbol Test Conditions Min. Typ. Max. Unit

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

STEP

DIR

tSTPL

tSTPH

tSU

RESETn

tEN

Output

State

ctive Undefined

STEP

(STRn)

X=don’t care

(SCK)

DIR

(SDI) X

C J

STEP/DIR Mode Serial Mode STEP/DIR

XX X

H H H

AD E

STRn

(STEP)

X=don’t care

SCK

(MS)

SDI

(DIR)

X X D14 X 51D X D0

Figure 1. Control Input Timing

Figure 2. Serial Data Timing

Figure 3. Interface Mode Change Timing

Interface Timing Diagrams

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

STRn

(STEP)

X=don’t care

SCK

(MS)

SDI

(DIR)

Serial Mode Step and Direction Mode

DIAG

(

Fault Present

)

DIAG

(

No Fault

)

XD0

D1X X

STEP

(STRn)

X=don’t care

(SCK)

DIR

(SDI) X

H H H H

Step and Direction Mode Serial Mode

X X

DIAG

(

Fault Present

)

DIAG

(

No Fault

)

Figure 4. Step and Direction to Serial Mode Change Acknowledge Timing

Figure 5. Serial to Step and Direction Mode Change Acknowledge Timing

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Functional Description

The A4992 is an automotive stepper motor driver suitable for

high temperature applications such as headlamp bending and

levelling, throttle control, and gas recirculation control. It is also

suitable for other low current stepper applications such as air con-

ditioning and venting. It provides a flexible microstepping motor

driver controlled with simple step and direction inputs. It can also

be switched into an optional serial interface mode, where it can

be configured and driven via an SPI compatible serial interface.

The two DMOS full-bridges are capable of driving bipolar step-

per motors in full-, half-, quarter-, eighth-, and sixteenth-step

modes, at up to 28 V, with phase current up to ±1.4 A but limited

by power dissipation and ambient temperature. For most appli-

cations typical phase current is up to ±750 mA. The current in

each phase of the stepper motor is regulated by a fixed-frequency

peak-detect PWM current control scheme operating in an adap-

tive mixed decay mode. This provides reduced audible motor

noise and increased step accuracy for a wide range of motors and

operating conditions.

The outputs are protected from short circuits and features for

open load and stalled rotor detection are included. Chip level pro-

tection includes hot thermal warning, overtemperature shutdown,

and overvoltage and undervoltage lockout.

Pin Functions

VBB Main motor supply and chip supply for internal regulators

and charge pump. Both VBB pins should be connected together

and each decoupled to ground with a low ESR electrolytic

capacitor and a good ceramic capacitor.

CP1, CP2 Pump capacitor connection for charge pump. Connect

a 100 nF (50 V) ceramic capacitor, between CP1 and CP2.

VCP Above supply voltage for high-side drive. A 100 nF (16 V)

ceramic capacitor should be connected between VCP and VBB to

provide the pump storage reservoir.

VREG Regulated supply for bridge gate drive. Should be decou-

pled to ground with a 470 nF (10V) ceramic capacitor.

AGND Analog reference ground. Quiet return for measurement

and input references. Connect to PGND. See recommendations in

Layout section.

PGND Digital and power ground. Connect to supply ground and

AGND. See recommendations in Layout section.

OAP, OAM Motor connection for phase A. Positive motor phase

current direction is defined as flowing from OAM to OAP.

OBP, OBM Motor connection for phase B. Positive motor phase

current direction is defined as flowing from OBM to OBP.

SENSA Phase A current sense. Connect sense resistor between

SENSA and PGND.

SENSB Phase B current sense. Connect sense resistor between

SENSB and PGND.

REF Reference input to set absolute maximum current level for

both phases. Defaults to internal reference when driven higher

than 2.5 V.

STEP (STRn) Step logic input with internal pull-down resistor.

Motor advances on rising edge. In serial mode, STEP pin takes

on the functions of STRn, the serial data strobe and serial access

enable input. When STRn is high any activity on MS (SCK func-

tion) or DIR (SDI function) is ignored.

DIR (SDI) Direction logic input with internal pull-down resistor.

Direction changes on next STEP rising edge. When DIR is high

the phase angle number is incremented by one on each rising

edge of STEP. In serial mode, DIR takes on the function of SDI,

the serial data input, accepting a 16-bit serial word input, with

MSB first.

MS (SCK) Microstep resolution select input with internal pull-

down resistor. In serial mode, MS takes on the function of SCK,

the serial clock input. Data is latched in from DIR (SDI func-

tion) on the rising edge of SCK. There must be 16 rising edges

per write and MS (SCK) must be held high when STEP (STRn)

changes.

RESETn Resets faults when pulsed low. Forces low-power

shutdown (sleep mode) when held low for more than the reset

shutdown width, tRSD. Can be pulled to VBB with 38 kΩ resistor.

DIAG Diagnostic open drain output, active low. Low indicates the

presence of a fault. External pull-up resistor required.

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Driving a Stepper Motor

A two-phase stepper motor is made to rotate by sequencing the

relative currents in each phase. In its simplest form each phase is

simply fully energized in turn by applying a voltage to the wind-

ing. For more precise control of the motor torque across tempera-

ture and voltage ranges, current control is required. For efficiency

this is usually accomplished using PWM techniques. In addition,

current control also allows the relative current in each phase to be

controlled providing more precise control over the motor move-

ment and hence improvements in torque ripple and mechanical

noise.

For bipolar stepper motors the current direction is significant

so the voltage applied to each phase must be reversible. This

requires the use of a full-bridge (also known as an H-bridge)

which can switch each phase connection to supply or ground.

Phase Current Control

In the A4992, current to each phase of the two-phase bipolar

stepper motor is controlled through a low impedance N-channel

DMOS full bridge. This allows efficient and precise control of

the phase current using fixed-frequency pulse width modulation

(PWM) switching. The full-bridge configuration provides full

control over the current direction during the PWM on-time and

the current decay mode during the PWM off-time. The A4992

automatically controls the bridge decay mode to provide the opti-

mum current control completely transparent to the user.

Each leg (high-side, low-side pair) of a bridge is protected from

shoot-through by a fixed dead time. This is the time between

switching off one FET and switching on the complementary

FET. Cross-conduction is prevented by lockout logic in each

driver pair.

The phase currents and in particular the relative phase currents

are defined by the on-board phase current table, which is shown

here in table 3. This table defines the two phase currents at each

microstep position. For each of the two phases, the current is

measured using a sense resistor, RSx, with voltage feedback to the

respective SENSx pin. The sense voltage is amplified by a fixed

gain and compared to the output of the digital-to-analog converter

(DAC) for that phase. The target current level is then defined by

the voltage from the DAC.

The maximum phase current, ISMAX, is defined by the sense

resistor and the reference input as:

ISMAX = VREF / (16 × RSx

)

where VREF is the voltage at the REF pin and RSx is value of the

sense resistor for that phase.

The actual current delivered to each phase at each step angle is

determined by the value of ISMAX and the contents of the phase

current table. For each phase, the value in the phase current table

is passed to the DAC, which uses ISMAX as the reference 100%

level (code 63) and reduces the current target depending on the

DAC code. The output from the DAC is used as the input to the

current comparators.

The current comparison is ignored at the start of the PWM on-

time for a duration referred to as the blank time. The blank time

is necessary to prevent any capacitive switching currents from

causing a peak current detection.

The PWM on-time starts at the beginning of each PWM period.

The current rises in the phase winding until the sense voltage

reaches the threshold voltage for the required current level. At

this point the PWM off-time starts and the bridge is switched into

fast decay. The sense voltage continues to be monitored. When

the sense voltage drops below the threshold voltage the bridge is

switched into slow decay for the remainder of the PWM period.

This mixed decay technique automatically adapts the current con-

trol to a wide range of motors and operating conditions in order

to minimize motor torque ripple and motor noise. It also provides

the lowest motor power dissipation and the highest motor effi-

ciency across a wide range of voltage and temperature conditions.

Step Angle and Direction Control

The relative phase currents are defined by the on-board phase

current table (see table 3). This table contains 64 lines and is

addressed by the step angle number, where step angle 0 cor-

responds to 0° or 360°. The step angle number is generated

internally by the step sequencer, which is controlled either by

the STEP and DIR inputs or by the step change value from the

serial input. The step angle number determines the motor posi-

tion within the 360° electrical cycle and a sequence of step angle

numbers determines the motor movement. Note that there are

four full mechanical steps per 360° electrical cycle.

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Each line of the phase current table has a 6-bit value, per phase,

to set the DAC level for each phase plus an additional bit, per

phase, to determine the current direction in each phase. The step

angle number sets the electrical angle of the stepper motor in

sixteenth microsteps, approximately equivalent to electrical steps

of 5.625°.

On first power-up or after a power-on reset, the step angle

number is set to 8, equivalent to the electrical 45° position. This

position is referred to as the home position. The maximum cur-

rent in each phase, ISMAX , is defined by the sense resistor and the

voltage at the REF pin, as described in the Phase Current Control

section, above. The phase currents for each entry in the phase

current table are expressed as a percentage of this maximum

phase current.

A pulse on the STEP input automatically increments the step

angle number when DIR is high and decrements the step angle

number when DIR is low. The magnitude of the resulting change

in angle is determined by the selected microstep mode. When

step and direction control mode is programmed, the microstep

mode is determined solely by the state of the MS input at power-

up or after a power-on reset. Half step is selected if MS is low,

and quarter step is selected if MS is high. When serial mode is

programmed, this allocation and three other pairs of step modes

are available, allowing full and eighth steps on STEP input (refer

to the Serial Interface section, below).

The serial interface can also be used to control the stepper motor

directly. This facility enables full control of the stepper motor

at any microstep resolution up to 1/16 microstep, plus the abil-

ity to change microstep resolution during operation, from one

microstep to the next. When using the serial interface to control

the stepper motor a step change value (6-bit) is input through the

serial interface to increment or decrement the step angle number.

The step change value is a two’s complement (2’s C) number,

where a positive value increments the step angle number and

a negative value decrements the step angle number. A single

step change in the step angle number is equivalent to a single

1/16 microstep. Therefore, for correct motor movement, the step

change value should be restricted to no greater than 16 steps posi-

tive or negative.

In both control input modes, the resulting step angle number is

used to determine the phase current value and current direction

for each phase based on the phase current table. The decay mode

is determined by the position in the phase current table and the

intended direction of rotation of the motor.

Diagnostics

The A4992 integrates several diagnostic features to protect the

driver and load, from both fault conditions and extreme operat-

ing environments. Some of these features automatically disable

the current drive to protect the outputs and the load. Others only

provide an indication of the likely fault status (see table 1).

A single open-drain diagnostic output pin, DIAG, provides mul-

tiple diagnostic signals. At power-up or after a power-on reset,

the DIAG pin outputs a simple Fault flag, which is low if a fault

is present. This Fault flag remains low while the fault is present

or if one of the latched faults (short circuit or serial write ) has

been detected.

In addition to the Fault flag, which signals all faults, the DIAG

output can be programmed through the serial interface to provide

four specific diagnostic signals:

• Stall signal, which goes low only when a stall is detected.

• Open load signal, which goes low only when an open load is

detected.

• Temperature signal, which goes low only when the chip tem-

perature rises above either the Hot Temperature Warning or the

Overtemperature thresholds.

• Supply voltage signal, which goes low only when:

▫ VBB goes above the VBB overvoltage threshold,

▫ VBB goes below the VBB undervoltage threshold, or

▫ VREG goes below the VREG undervoltage threshold.

Table 1. Fault Table

Diagnostic Action Latched

VBB Overvoltage Disable outputs, set Fault flag No

VBB Undervoltage Set Fault flag No

VREG Undervoltage Disable outputs, set Fault flag No

Power-On Reset Power-down, full reset No

Temperature Warning Set Fault flag No

Overtemperature Disable outputs, set Fault flag No

Bridge Short Disable outputs, set Fault flag Yes

Bridge Open Set Fault flag No

Stall Detect Set Fault flag No

Serial Write Fault Set Fault flag Yes

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

System Diagnostics

At the system level the supply voltages and chip temperature are

monitored.

Supply Voltage Monitors

The motor supply, VBB , and the regulator output, VREG , are

monitored. The motor supply is monitored for overvoltage and

undervoltage, and the regulator output for undervoltage, as fol-

lows:

• If the motor supply voltage, VBB , goes above the VBB Over-

voltage Threshold, the A4992 will disable the outputs and will

indicate the fault. If the motor supply voltage then goes below

that threshold, the outputs will be re-enabled and the Fault flag

removed.

• If the motor supply voltage, VBB , goes below the VBB Under-

voltage Threshold, the A4992 will indicate the fault and reduce

the VREG Undervoltage Threshold to the low level. When

the motor supply voltage goes above the VBB Undervoltage

Threshold the VREG Undervoltage Threshold will be increased

to the high level and the Fault flag removed.

• If the motor supply voltage, VBB , goes below the VBB power-

on reset threshold, the A4992 will be completely disabled ex-

cept to monitor the motor supply voltage level. When the motor

supply voltage rises above the VBB Power-On Reset Threshold,

a power-on reset will take place and all registers will be reset to

the default state.

• If the output of the regulator, VREG

, goes below the VREG

Undervoltage Threshold, the A4992 will disable the outputs

and indicate the fault. When the regulator output rises above

that threshold the outputs will be re-enabled and the Fault flag

removed.

The VREG Undervoltage Threshold level is determined by the

state of the VBB undervoltage monitor. If VBB falls causing a

VBB undervoltage fault, then the VREG threshold is reduced to

the low level, VREGUVL. When VBB is above the VBB Under-

voltage Threshold the VREG Undervoltage Threshold is set to

the high level, VREGUVH. This allows the A4992 to continue to

drive a stepper motor with a motor supply (VBB) voltage as low

as 3.8 V without disabling the outputs. By retaining the higher

threshold when VBB is above the VBB Undervoltage Threshold,

the A4992 also provides protection for its outputs from excessive

power dissipation during a high voltage transient on VBB when

an independent VREG undervoltage condition is present.

Note that the point at which the A4992 stops driving the motor

will always be less than 3.8 V. The maximum value for the low

level VREG undervoltage is 3.5 V and for the VREG drop out,

VREGDO , is 200 mV. This means that the VREG undervoltage

will never occur until VBB falls below 3.7 V, giving a 100 mV

margin for noise. Typically the VREG undervoltage will occur

when VBB drops below 3.45 V. The A4992 will continue with full

PWM current control and all output fault detection right down to

the point at which the VREG undervoltage occurs.

Figures 6 and 7 show how the undervoltage thresholds change

when a typical cold crank transient occurs.

Figure 6. Response to an undervoltage transient

Figure 7. Expanded view of undervoltage transient response

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

The standard ISO7637 pulse 4 is shown for reference in figure 6.

The VBB transient shown is lower than the standard ISO pulse

due to the forward voltage of a reverse polarity protection diode

and switching transients.

Figure 7 provides more detail of the time around which the VBB

undervoltage is detected, and shows the VREG voltage follow-

ing below the VBB voltage by the maximum offset voltage of

the VREG regulator. Typically this dropout will be less than the

200 mV shown.

When VBB drops below the falling VBB Undervoltage Thresh-

old (at 1.2 ms and 5.6 V in figure 7), the VREG Undervoltage

Threshold drops from high, 4.8 V (typ), to low, 3.35 V (typ). At

the same time, the VBB Undervoltage Threshold increases by the

VBB Threshold Hysteresis, 760 mV, (typ) and the Fault flag is

active.

This state remains until VBB increases above the rising VBB

undervoltage threshold (at 127 ms and 6.4 V in figure 6). At this

point the VREG Undervoltage Threshold is increased back to the

high threshold value of 4.8 V (typ) and the reverse hysteresis is

applied to the VBB Undervoltage Threshold causing it to drop

back to the falling level of 5.5 V (typ). The Fault flag goes inac-

tive.

When a power-on reset occurs, or the A4992 is activated from

sleep mode by taking RESETn high, then the VREG Under-

voltage Threshold is initially set to the high level, VREGUVH .

(A power-on reset occurs either when power is first applied or

when the motor supply voltage drops below the VBB Power-On

Reset Threshold.) The VREG threshold will remain at the high

level, irrespective of the state of VBB, until the VBB voltage has

exceeded the VBB Undervoltage Threshold for the first time.

After this has happened, the VREG Undervoltage Threshold is

then determined by the state of the VBB undervoltage monitor

output. When applying power, or when activating from sleep

mode, the outputs should remain inactive for at least the Wake-

Up from Reset time, tEN , to allow the internal charge pump and

regulator to reach their full operating state.

The VBB and VREG undervoltage monitor system is designed to

allow the A4992 to continue operating safely during the extreme

motor supply voltage drop caused by cold cranking with a weak

battery when a reverse battery protection diode is also present.

During low voltage transients the A4992 will continue to step a

motor. However, current control will not achieve the same accu-

racy as specified with a motor supply voltage greater than 7 V. In

fact a low motor supply voltage may not provide sufficient drive

to allow the motor current to reach its normal operating level,

especially if the motor is rotating and a back EMF is present. It is

therefore recommended that, when a VBB undervoltage condition

is indicated, the motor should be held stationary. This will help

ensure that the motor does not slip and that the system retains

some degree of control over the motor position, thus avoiding the

need to recalibrate the motor position.

The output drive FETs of the A4992 remain protected from short

circuits down to the VREG undervoltage level. However, the

overcurrent thresholds cannot be ensured to meet the precision

specified at higher supply voltage. In addition the open load

detection may indicate a fault and the stall detection is not likely

to correctly identify a motor stall condition when VBB is below

the VBB undervoltage level.

Temperature Monitors

Two temperature thresholds are provided, a hot warning and an

overtemperature shutdown.

• If the chip temperature rises above the Hot Temperature Warn-

ing Threshold the Fault flag will go low. No action will be taken

by the A4992. When the temperature drops below the Hot Tem-

perature Warning Threshold, the Fault flag will go high.

• If the chip temperature rises above the Overtemperature Shut-

down threshold the Fault flag will go low and the A4992 will

disable the outputs to try to prevent a further increase in the

chip temperature. When the temperature drops below the over-

temperature threshold the Fault flag will go high and the outputs

will be re-enabled.

Bridge and Output Diagnostics

The A4992 includes monitors that can detect a short to supply or

a short to ground at the motor phase connections. These condi-

tions are detected by monitoring the current from the motor

phase connections through the bridge to the motor supply and to

ground. Low current comparators and timers are provided to help

detect possible open load conditions.

Short to Supply

A short from any of the motor connections to the motor supply,

VBB, is detected by monitoring the voltage across the low-side

current sense resistor in each bridge. This gives a direct measure-

ment of the current through the low-side of the bridge.

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

When a low-side FET is in the on state, the voltage across the

sense resistor, under normal operating conditions, should never

be more than the maximum sense voltage, VSMAX . In this state,

an overcurrent is determined to exist when the voltage across the

sense resistor exceeds VOCL , typically 2 × VSMAX . This overcur-

rent must be continuously present for at least the Overcurrent

Fault Delay time, tSCT , before the short fault is confirmed by

driving the DIAG output low if the Fault flag is selected. The

output is switched off and remains off until a fault reset occurs.

The actual overcurrent that VOCL represents is determined by the

value of the sense resistor and is typically 2 × ISMAX .

Short to Ground

A short from any of the motor connections to ground is detected

by directly monitoring the current through each of the high-side

FETs in each bridge. When a high-side FET is in the on state the

maximum current is typically always less than 1.4 A. In this state,

an overcurrent is determined to exist when the current through

the active high-side FET exceeds the High-Side Overcurrent

Threshold, IOCH .

This overcurrent must be present for at least the Overcurrent

Fault Delay time, tSCT , before the short fault is confirmed by

driving the DIAG output low if the Fault flag is selected. The

output is switched off and remains off until a fault reset occurs.

Note that when a short to ground is present the current through

the high-side FET is limited to the High-Side Current Limit,

ILIMH

, during the Overcurrent Fault Delay time. This prevents

large negative transients at the phase output pins when the out-

puts are switched off.

Shorted Load

A short across the load is indicated by concurrent short faults on

both high side and low side.

Short Fault Blanking

All overcurrent conditions are ignored for the duration of the

Overcurrent Fault Delay time, tSCT . The short detection delay

timer is started when an overcurrent first occurs. If the overcur-

rent is still present at the end of the short detection delay time

then a short fault will be generated and latched. If the overcurrent

goes away before the short detection delay time is complete then

the timer is reset and no fault is generated.

This prevents false short detection caused by supply and load

transients. It also prevents false short detection from the current

transients generated by the motor or wiring capacitance when a

FET is first switched on.

Short Fault Reset and Retry

When a short circuit has been detected, all outputs for the faulty

phase are disabled until: the next rising edge on the STEP input,

or the RESETn input is pulsed low, or a serial write is completed.

At the next step command, or after a fault reset, the Fault flag

is cleared, the outputs are re-enabled, and the voltage across the

FET is resampled.

While the fault persists the A4992 will continue this cycle,

enabling the outputs for a short period then disabling the out-

puts. This allows the A4992 to handle a continuous short circuit

without damage. If, while stepping rapidly, a short circuit appears

and no action is taken, the repeated short circuit current pulses

will eventually cause the temperature of the A4992 to rise and an

overtemperature fault will occur.

Open Load Detection

Possible open load conditions are detected by monitoring the

phase current when the phase DAC values are greater than 31.

The open load current threshold, IOL , is defined by the OL bit in

the diagnostic Configuration register as a percentage of the maxi-

mum (100%) phase current, ISMAX

The open load current monitor is only active after a Blank

Time from the start of a PWM cycle. An open load can only be

detected if the DAC value for the phase is greater than 31 and the

current has not exceeded the open load current threshold for more

than 15 PWM cycles. The A4992 continues to drive the bridge

outputs under an open load condition and clears the Fault flag as

soon as the phase current exceeds the open load current threshold

or the DAC value is less than 32.

Stall Detection

For all motors it is possible to determine the mechanical state of

the motor by monitoring the back EMF generated in the motor

phase winding. A stalled motor condition is when the phase cur-

rents are being sequenced to step the motor but the motor remains

stationary. This can be due to a mechanical blockage such as an

end stop or the step sequence exceeding the motor capability for

the attached load.

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

A PWM monitor feature is included in the A4992 to assist in

detecting the stall condition of the stepper motor. This feature

uses the indirect effect of the back EMF on the current rise quad-

rant to determine the point at which a stall occurs.

When a motor is running normally at speed, the back EMF,

generated by the magnetic poles in the motor passing the phase

windings, acts against the supply voltage and reduces the rise

rate of the phase current, as shown in figure 8. The PWM cur-

rent control does not activate until the current reaches the set

trip level for the microstep position. When a motor is stopped,

as in a stall condition, the back EMF is reduced. This allows the

current to rise to the limit faster and the PWM current control to

activate sooner. Assuming a constant step rate and motor load this

results in an increase in the quantity of PWM cycles for each step

of the motor. The A4992 uses this difference to detect a motor

changing from continuous stepping to being stalled.

Two PWM counters, one for each phase, accumulate the count of

PWM cycles when the phase current is stepped from zero to full

current. At the end of each phase current rise, the counter for that

phase is compared to the counter for the previous current rise in

the opposite phase (see figure 9). If the difference is greater than

the PWM count difference in the Configuration register, then the

ST fault signal will go low.

This stall detection scheme assumes two factors:

• The motor must be stepping fast enough for the back EMF to

reduce the phase current slew rate. Stall detection reliability

improves as the current slew rate reduces.

• The motor is not being stepped in full step.

Although stall detection cannot be guaranteed using this detection

method, good stall detection reliability can be achieved by careful

selection of motor speed, count difference, and by conforming to

the above factors.

Effect of stall

conditionNormal running

condition

Increased number o

PWM cycles at each

microstep

Figure 8. Effect of stall condition on current rise

Figure 9. Stall detect by PWM count compare

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

A three wire synchronous serial interface, compatible with SPI,

can be used to control all features of the A4992.

The A4992 powers-up in the default step and direction control

mode. The serial mode is only available following a specific

sequence on the DIR and MS pins when STEP is high (see

figure 3). The sequence starts when STEP is high and MS is held

high and DIR is held low for longer than the Sequence Minimum

Hold Time, tSSH . MS must then be held low and DIR high for

longer than tSSH , followed by holding MS high and DIR low for

longer than tSSH . The final step is to hold DIR high for tSSH

, at

which time the A4992 will enter the serial control mode. If STEP

is taken low at any time during the sequence then the sequence is

reset and the sequence of MS and DIR must be repeated.

When the sequence is accepted by the A4992, the DIAG output

will change state for the Serial Mode Acknowledge Pulse dura-

tion, tSAP , to indicate that the switch was successful. For example

if a fault is present when switching between modes, DIAG will

be low during the sequence and will first go high, to acknowledge

the mode change, then go low after tSAP . If no fault is present

DIAG will be high during the sequence and will first go low to

acknowledge the mode change, then go high after tSAP .

When in serial control mode the function of the STEP, DIR, and

MS pins change. STEP assumes the function of STRn, the serial

data strobe input, DIR functions as SDI, the serial data input, and

MS as SCK, the serial clock.

The A4992 will remain in the serial mode as long as the SER bit

remains set to 1. If a serial transfer occurs when SER is 0, then

the A4992 will revert to the step and direction control mode after

the Serial Mode Exit Time, tSSEX

, following the rising edge of

STRn. The STRn, SDI, and SCK inputs will then revert to their

default functions, STEP, DIR, and MS, respectively. The DIAG

output will change state for tSAP to indicate that the switch was

successful.

Note that the DIAG output is inverted to acknowledge a state

change. This means that, if the DIAG output is in the default fault

output mode, and a fault occurs or is removed during the time the

DIAG pin is inverted, then it will change state part way through

the acknowledge pulse time. If this occurs, and it is not clear that

the mode has changed, then the sequence must be reset before

entering the sequence to change from the step and direction mode

to the serial mode. A serial write can then be made to reset back

to step and direction mode.

The A4992 can be operated without the serial interface, by using

the default settings and the STEP and DIR inputs. Application-

specific configurations are only possible, however, by setting the

appropriate register bits through the serial interface. In addition to

setting the configuration bits, the serial interface can also be used

to control the motor directly.

The serial interface timing requirements are specified in the

Electrical Characteristics table, and illustrated in the Serial Data

Timing diagram (figure 2). Data is received on SDI and clocked

Serial Interface

Table 2. Serial Register Definition*

15 14 13 12 11 109876543210

Configuration Register

Config 0SER – TSC OL CD4 CD3 CD2 CD1 CD0 MS1 MS0 HLR TBK FRQ1 FRQ0

000000000000001

Run Register

Run 1SER – DG2 DG1 DG0 SR DIS – – SC5 SC4 SC3 SC2 SC1 SC0

000000000000000

*Power-on reset value shown below each input register bit.

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

through a shift register on the rising edge of the clock signal input

on SCK. STRn is normally held high, and is only brought low

to initiate a serial transfer. No data is clocked through the shift

When 16 data bits have been clocked into the shift register, STRn

must be taken high to latch the data into the selected register.

When this occurs, the internal control circuits act on the new

data. If fewer than, or greater than 16 rising edges of the SCK are

received before STRn goes high then the sequence is considered

invalid and a serial write fault condition is set. This fault condi-

tion can be cleared by a subsequent valid serial write and by a

power-on-reset or by a RESETn low pulse.

Configuration and Run Registers

The serial data word is 16 bits, input MSB first, and the first bit

selects which register is written.

• The first register, selected when the MSB is 0, is the Configura-

tion register, containing system and diagnostic parameters.

• The second register, selected when the MSB is 1, is the Run

tor movement and phase current.

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Config 0 SER – TSC OL CD4 CD3 CD2 CD1 CD0 MS1 MS0 HLR TBK FRQ1 FRQ0

000000000000001

MS[1..0] Microstep modes for MS input control

MS1 MS0 Microstep Modes Default

MS = Low MS = High

0 0 Half Quarter D

0 1 Full Half

1 0 Full Quarter

1 1 Half Eighth

FRQ[1..0] Frequency, assumes 4 MHz clock

FRQ 1 FRQ 0 Period / Frequency Default

0 0 60 µs / 16.7 kHz

0 1 46 µs / 21.7 kHz D

1 0 40 µs / 25.0 kHz

1 1 32 µs / 31.3 kHz

SER Selects serial or step and direction mode

following serial transfer

SER Control Mode Select Default

0 Switch to step and direction mode

1 Remain in serial mode D

TSC Overcurrent fault delay, assumes

4 MHz clock

TSC Detect Delay Time Default

0 2 µs D

1 4 µs

HLR Selects slow decay recirculation path

HLR Recirculation Path Default

0 High side D

1 Low side

TBK Blank Time, assumes 4 MHz clock

TBK Blank Time Default

0 3.5 µs D

1 1.5 µs

OL Open load current threshold as a

percentage of maximum current defined

by ISMAX

OL Open Load Current Default

0 20% D

1 30%

CD[4..0] PWM count difference for ST detection

0 = Stall detect disabled

Default to 0.

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

DG[2..0] Selects signal routed to DIAG output

DG2 DG1 DG0 Signal on DIAG pin

(low true) Default

0 0 0 All Faults D

0 0 1

VBB and VREG

undervoltage, and

VBB overvoltage

0 1 0 Open load

0 1 1 Temperature warning,

and overtemperature

1 X X Stall

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Run 1SER – DG2 DG1 DG0 SR DIS – – SC5 SC4 SC3 SC2 SC1 SC0

000000000000000

SER Selects serial or step and direction mode

following serial transfer

SER Control Mode Select Default

0 Switch to step and direction mode D

1 Remain in serial mode

SR Synchronous rectification

SR Synchronous rectification Default

0 Synchronous D

1 Diode recirculation

DIS Phase current disable

DIS Phase Outputs Default

0 Output bridges enabled D

1 Output bridges disabled

SC[5..0] Step change number

2’s complement format

Positive value increases step angle number

Negative value decreases step angle number

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Applications Information

Motor Movement Control

The A4992 provides two independent methods to control the

movement of a stepper motor. The simpler is the step and direc-

tion method, which only requires two control signals to control

the stepper motor in either direction. The other method is through

the serial interface, which provides more flexible control capa-

bility. Both methods can be used together (although it is not

common), provided the timing restrictions of the STEP input in

relation to the STRn input are preserved.

Phase Table and Phase Diagram

The key to understanding both of the available control methods

lies in understanding the on-board phase current table, shown

here as table 3. This table contains the relative phase current

magnitude and direction for each of the two motor phases at

each microstep position. The maximum resolution of the A4992

is 1/16 microstep. That is 16 microsteps per full step. There are 4

full steps per electrical cycle, so the phase current table has 64

microstep entries. The entries are numbered from 0 to 63. This

number represents the phase angle within the full 360° electrical

cycle and is called the step angle number. This is illustrated in

figure 10.

Figure 10 shows the contents of the phase current table as a phase

diagram. The phase B current, IB, from the phase current table, is

plotted on horizontal axis and the phase A current, IA, is plotted

on the vertical axis. The resultant motor current at each microstep

is shown as numbered radial arrows. The number shown cor-

responds to the 1/16 microstep step angle number in the phase

current table.

Figure 11 shows an example of calculating the resultant motor

current magnitude and angle for step number 28. The target is to

have the magnitude of the resultant motor current be 100% at all

microstep positions. The relative phase currents from the phase

current table are:

IA = 37.50%

IB = –92.19%

Assuming a full scale (100%) current of 1A means that the two

phase currents are:

IA = 0.3750 A

IB = -0.9219 A

The magnitude of the resultant will be the square root of the sum

of the squares of these two currents:

9953 (A).08499.01406.0||

=+=+=

III

So the resultant current magnitude is 99.53% of full scale. This

is within 0.5% of the target (100%) and is well within the ±5%

accuracy of the A4992.

15 16 17

43 44 45 46 47 48 49 50 51 52

Figure 10. A4992 Phase Current table as a phase diagram; values shown

are referred to as the step angle number

Figure 11. Calculation of resultant motor current

A28

IB28

=–92.19%

157.9°

=37.5%

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Table 3. Phase Current Table

Step Angle Number Phase Current

(% of ISMAX)

Step

Angle Phase DAC Step Angle Number Phase Current

(% of ISMAX)

Step

Angle Phase DAC

Full 1/2 1/4 1/8 1/16 A B A B A B Full 1/2 1/4 1/8 1/16 A B A B A B

0 0 0 0 0.00 100.00 0.0 0 0 0 63 4 8 16 32 0.00 –100.00 180.0 0 1 0 63

1 9.38 100.00 5.4 0 0 5 63 33 –9.38 –100.00 185.4 1 1 5 63

1 2 18.75 98.44 10.8 0 0 11 62 17 34 –18.75 –98.44 190.8 1 1 11 62

3 29.69 95.31 17.3 0 0 18 60 35 –29.69 –95.31 197.3 1 1 18 60

1 2 4 37.50 92.19 22.1 0 0 23 58 9 18 36 –37.50 –92.19 202.1 1 1 23 58

5 46.88 87.50 28.2 0 0 29 55 37 –46.88 –87.50 208.2 1 1 29 55

3 6 56.25 82.81 34.2 0 0 35 52 19 38 –56.25 –82.81 214.2 1 1 35 52

7 64.06 76.56 39.9 0 0 40 48 39 –64.06 –76.56 219.9 1 1 40 48

0 1 2 4 8 70.31 70.31 45.0 0 0 44 44 2 5 10 20 40 –70.31 –70.31 225.0 1 1 44 44

9 76.56 64.06 50.1 0 0 48 40 41 –76.56 –64.06 230.1 1 1 48 40

5 10 82.81 56.25 55.8 0 0 52 35 21 42 –82.81 –56.25 235.8 1 1 52 35

11 87.50 46.88 61.8 0 0 55 29 43 –87.50 –46.88 241.8 1 1 55 29

3 6 12 92.19 37.50 67.9 0 0 58 23 11 22 44 –92.19 –37.50 247.9 1 1 58 23

13 95.31 29.69 72.7 0 0 60 18 45 –95.31 –29.69 252.7 1 1 60 18

7 14 98.44 18.75 79.2 0 0 62 11 23 46 –98.44 –18.75 259.2 1 1 62 11

15 100.00 9.38 84.6 0 0 63 5 47 –100.00 –9.38 264.6 1 1 63 5

2 4 8 16 100.00 0.00 90.0 0 0 63 0 6 12 24 48 –100.00 0.00 270.0 1 1 63 0

17 100.00 –9.38 95.4 0 1 63 5 49 –100.00 9.38 275.4 1 0 63 5

9 18 98.44 –18.75 100.8 0 1 62 11 25 50 –98.44 18.75 280.8 1 0 62 11

19 95.31 –29.69 107.3 0 1 60 18 51 –95.31 29.69 287.3 1 0 60 18

5 10 20 92.19 –37.50 112.1 0 1 58 23 13 26 52 –92.19 37.50 292.1 1 0 58 23

21 87.50 –46.88 118.2 0 1 55 29 53 –87.50 46.88 298.2 1 0 55 29

11 22 82.81 –56.25 124.2 0 1 52 35 27 54 –82.81 56.25 304.2 1 0 52 35

23 76.56 –64.06 129.9 0 1 48 40 55 –76.56 64.06 309.9 1 0 48 40

1 3 6 12 24 70.31 –70.31 135.0 0 1 44 44 3 7 14 28 56 –70.31 70.31 315.0 1 0 44 44

25 64.06 –76.56 140.1 0 1 40 48 57 –64.06 76.56 320.1 1 0 40 48

13 26 56.25 –82.81 145.8 0 1 35 52 29 58 –56.25 82.81 325.8 1 0 35 52

27 46.88 –87.50 151.8 0 1 29 55 59 –46.88 87.50 331.8 1 0 29 55

7 14 28 37.50 –92.19 157.9 0 1 23 58 15 30 60 –37.50 92.19 337.9 1 0 23 58

29 29.69 –95.31 162.7 0 1 18 60 61 –29.69 95.31 342.7 1 0 18 60

15 30 18.75 –98.44 169.2 0 1 11 62 31 62 –18.75 98.44 349.2 1 0 11 62

31 9.38 –100.00 174.6 0 1 5 63 63 –9.38 100.00 354.6 1 0 5 63

4 8 16 32 0.00 –100.00 180.0 0 1 0 63 0 0 0 0 0.00 100.00 0.0 0 0 0 63

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

The reference angle, zero degrees (0°), within the full electrical

cycle (360°), is defined as the angle where IB is at +100% and IA

is zero. Each full step is represented by 90° in the electrical cycle

so each one-sixteenth microstep is: 90°/16 steps = 5.625°. The

target angle of each microstep position with the electrical cycle

is determined by the product of the Step angle number and the

angle for a single microstep. So for the example of figure 11:

°=°×= 5.157625.528

)(28 TARGET

The actual angle is calculated using basic trigonometry as:













+= −

)(28 tan180

ACTUAL I

( )

°=−+= 9.1571.22180

So the angle error is only 0.4°. Equivalent to about 0.1% error in

360° and well within the current accuracy of the A4992.

Note that each phase current in the A4992 is defined by a 6-bit

DAC. This means that the smallest resolution of the DAC is

100 / 64 = 1.56% of the full scale, so the A4992 cannot produce

a resultant motor current of exactly 100% at each microstep. Nor

can it produce an exact microstep angle. However, as can be seen

from the calculations above, the results for both are well within

the specified accuracy of the A4992 current control. The resultant

motor current angle and magnitude are also more than precise

enough for all but the highest precision stepper motors.

With the phase current table, control of a stepper motor is simply

a matter of increasing or decreasing the Step angle number to

move around the phase diagram of figure 10. This can be in

predefined multiples using the STEP input, or it can be variable

using the serial interface.

Using Step and Direction Control

The STEP input moves the motor at the microstep resolution

defined by the MS0 and MS1 bits and the logic level of the MS

input. The DIR input defines the motor direction. These inputs

define the output of a translator which determines the required

step angle number in the phase current table.

The MS input allows two microstep resolutions to be selected.

The default combination, reset at power-on, is half step when

MS is low, and quarter step when MS is high. The two resolution

combinations can be changed using the MS0 and MS1 bits in the

Configuration register through the serial interface. The combina-

tions available are as shown in table 4.

Note that the microstep selection is only used with the STEP

input. It has no effect when the motor is fully controlled through

the serial interface. 1/16 microstepping is only possible using the

serial interface.

In eighth-step mode the translator simply increments or decre-

ments the step angle number by two on each rising edge of the

STEP input depending on the logic state of the DIR input. In the

other three microstep resolution modes the translator outputs spe-

cific step angle numbers as defined in the phase current table.

Full step uses four of the entries in the phase current table. These

are 8, 24, 40, and 56 as shown in figure 12. Note that the four

positions selected for full step are not the points at which only

one current is active, as would be the case in a simple on-off full

step driver. There are two advantages in using these positions

rather than the single full current positions. With both phases

active, the power dissipation is shared between two drivers. This

slightly improves the ability to dissipate the heat generated and

reduces the stress on each driver. The second reason is that the

holding torque is slightly improved because the forces holding

the motor are mainly rotational rather than mainly radial.

Table 4. Microstep Mode Selection

(Serial Mode)

MS1 MS0 Microstep Mode

MS = Low MS = High

0 0 Half Step Quarter Step

0 1 Full Step Half Step

1 0 Full Step Quarter Step

1 1 Half Step Eighth Step

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Half step uses eight of the entries in the phase current table.

These are 0, 8, 16, 24, 32, 40, 48, and 56 as shown in figure 13.

Quarter step uses sixteen of the entries in the phase current table.

These are 0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 52, 56,

and 60 as shown in figure 14.

In half step and in quarter step, the single phase active positions

are used to preserve symmetry. However, if the motor is required

to stop with a significant holding torque for any length of time

it is recommended that the 45° positions be used; those are step

angle numbers 8, 24, 40, and 56, as used with full-step resolution.

Table 5 summarizes the step angle numbers used for the four

resolutions available when using the STEP input to control the

output of the A4992.

The microstep select inputs can be changed between each rising

edge of the STEP input. The only restriction is that the MS logic

input must comply with the set-up and hold timing constraints.

When the microstep resolution changes, the A4992 moves to the

next available step angle number on the next rising edge of the

STEP input. For example if the microstep mode is eighth and the

present step angle is 58 then with the direction forwards (increas-

ing step angle), changing to quarter step will cause the phase

number to go to 60 on the next rising edge of the STEP input. If

the microstep mode is changed to half step then the phase number

will go to 0 on the next rising edge of the STEP input. If the

microstep mode is changed to full step then the phase number

will go to 8 on the next rising edge of the STEP input.

Control Through the Serial Interface

The A4992 provides the ability to directly control the motor

movement using only the serial interface by directly increasing or

decreasing the step angle number. Note that the maximum value

of the step angle number is 63 and the minimum number is 0.

Therefore, any increase or reduction in the microstep number is

performed using modulo 64 arithmetic. This means that increas-

ing a step angle number of 63 by 1 will produce a step angle

number of 0. Increasing by two from 63 will produce 1 and so on.

Similarly in the reverse direction, reducing a step angle number

Figure 12. Full-step phase diagram using STEP input

Figure 13. Half-step phase diagram using STEP input

Figure 14. Quarter-step phase diagram using STEP input

Table 5. Step Angle Number Allocation

Mode Step Angle

Full Step 8, 24, 40, 56

Half Step 0, 8, 16, 24, 32, 40, 48,56

Quarter Step 0, 4, 8, 12, 16, 20, 24, 28,

32, 36, 40, 44, 48, 52, 56, 60

Eighth Step

0, 2, 4, 6, 8, 10, 12, 14, 16

18, 20, 22, 24, 26, 28, 32, 32,

34, 36, 38, 40, 42, 44, 46, 48,

50, 52, 54, 56, 58, 60, 62

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

of 0 by 1 will produce a step angle number of 63. Decreasing by

two from 0 will produce 62 and so on.

The least significant six bits of the Run register, bits 0 to 5, are

the step change number, SC[5..0]. This number is a two’s com-

plement number that is added to the step angle number causing it

to increase or decrease. Two’s complement is the natural integer

number system for most microcontrollers. This allows standard

arithmetic operators to be used, within the microcontroller, to

determine the size of the next step increment. Table 6 shows the

binary equivalent of each decimal number between 16 and +16.

Each increase in the step angle number represents a forwards

movement of one eighth microstep. Each decrease in the step

angle number represents a reverse movement of one eighth

microstep.

To move the motor one full step, the step angle number must be

increased or decreased by 16. To move the motor one half step,

the step angle number must be increased or decreased by 8. For

quarter step the increase or decrease is 4 and for eighth step, 2.

So, for example, to continuously move the motor forwards in

quarter-step increments, the number 4 (000100) is repeatedly

written to SC[5..0] through the serial interface Run register (see

figure 15). To move the motor backwards in quarter step incre-

ments, the number -4 (111100) is repeatedly written to SC[5..0]

(see figure 16). The remaining bits in the Run register should be

set for the required configuration and sent with the step change

number each time.

Table 6. Two’s Complements

Decimal

2’s

Complement Decimal

2’s

Complement

0 000000 – –

1 000001 –1 111111

2 000010 –2 111110

3 000011 –3 111101

4 000100 –4 111100

5 000101 –5 111011

6 000110 –6 111010

7 000111 –7 111001

8 001000 –8 111000

9 001001 –9 110111

10 001010 –10 110110

11 001011 –11 110101

12 001100 –12 110100

13 001101 –13 110011

14 001110 –14 110010

15 001111 –15 110001

16 010000 –16 110000

SDI

SCK

STRn

-4

SDI

SCK

STRn

STEP

Figure 8. Serial interface sequence for quarter step in forward direction

Figure 9. Serial interface sequence for quarter step in reverse direction

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

The step rate is controlled by the timing of the serial interface.

It is the inverse of the step time, tSTEP

, shown in figure 15. The

motor step only takes place when the STRn goes from low to

high when writing to the Run register. The motor step rate is

therefore determined by the timing of the rising edge of the STRn

input. The clock rate of the serial interface, defined by the fre-

quency of the SCK input, has no effect on the step rate.

Layout

The printed circuit board (PCB) should use a higher weight cop-

per thickness than a standard small signal or digital board. This

helps to reduce the impedance of the copper traces when conduct-

ing high currents. PCB traces carrying switching currents should

be as wide and short as possible to reduce the inductance of the

trace. This will help reduce any voltage transients caused by cur-

rent switching during PWM current control.

For optimum thermal performance, the exposed thermal pad on

the underside of the A4992 should be soldered directly onto the

board. A solid ground plane should be added to the opposite side

of the board and multiple vias through the board placed in the

area under the thermal pad.

Decoupling

All supplies should be decoupled with an electrolytic capaci-

tor in parallel with a ceramic capacitor. The ceramic capacitor

should have a value of 100 nF and should be placed as close as

possible to the associated supply and ground pins of the A4992.

The electrolytic capacitor connected to VBB should be rated to at

least 1.5 times the maximum voltage and selected to support the

maximum ripple current provided to the motor. The value of the

capacitor is unimportant but should be the lowest value with the

necessary ripple current capability.

The pump capacitor between CP1 and CP2, the pump storage

capacitor between VCP and VBB, and the compensation capaci-

tor between VREG and ground should be connected as close as

possible to the respective pins of the A4992.

Grounding

A star ground system, with the common star point located close

to the A4992 is recommended. On the 20-lead TSSOP package,

the reference ground, AGND (pin 6), and the power ground,

PGND (pin 9), must be connected together externally. The copper

ground plane located under the exposed thermal pad is typically

used as the star ground point.

Current Sense Resistor

To minimize inaccuracies caused by ground-trace IR drops in

sensing the output current level, the current-sense resistors ( RSx

)

should have an independent ground return to the star ground

point. This path should be as short as possible. For low-value

sense resistors the IR drop in the PCB trace to the sense resis-

tor can be significant and should be taken into account. Surface

mount chip resistors are recommended to minimize contact

resistance and parasitic inductance. The value, RS, of the sense

resistors is given by:

SMAX

REF

R×

=16

There is no restriction on the value of RS or VREF , other than the

range of VREF over which the output current precision is guar-

anteed. However, it is recommended that the value of VREF be

kept as high as possible to improve the current accuracy. Table 7

provides increasing values of ISMAX for suggested values of VREF

and standard E96 values of RS

Table 7. Suggested Values

ISMAX

(mA)

(mΩ)

VREF

(V)

100 499 0.8

200 499 1.6

300 417 2.0

405 309 2.0

501 249 2.0

610 205 2.0

702 178 2.0

812 154 2.0

912 137 2.0

1008 124 2.0

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Package LP, 20-Pin TSSOP

With Exposed Thermal Pad

1.20 MAX

0.15

0.00

0.30

0.19

0.20

0.09

8º

0º

0.60 ±0.15

1.00 REF

SEATING

PLANE

C0.10

20X

0.65 BSC

0.25 BSC

6.50±0.10

4.40±0.103.00 3.00

4.20

6.40±0.20

GAUGE PLANE

SEATING PLANE

ATerminal #1 mark area

For Reference Only; not for tooling use (reference MO-153 ACT)

Dimensions in millimeters

Dimensions exclusive of mold flash, gate burrs, and dambar protrusions

Exact case and lead configuration at supplier discretion within limits shown

0.45

1.70

PCB Layout Reference View

6.10

0.65

CExposed thermal pad (bottom surface); dimensions may vary with device

Reference land pattern layout (reference IPC7351 SOP65P640X110-21M);

All pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary

to meet application process requirements and PCB layout tolerances; when

mounting on a multilayer PCB, thermal vias at the exposed thermal pad

land can improve thermal dissipation (reference EIA/JEDEC Standard

JESD51-5)

Automotive Stepper Driver

A4992

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Revision History

Revision Revision Date Description of Revision

1 April 22, 2014 Revised TtJ spec. in Abs. Max. Ratings table

Allegro MicroSystems, LLC reserves the right to make, from time to time, such departures from the detail specifications as may be required to

permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that

the information being relied upon is current.

Allegro’s products are not to be used in any devices or systems, including but not limited to life support devices or systems, in which a failure of

Allegro’s product can reasonably be expected to cause bodily harm.

The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, LLC assumes no responsibility for its

use; nor for any infringement of patents or other rights of third parties which may result from its use.

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