LM1876
LM1876 Overture Audio Power Amplifier Series Dual 20W Audio Power
Amplifier with Mute and Standby Modes
Literature Number: SNAS097B
LM1876 Overture™Audio Power Amplifier Series
Dual 20W Audio Power Amplifier with Mute and Standby
Modes
General Description
The LM1876 is a stereo audio amplifier capable of delivering
typically 20W per channel of continuous average output
power into a 4Ωor 8Ωload with less than 0.1% THD+N.
Each amplifier has an independent smooth transition fade-
in/out mute and a power conserving standby mode which
can be controlled by external logic.
The performance of the LM1876, utilizing its Self Peak In-
stantaneous Temperature (˚Ke) (SPiKe™) protection cir-
cuitry, places it in a class above discrete and hybrid amplifi-
ers by providing an inherently, dynamically protected Safe
Operating Area (SOA). SPiKe protection means that these
parts are safeguarded at the output against overvoltage,
undervoltage, overloads, including thermal runaway and in-
stantaneous temperature peaks.
Key Specifications
jTHD+N at 1kHz at 2 x 15W continuous average
output power into 4Ωor 8Ω: 0.1% (max)
jTHD+N at 1kHz at continuous average
output power of 2 x 20W into 8Ω: 0.009% (typ)
jStandby current: 4.2mA (typ)
Features
nSPiKe protection
nMinimal amount of external components necessary
nQuiet fade-in/out mute mode
nStandby-mode
nIsolated 15-lead TO-220 package
nNon-Isolated 15-lead TO-220 package
nWide supply range 20V - 64V
Applications
nHigh-end stereo TVs
nComponent stereo
nCompact stereo
Connection Diagram
Plastic Package
01207202
Top View
Isolated Package
Order Number LM1876TF
See NS Package Number TF15B
Non-Isolated Package
Order Number LM1876T
See NS Package Number TA15A
SPiKe™Protection and Overture™are trademarks of National Semiconductor Corporation.
July 2003
LM1876 Overture
™
Audio Power Amplifier Series
Dual 20W Audio Power Amplifier with Mute and Standby Modes
© 2003 National Semiconductor Corporation DS012072 www.national.com
Typical Application
Note: Numbers in parentheses represent pinout for amplifier B. *Optional component dependent upon specific design requirements.
01207201
FIGURE 1. Typical Audio Amplifier Application Circuit
LM1876
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Absolute Maximum Ratings (Notes 4,
5)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage |V
CC
|+|V
EE
|
(No Input) 64V
Supply Voltage |V
CC
|+|V
EE
|
(with Input) 64V
Common Mode Input Voltage (V
CC
or V
EE
) and
|V
CC
|+|V
EE
|≤
54V
Differential Input Voltage 54V
Output Current Internally Limited
Power Dissipation (Note 6) 62.5W
ESD Susceptability (Note 7) 2000V
Junction Temperature (Note 8) 150˚C
Thermal Resistance
Isolated TF-Package
θ
JC
2˚C/W
Non-Isolated T-Package
θ
JC
1˚C/W
Soldering Information
TF Package (10 sec.) 260˚C
Storage Temperature −40˚C to +150˚C
Operating Ratings (Notes 4, 5)
Temperature Range
T
MIN
≤T
A
≤T
MAX
−20˚C ≤T
A
≤
+85˚C
Supply Voltage |V
CC
|+|V
EE
| (Note
1) 20V to 64V
Electrical Characteristics (Notes 4, 5)
The following specifications apply for V
CC
= +22V, V
EE
= −22V with R
L
=8Ωunless otherwise specified. Limits apply for T
A
=
25˚C.
Symbol Parameter Conditions LM1876 Units
(Limits)
Typical Limit
(Note 9) (Note 10)
|V
CC
| + Power Supply Voltage GND − V
EE
≥9V 20 V (min)
|V
EE
| (Note 11) 64 V (max)
P
O
Output Power THD + N = 0.1% (max),
(Note 3) (Continuous Average) f = 1 kHz
|V
CC
|=|V
EE
| = 22V, R
L
=8Ω20 15 W/ch (min)
|V
CC
|=|V
EE
| = 20V, R
L
=4Ω(Note 13) 22 15 W/ch (min)
THD + N Total Harmonic Distortion 15 W/ch, R
L
=8Ω0.08 %
Plus Noise 15 W/ch, R
L
=4Ω,|V
CC
|=|V
EE
| = 20V 0.1 %
20 Hz ≤f≤20 kHz, A
V
=26dB
X
talk
Channel Separation f = 1 kHz, V
O
= 10.9 Vrms 80 dB
SR
(Note 3)
Slew Rate V
IN
= 1.414 Vrms, t
rise
= 2 ns 18 12 V/µs (min)
I
total
Total Quiescent Power Both Amplifiers V
CM
= 0V,
(Note 2) Supply Current V
O
= 0V, I
O
=0mA
Standby: Off 50 80 mA (max)
Standby: On 4.2 6 mA (max)
V
OS
(Note 2)
Input Offset Voltage V
CM
= 0V, I
O
= 0 mA 2.0 15 mV (max)
I
B
Input Bias Current V
CM
= 0V, I
O
= 0 mA 0.2 0.5 µA (max)
I
OS
Input Offset Current V
CM
= 0V, I
O
= 0 mA 0.002 0.2 µA (max)
I
O
Output Current Limit |V
CC
|=|V
EE
| = 10V, t
ON
= 10 ms, 3.5 2.9 Apk (min)
V
O
=0V
V
OD
Output Dropout Voltage |V
CC
–V
O
|, V
CC
= 20V, I
O
= +100 mA 1.8 2.3 V (max)
(Note 2) (Note 12) |V
O
–V
EE
|, V
EE
= −20V, I
O
= −100 mA 2.5 3.2 V (max)
PSRR Power Supply Rejection Ratio V
CC
= 25V to 10V, V
EE
= −25V, 115 85 dB (min)
(Note 2) V
CM
= 0V, I
O
=0mA
V
CC
= 25V, V
EE
= −25V to −10V 110 85 dB (min)
V
CM
= 0V, I
O
=0mA
LM1876
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Electrical Characteristics (Notes 4, 5) (Continued)
The following specifications apply for V
CC
= +22V, V
EE
= −22V with R
L
=8Ωunless otherwise specified. Limits apply for T
A
=
25˚C.
Symbol Parameter Conditions LM1876 Units
(Limits)
Typical Limit
(Note 9) (Note 10)
CMRR Common Mode Rejection Ratio V
CC
= 35V to 10V, V
EE
= −10V to −35V, 110 80 dB (min)
(Note 2) V
CM
= 10V to −10V, I
O
=0mA
A
VOL
(Note 2)
Open Loop Voltage Gain R
L
=2kΩ,∆V
O
= 20 V 110 90 dB (min)
GBWP Gain Bandwidth Product f
O
= 100 kHz, V
IN
= 50 mVrms 7.5 5 MHz (min)
e
IN
Input Noise IHF — A Weighting Filter 2.0 8 µV (max)
(Note 3) R
IN
= 600Ω(Input Referred)
SNR Signal-to-Noise Ratio P
O
= 1W, A — Weighted, 98 dB
Measured at 1 kHz, R
S
=25Ω
P
O
= 15W, A — Weighted 108 dB
Measured at 1 kHz, R
S
=25Ω
A
M
Mute Attenuation Pin 6,11 at 2.5V 115 80 dB (min)
Standby
Pin
V
IL
Standby Low Input Voltage Not in Standby Mode 0.8 V (max)
V
IH
Standby High Input Voltage In Standby Mode 2.0 2.5 V (min)
Mute pin
V
IL
Mute Low Input Voltage Outputs Not Muted 0.8 V (max)
V
IH
Mute High Input Voltage Outputs Muted 2.0 2.5 V (min)
Note 1: Operation is guaranteed up to 64V, however, distortion may be introduced from SPiKe Protection Circuitry if proper thermal considerations are not taken
into account. Refer to the Application Information section for a complete explanation.
Note 2: DC Electrical Test; Refer to Test Circuit #1.
Note 3: AC Electrical Test; Refer to Test Circuit #2.
Note 4: All voltages are measured with respect to the GND pins (5, 10), unless otherwise specified.
Note 5: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. Electrical Characteristics state DC and AC electrical specifications under particular test conditions which
guarantee specific performance limits. This assumes that the device is within the Operating Ratings. Specifications are not guaranteed for parameters where no limit
is given, however, the typical value is a good indication of device performance.
Note 6: For operating at case temperatures above 25˚C, the device must be derated based on a 150˚C maximum junction temperature and a thermal resistance
of θJC = 2˚C/W (junction to case) for the TF package and θJC = 1˚C/W for the T package. Refer to the section Determining the Correct Heat Sink in the Application
Information section.
Note 7: Human body model, 100 pF discharged through a 1.5 kΩresistor.
Note 8: The operating junction temperature maximum is 150˚C, however, the instantaneous Safe Operating Area temperature is 250˚C.
Note 9: Typicals are measured at 25˚C and represent the parametric norm.
Note 10: Limits are guarantees that all parts are tested in production to meet the stated values.
Note 11: VEE must have at least −9V at its pin with reference to ground in order for the under-voltage protection circuitry to be disabled. In addition, the voltage
differential between VCC and VEE must be greater than 14V.
Note 12: The output dropout voltage, VOD, is the supply voltage minus the clipping voltage. Refer to the Clipping Voltage vs. Supply Voltage graph in the Typical
Performance Characteristics section.
Note 13: Fora4Ωload, and with ±20V supplies, the LM1876 can deliver typically 22W of continuous average output power with less than 0.1% (THD + N). With
supplies above ±20V, the LM1876 cannot deliver more than 22W into a 4Ωdue to current limiting of the output transistors. Thus, increasing the power supply above
±20V will only increase the internal power dissipation, not the possible output power. Increased power dissipation will require a larger heat sink as explained in the
Application Information section.
LM1876
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Single Supply Application Circuit
Note: *Optional components dependent upon specific design requirements.
Auxiliary Amplifier Application
Circuit
01207206
FIGURE 3. Single Supply Amplifier Application Circuit
01207207
FIGURE 4. Special Audio Amplifier Application Circuit
LM1876
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Equivalent Schematic (excluding active
protection circuitry)
LM1876 (per Amp)
01207208
LM1876
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External Components Description
Components Functional Description
1R
B
Prevents currents from entering the amplifier’s non-inverting input which may be passed through to the load
upon power down of the system due to the low input impedance of the circuitry when the undervoltage
circuitry is off. This phenomenon occurs when the supply voltages are below 1.5V.
2R
i
Inverting input resistance to provide AC gain in conjunction with R
f
.
3R
f
Feedback resistance to provide AC gain in conjunction with R
i
.
4C
i
(Note 14)
Feedback capacitor which ensures unity gain at DC. Also creates a highpass filter with R
i
at f
C
= 1/(2πR
i
C
i
).
5C
S
Provides power supply filtering and bypassing. Refer to the Supply Bypassing application section for proper
placement and selection of bypass capacitors.
6R
V
(Note 14)
Acts as a volume control by setting the input voltage level.
7R
IN
(Note 14)
Sets the amplifier’s input terminals DC bias point when C
IN
is present in the circuit. Also works with C
IN
to
create a highpass filter at f
C
= 1/(2πR
IN
C
IN
). Refer to Figure 4.
8C
IN
(Note 14)
Input capacitor which blocks the input signal’s DC offsets from being passed onto the amplifier’s inputs.
9R
SN
(Note 14)
Works with C
SN
to stabilize the output stage by creating a pole that reduces high frequency instabilities.
10 C
SN
(Note 14)
Works with R
SN
to stabilize the output stage by creating a pole that reduces high frequency instabilities.
The pole is set at f
C
= 1/(2πR
SN
C
SN
). Refer to Figure 4.
11 L (Note 14) Provides high impedance at high frequencies so that R may decouple a highly capacitive load and reduce
the Q of the series resonant circuit. Also provides a low impedance at low frequencies to short out R and
pass audio signals to the load. Refer to Figure 4.
12 R (Note 14)
13 R
A
Provides DC voltage biasing for the transistor Q1 in single supply operation.
14 C
A
Provides bias filtering for single supply operation.
15 R
INP
(Note 14)
Limits the voltage difference between the amplifier’s inputs for single supply operation. Refer to the Clicks
and Pops application section for a more detailed explanation of the function of R
INP
.
16 R
BI
Provides input bias current for single supply operation. Refer to the Clicks and Pops application section for
a more detailed explanation of the function of R
BI
.
17 R
E
Establishes a fixed DC current for the transistor Q1 in single supply operation. This resistor stabilizes the
half-supply point along with C
A
.
Note 14: Optional components dependent upon specific design requirements.
Typical Performance Characteristics
THD+NvsFrequency THD+NvsFrequency
01207213 01207214
LM1876
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Typical Performance Characteristics (Continued)
THD+NvsFrequency
THD+Nvs
Output Power
01207215 01207216
THD+Nvs
Output Power
THD+Nvs
Output Power
01207217 01207218
THD+Nvs
Output Power
THD+Nvs
Output Power
01207219 01207220
LM1876
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Typical Performance Characteristics (Continued)
THD+Nvs
Output Power
Clipping Voltage vs
Supply Voltage
01207221 01207222
Clipping Voltage vs
Supply Voltage
Clipping Voltage vs
Supply Voltage
01207223 01207224
Output Power vs
Load Resistance
Power Dissipation vs
Output Power
01207225 01207226
LM1876
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Typical Performance Characteristics (Continued)
Power Dissipation vs
Output Power
Output Power vs
Supply Voltage
01207227 01207228
Output Mute vs
Mute Pin Voltage
Output Mute vs
Mute Pin Voltage
01207229 01207230
Channel Separation vs
Frequency Pulse Response
01207231 01207232
LM1876
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Typical Performance Characteristics (Continued)
Large Signal Response
Power Supply
Rejection Ratio
01207233 01207234
Common-Mode
Rejection Ratio
Open Loop
Frequency Response
01207235 01207236
Safe Area
SPiKe Protection
Response
01207237 01207238
LM1876
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Typical Performance Characteristics (Continued)
Supply Current vs
Supply Voltage
Pulse Thermal
Resistance
01207239 01207240
Pulse Thermal
Resistance
Supply Current vs
Output Voltage
01207241 01207242
Pulse Power Limit Pulse Power Limit
01207243 01207244
LM1876
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Typical Performance Characteristics (Continued)
Supply Current vs
Case Temperature
Supply Current (I
CC
)vs
Standby Pin Voltage
01207245 01207246
Supply Current (I
EE
)vs
Standby Pin Voltage
Input Bias Current vs
Case Temperature
01207247 01207248
Output Power/Channel vs
Supply Voltage
f = 1kHz, R
L
=4Ω, 80kHz BW
Output Power/Channel vs
Supply Voltage
f = 1kHz, R
L
=6Ω, 80kHz BW
01207252 01207253
LM1876
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Typical Performance Characteristics (Continued)
Output Power/Channel vs
Supply Voltage
f = 1kHz, R
L
=8Ω, 80kHz BW
01207254
LM1876
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Application Information
MUTE MODE
By placing a logic-high voltage on the mute pins, the signal
going into the amplifiers will be muted. If the mute pins are
left floating or connected to a logic-low voltage, the amplifi-
ers will be in a non-muted state. There are two mute pins,
one for each amplifier, so that one channel can be muted
without muting the other if the application requires such a
configuration. Refer to the Typical Performance Character-
istics section for curves concerning Mute Attenuation vs
Mute Pin Voltage.
STANDBY MODE
The standby mode of the LM1876 allows the user to drasti-
cally reduce power consumption when the amplifiers are
idle. By placing a logic-high voltage on the standby pins, the
amplifiers will go into Standby Mode. In this mode, the
current drawn from the V
CC
supply is typically less than 10
µA total for both amplifiers. The current drawn from the V
EE
supply is typically 4.2 mA. Clearly, there is a significant
reduction in idle power consumption when using the standby
mode. There are two Standby pins, so that one channel can
be put in standby mode without putting the other amplifier in
standby if the application requires such flexibility. Refer to
the Typical Performance Characteristics section for
curves showing Supply Current vs. Standby Pin Voltage for
both supplies.
UNDER-VOLTAGE PROTECTION
Upon system power-up, the under-voltage protection cir-
cuitry allows the power supplies and their corresponding
capacitors to come up close to their full values before turning
on the LM1876 such that no DC output spikes occur. Upon
turn-off, the output of the LM1876 is brought to ground
before the power supplies such that no transients occur at
power-down.
OVER-VOLTAGE PROTECTION
The LM1876 contains over-voltage protection circuitry that
limits the output current to approximately 3.5 Apk while also
providing voltage clamping, though not through internal
clamping diodes. The clamping effect is quite the same,
however, the output transistors are designed to work alter-
nately by sinking large current spikes.
SPiKe PROTECTION
The LM1876 is protected from instantaneous peak-
temperature stressing of the power transistor array. The Safe
Operating graph in the Typical Performance Characteris-
tics section shows the area of device operation where
SPiKe Protection Circuitry is not enabled. The waveform to
the right of the SOA graph exemplifies how the dynamic
protection will cause waveform distortion when enabled.
THERMAL PROTECTION
The LM1876 has a sophisticated thermal protection scheme
to prevent long-term thermal stress of the device. When the
temperature on the die reaches 165˚C, the LM1876 shuts
down. It starts operating again when the die temperature
drops to about 155˚C, but if the temperature again begins to
rise, shutdown will occur again at 165˚C. Therefore, the
device is allowed to heat up to a relatively high temperature
if the fault condition is temporary, but a sustained fault will
cause the device to cycle in a Schmitt Trigger fashion be-
tween the thermal shutdown temperature limits of 165˚C and
155˚C. This greatly reduces the stress imposed on the IC by
thermal cycling, which in turn improves its reliability under
sustained fault conditions.
Since the die temperature is directly dependent upon the
heat sink used, the heat sink should be chosen such that
thermal shutdown will not be reached during normal opera-
tion. Using the best heat sink possible within the cost and
space constraints of the system will improve the long-term
reliability of any power semiconductor device, as discussed
in the Determining the Correct Heat Sink Section.
DETERMlNlNG MAXIMUM POWER DISSIPATION
Power dissipation within the integrated circuit package is a
very important parameter requiring a thorough understand-
ing if optimum power output is to be obtained. An incorrect
maximum power dissipation calculation may result in inad-
equate heat sinking causing thermal shutdown and thus
limiting the output power.
Equation (1) exemplifies the theoretical maximum power
dissipation point of each amplifier where V
CC
is the total
supply voltage.
P
DMAX
=V
CC
2/2π
2
R
L
(1)
Thus by knowing the total supply voltage and rated output
load, the maximum power dissipation point can be calcu-
lated. The package dissipation is twice the number which
results from equation (1) since there are two amplifiers in
each LM1876. Refer to the graphs of Power Dissipation
versus Output Power in the Typical Performance Charac-
teristics section which show the actual full range of power
dissipation not just the maximum theoretical point that re-
sults from equation (1).
DETERMINING THE CORRECT HEAT SINK
The choice of a heat sink for a high-power audio amplifier is
made entirely to keep the die temperature at a level such
that the thermal protection circuitry does not operate under
normal circumstances.
The thermal resistance from the die (junction) to the outside
air (ambient) is a combination of three thermal resistances,
θ
JC
,θ
CS
, and θ
SA
. In addition, the thermal resistance, θ
JC
(junction to case), of the LM1876TF is 2˚C/W and the
LM1876T is 1˚C/W. Using Thermalloy Thermacote thermal
compound, the thermal resistance, θ
CS
(case to sink), is
about 0.2˚C/W. Since convection heat flow (power dissipa-
tion) is analogous to current flow, thermal resistance is
analogous to electrical resistance, and temperature drops
are analogous to voltage drops, the power dissipation out of
the LM1876 is equal to the following:
P
DMAX
=(T
JMAX
−T
AMB
)/θ
JA
(2)
where T
JMAX
= 150˚C, T
AMB
is the system ambient tempera-
ture and θ
JA
=θ
JC
+θ
CS
+θ
SA
.
Once the maximum package power dissipation has been
calculated using equation (1), the maximum thermal resis-
tance, θ
SA
, (heat sink to ambient) in ˚C/W for a heat sink can
be calculated. This calculation is made using equation (3)
which is derived by solving for θ
SA
in equation (2).
θ
SA
= [(T
JMAX
−T
AMB
)−P
DMAX
(θ
JC
+θ
CS
)]/P
DMAX
(3)
Again it must be noted that the value of θ
SA
is dependent
upon the system designer’s amplifier requirements. If the
ambient temperature that the audio amplifier is to be working
under is higher than 25˚C, then the thermal resistance for the
heat sink, given all other things are equal, will need to be
smaller.
LM1876
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Application Information (Continued)
SUPPLY BYPASSING
The LM1876 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM1876 should have its supply leads bypassed with low-
inductance capacitors having short leads that are located
close to the package terminals. Inadequate power supply
bypassing will manifest itself by a low frequency oscillation
known as “motorboating” or by high frequency instabilities.
These instabilities can be eliminated through multiple by-
passing utilizing a large tantalum or electrolytic capacitor (10
µF or larger) which is used to absorb low frequency varia-
tions and a small ceramic capacitor (0.1 µF) to prevent any
high frequency feedback through the power supply lines.
If adequate bypassing is not provided, the current in the
supply leads which is a rectified component of the load
current may be fed back into internal circuitry. This signal
causes distortion at high frequencies requiring that the sup-
plies be bypassed at the package terminals with an electro-
lytic capacitor of 470 µF or more.
BRIDGED AMPLIFIER APPLICATION
The LM1876 has two operational amplifiers internally, allow-
ing for a few different amplifier configurations. One of these
configurations is referred to as “bridged mode” and involves
driving the load differentially through the LM1876’s outputs.
This configuration is shown in Figure 2. Bridged mode op-
eration is different from the classical single-ended amplifier
configuration where one side of its load is connected to
ground.
A bridge amplifier design has a distinct advantage over the
single-ended configuration, as it provides differential drive to
the load, thus doubling output swing for a specified supply
voltage. Consequently, theoretically four times the output
power is possible as compared to a single-ended amplifier
under the same conditions. This increase in attainable output
power assumes that the amplifier is not current limited or
clipped.
A direct consequence of the increased power delivered to
the load by a bridge amplifier is an increase in internal power
dissipation. For each operational amplifier in a bridge con-
figuration, the internal power dissipation will increase by a
factor of two over the single ended dissipation. Thus, for an
audio power amplifier such as the LM1876, which has two
operational amplifiers in one package, the package dissipa-
tion will increase by a factor of four. To calculate the
LM1876’s maximum power dissipation point for a bridged
load, multiply equation (1) by a factor of four.
This value of P
DMAX
can be used to calculate the correct size
heat sink for a bridged amplifier application. Since the inter-
nal dissipation for a given power supply and load is in-
creased by using bridged-mode, the heatsink’s θ
SA
will have
to decrease accordingly as shown by equation (3). Refer to
the section, Determining the Correct Heat Sink, for a more
detailed discussion of proper heat sinking for a given appli-
cation.
SINGLE-SUPPLY AMPLIFIER APPLICATION
The typical application of the LM1876 is a split supply am-
plifier. But as shown in Figure 3, the LM1876 can also be
used in a single power supply configuration. This involves
using some external components to create a half-supply bias
which is used as the reference for the inputs and outputs.
Thus, the signal will swing around half-supply much like it
swings around ground in a split-supply application. Along
with proper circuit biasing, a few other considerations must
be accounted for to take advantage of all of the LM1876
functions.
The LM1876 possesses a mute and standby function with
internal logic gates that are half-supply referenced. Thus, to
enable either the Mute or Standby function, the voltage at
these pins must be a minimum of 2.5V above half-supply. In
single-supply systems, devices such as microprocessors
and simple logic circuits used to control the mute and
standby functions, are usually referenced to ground, not
half-supply. Thus, to use these devices to control the logic
circuitry of the LM1876, a “level shifter,” like the one shown in
Figure 5, must be employed. A level shifter is not needed in
a split-supply configuration since ground is also half-supply.
When the voltage at the Logic Input node is 0V, the 2N3904
is “off” and thus resistor R
c
pulls up mute or standby input to
the supply. This enables the mute or standby function. When
the Logic Input is 5V, the 2N3904 is “on” and consequently,
the voltage at the collector is essentially 0V. This will disable
the mute or standby function, and thus the amplifier will be in
its normal mode of operation. R
shift
, along with C
shift
, creates
an RC time constant that reduces transients when the mute
or standby functions are enabled or disabled. Additionally,
R
shift
limits the current supplied by the internal logic gates of
the LM1876 which insures device reliability. Refer to the
Mute Mode and Standby Mode sections in the Application
Information section for a more detailed description of these
functions.
CLICKS AND POPS
In the typical application of the LM1876 as a split-supply
audio power amplifier, the IC exhibits excellent “click” and
“pop” performance when utilizing the mute and standby
modes. In addition, the device employs Under-Voltage Pro-
tection, which eliminates unwanted power-up and power-
down transients. The basis for these functions are a stable
and constant half-supply potential. In a split-supply applica-
tion, ground is the stable half-supply potential. But in a
single-supply application, the half-supply needs to charge up
just like the supply rail, V
CC
. This makes the task of attaining
a clickless and popless turn-on more challenging. Any un-
even charging of the amplifier inputs will result in output
clicks and pops due to the differential input topology of the
LM1876.
01207212
FIGURE 5. Level Shift Circuit
LM1876
www.national.com17
Application Information (Continued)
To achieve a transient free power-up and power-down, the
voltage seen at the input terminals should be ideally the
same. Such a signal will be common-mode in nature, and
will be rejected by the LM1876. In Figure 3, the resistor R
INP
serves to keep the inputs at the same potential by limiting the
voltage difference possible between the two nodes. This
should significantly reduce any type of turn-on pop, due to an
uneven charging of the amplifier inputs. This charging is
based on a specific application loading and thus, the system
designer may need to adjust these values for optimal perfor-
mance.
As shown in Figure 3, the resistors labeled R
BI
help bias up
the LM1876 off the half-supply node at the emitter of the
2N3904. But due to the input and output coupling capacitors
in the circuit, along with the negative feedback, there are two
different values of R
BI
, namely 10 kΩand 200 kΩ. These
resistors bring up the inputs at the same rate resulting in a
popless turn-on. Adjusting these resistors values slightly
may reduce pops resulting from power supplies that ramp
extremely quick or exhibit overshoot during system turn-on.
AUDIO POWER AMPLlFIER DESIGN
Design a 15W/8ΩAudio Amplifier
Given:
Power Output 15 Wrms
Load Impedance 8Ω
Input Level 1 Vrms(max)
Input Impedance 47 kΩ
Bandwidth 20 Hz−20 kHz
±0.25 dB
A designer must first determine the power supply require-
ments in terms of both voltage and current needed to obtain
the specified output power. V
OPEAK
can be determined from
equation (4) and I
OPEAK
from equation (5).
(4)
(5)
To determine the maximum supply voltage the following
conditions must be considered. Add the dropout voltage to
the peak output swing V
OPEAK
, to get the supply rail at a
current of I
OPEAK
. The regulation of the supply determines
the unloaded voltage which is usually about 15% higher. The
supply voltage will also rise 10% during high line conditions.
Therefore the maximum supply voltage is obtained from the
following equation.
Max supplies ≈±(V
OPEAK
+V
OD
) (1 + regulation) (1.1)
For 15W of output power into an 8Ωload, the required
V
OPEAK
is 15.49V. A minimum supply rail of 20.5V results
from adding V
OPEAK
and V
OD
. With regulation, the maximum
supplies are ±26V and the required I
OPEAK
is 1.94A from
equation (5). It should be noted that for a dual 15W amplifier
into an 8Ωload the I
OPEAK
drawn from the supplies is twice
1.94 Apk or 3.88 Apk. At this point it is a good idea to check
the Power Output vs Supply Voltage to ensure that the
required output power is obtainable from the device while
maintaining low THD+N. In addition, the designer should
verify that with the required power supply voltage and load
impedance, that the required heatsink value θ
SA
is feasible
given system cost and size constraints. Once the heatsink
issues have been addressed, the required gain can be de-
termined from Equation (6).
(6)
From equation 6, the minimum A
V
is: A
V
≥11.
By selecting a gain of 21, and with a feedback resistor, R
f
=
20 kΩ, the value of R
i
follows from equation (7).
R
i
=R
f
(A
V
− 1) (7)
Thus with R
i
=1kΩa non-inverting gain of 21 will result.
Since the desired input impedance was 47 kΩ, a value of 47
kΩwas selected for R
IN
. The final design step is to address
the bandwidth requirements which must be stated as a pair
of −3 dB frequency points. Five times away from a −3 dB
point is 0.17 dB down from passband response which is
better than the required ±0.25 dB specified. This fact results
in a low and high frequency pole of 4 Hz and 100 kHz
respectively. As stated in the External Components sec-
tion, R
i
in conjunction with C
i
create a high-pass filter.
C
i
≥1/(2π*1kΩ*4 Hz) = 39.8 µF; use 39 µF.
The high frequency pole is determined by the product of the
desired high frequency pole, f
H
, and the gain, A
V
. With a
A
V
= 21 and f
H
= 100 kHz, the resulting GBWP is 2.1 MHz,
which is less than the guaranteed minimum GBWP of the
LM1876 of 5 MHz. This will ensure that the high frequency
response of the amplifier will be no worse than 0.17 dB down
at 20 kHz which is well within the bandwidth requirements of
the design.
LM1876
www.national.com 18
Physical Dimensions inches (millimeters) unless otherwise noted
Isolated TO-220 15-Lead Package
Order Number LM1876TF
NS Package Number TF15B
Non-Isolated TO-220 15-Lead Package
Order Number LM1876T
NS Package Number TA15A
LM1876
www.national.com19
Notes
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
National Semiconductor
Americas Customer
Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959
National Semiconductor
Europe Customer Support Center
Fax: +49 (0) 180-530 85 86
Email: europe.support@nsc.com
Deutsch Tel: +49 (0) 69 9508 6208
English Tel: +44 (0) 870 24 0 2171
Français Tel: +33 (0) 1 41 91 8790
National Semiconductor
Asia Pacific Customer
Support Center
Email: ap.support@nsc.com
National Semiconductor
Japan Customer Support Center
Fax: 81-3-5639-7507
Email: jpn.feedback@nsc.com
Tel: 81-3-5639-7560
www.national.com
LM1876 Overture
™
Audio Power Amplifier Series
Dual 20W Audio Power Amplifier with Mute and Standby Modes
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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