
Wide Input Voltage, Synchronous USB Buck Regulator
with Remote Load Regulation
A8652,
A8653
12
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
FUNCTIONAL DESCRIPTION
Overview
The A8652/53 is a synchronous PWM buck regulator that inte-
grates low RDS(on) high-side and low-side N-channel MOSFETs.
It is designed to remain operational when input voltage falls as
low as 2.6 V. The A8652/53 employs peak current mode control
to provide superior line and load regulation, pulse-by-pulse cur-
rent limit, fast transient response and simple compensation. The
A8652/53 incorporates a Cable Drop Compensation (Remote
Load Regulation) function in its current mode control architec-
ture to adjust the output voltage according to the load current,
offsetting the voltage drop introduced by the wiring harness. The
reference voltage in the feedback loop is adjusted relative to the
voltage across the sensing resistor at the load side. When the
load current increases, it causes the reference voltage at the error
amplifier to increase and the output voltage to follow. The gain of
the voltage correction is configurable using the GADJ and IADJ
pins. Such features provide flexibility in setting the amount of
output voltage correction and the load current limit.
The features of the A8652/53 include Remote Load Regula-
tion, an internal precision reference, an adjustable switching
frequency, a transconductance error amplifier, an enable input,
integrated top and bottom switching MOSFETs, adjustable soft-
start time, pre-bias startup, and a Power OK output. Protection
features of A8652/53 include VIN undervoltage lockout, pulse-by-
pulse overcurrent protection, BOOT overvoltage and undervolt-
age protection, hiccup mode short-circuit protection, dynamic
overvoltage protection, and thermal shutdown. In addition, the
A8652/53 provides open-circuit, adjacent pin short-circuit, and
pin-to-ground short-circuit protection.
Reference Voltage
The A8652/53 incorporates an internal precision reference that
allows output voltages as low as 0.8 V. The accuracy of the
internal reference is ±1% from –40°C to 125°C and ±1.5% across
from –40°C to 150°C when the Remote Load Regulation is dis-
abled. The output voltage of the regulator is programmed with a
resistor divider between VOUT and the FB pin of the A8652/53.
Oscillator/Switching Frequency and
Synchronization
The PWM switching frequency of the A8652/53 is adjustable
from 100 kHz to 2.2 MHz and has an accuracy of about ±10%
over the operating temperature range. Connecting a resistor
from the FSET/SYNC pin to GND, as shown in the Applications
Schematic, sets the switching frequency. An FSET resistor with
±1% tolerance is recommended. A graph of switching frequency
versus FSET resistor value is shown in the Component Selection
section of this datasheet. The A8652/53 will suspend operation if
the FSET pin is shorted to GND or left open.
FSET/SYNC pin also can be used as a synchronization input that
accepts an external clock to switch the A8652/53 from 100 kHz
to 2.2 MHz and scales the slope compensation according to the
synchronization frequency. When being used as a synchroniza-
tion input, the applied clock pulses must satisfy the pulse width,
duty cycle, and rise/fall time requirements shown in the Electrical
Characteristics shown in this datasheet.
Remote Load Regulation Control and
Transconductance Error Amplifier
The Remote Load Regulation control in the A8652/53 provides
improved load regulation at the remote load by increasing the
voltage reference of the error amplifier to correct for the voltage
drop introduced by wiring harness to the load. The amount of
voltage correction is user-programmable with external configu-
ration resistors, allowing the A8652/53 to be applied to wiring
harnesses that have up to 750 mV IR drops at full load. The
Remote Load Regulation controller has a variety of protection
features, including a load-side current limit, a maximum regula-
tion voltage, and protection in the event of open pin or shorted
pin conditions.
The Remote Load Regulation voltage correction and protection
features interface with the error amplifier, which is a four-termi-
nal input device with three positive inputs and one negative input,
as shown in Figure 1. The negative input is simply connected to
the FB pin and is used to sense the feedback voltage for regula-
tion. The error amplifier performs an “analog OR” selection
between its positive inputs, operating according to the positive
input with the lowest potential. The three positive inputs are used
for soft-start, steady-state regulation, and the 15% maximum
regulation voltage. The error amplifier regulates to the soft-start
pin voltage minus 400 mV during startup, the sum of A8652/53
internal reference (VREF) and the Remote Load Regulation
correction (REF_ADJ) during normal operation, or the 920 mV
maximum.