LM4951A
LM4951A Wide Voltage Range 1.8 Watt Audio Amplifier With Short Circuit
Protection
Literature Number: SNAS453B
September 5, 2008
LM4951A
Wide Voltage Range 1.8 Watt Audio Amplifier With Short
Circuit Protection
General Description
The LM4951A is an audio power amplifier designed for ap-
plications with supply voltages ranging from 2.7V up to 9V.
The LM4951A is capable of delivering 1.8W continuous av-
erage power with less than 1% THD+N into a bridge connect-
ed 8Ω load when operating from a 7.5VDC power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4951A does not require boot-
strap capacitors, or snubber circuits.
The LM4951A features a low-power consumption active-low
shutdown mode. Additionally, the LM4951A features an in-
ternal thermal shutdown protection mechanism and short
circuit protection.
The LM4951A contains advanced pop & click circuitry that
eliminates noises which would otherwise occur during turn-on
and turn-off transitions.
The LM4951A is unity-gain stable and can be configured by
external gain-setting resistors.
Key Specifications
■ Wide Voltage Range 2.7V to 9V
■ Quiescent Power Supply Current
(VDD = 7.5V) 2.5mA (typ)
■ Power Output BTL at 7.5V,
1% THD 1.8W (typ)
■ Shutdown Current 0.01µA (typ)
■ Fast Turn on Time 25ms (typ)
Features
■Pop & click circuitry eliminates noise during turn-on and
turn-off transitions
■Wide supply voltage range: 2.7V to 9V
■Low current, active-low shutdown mode
■Low quiescent current
■Thermal shutdown protection
■Short circuit protection
■Unity-gain stable
■External gain configuration capability
Applications
■Portable devices
■Cell phones
■Laptop computers
■Computer speaker systems
■MP3 player speakers
Boomer® is a registered trademark of National Semiconductor Corporation.
© 2008 National Semiconductor Corporation 300578 www.national.com
LM4951A Wide Voltage Range 1.8 Watt Audio Amplifier With Short Circuit Protection
Typical Application
300578f4
FIGURE 1. Typical Bridge-Tied-Load (BTL) Audio Amplifier Application Circuit
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LM4951A
Connection Diagrams
SD Package
30057829
Top View
Order Number LM4951ASD
See NS Package Number SDC10A
SD Package Marking
30057831
Top View
U = Fab site code
Z = Assembly plant code
XY = Date code
TT = Die traceability
4951A = LM4951A
SD = Package code
Ordering Information
Order Number Package Package DWG # Transport Media MSL Level Green Status Features
LM4951ASD 10 Lead LLP SDC10A 1000 units in Tape and Reel 1 RoH and no Sb/Br
LM4951ASDX 10 Lead LLP SDC10A 4500 units in Tape and Reel 1 RoH and no Sb/Br
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LM4951A
TABLE 1. Pin Name and Function
Pin Number Name Function Type
1 Bypass
½ supply reference voltage bypass output. See sections POWER
SUPPLY BYPASSING and SELECTING EXTERNAL COMPONENTS for
more information.
Analog Output
2 Shutdown Shutdown control active low signal. A logic low voltage will put the
LM4951A into Shutdown mode. Digital Input
3CCHG
Input capacitor charge to decrease turn on time. See section Selecting A
Value for RC for more information. Analog Output
4 NC No connection to die. Pin can be connected to any potential. No Connect
5 VIN Single-ended signal input pin. Analog Input
6 VO- Inverting output of amplifier. Analog Output
7 GND Ground connection. Ground
8 NC No connection to die. Pin can be connected to any potential. No Connect
9 VDD Power supply. Power
10 VO+ Non-Inverting output of amplifier. Analog Output
Exposed DAP NC No connect. Pin must be electrically isolated (floating) or connected to
GND. No Connect
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LM4951A
Absolute Maximum Ratings (Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage 9.5V
Storage Temperature −65°C to +150°C
Input Voltage −0.3V to VDD + 0.3V
Power Dissipation (Note 3) Internally limited
ESD Rating (Note 4) 2000V
ESD Rating (Note 5) 200V
Junction Temperature (TJMAX)150°C
Thermal Resistance
θJA (LLP) (Note 3) 73°C/W
Soldering Information
See AN-1187 'Leadless Leadframe
Packaging (LLP).'
Operating Ratings (Notes 1, 2)
Temperature Range
TMIN ≤ TA ≤ TMAX −40°C ≤ T A ≤ +85°C
Supply Voltage 2.7V ≤ VDD ≤ 9V
Electrical Characteristics VDD = 7.5V (Notes 1, 2)
The following specifications apply for VDD = 7.5V, AV-BTL = 6dB, RL = 8Ω unless otherwise specified. Limits apply for TA = 25°C.
Symbol Parameter Conditions
LM4951A Units
(Limits)
Typical
(Note 6)
Limit
(Note 7)
IDD Quiescent Power Supply Current VIN = 0V, IO = 0A, RL = 8Ω BTL 2.5 4.5 mA (max)
ISD Shutdown Current VSD = GND (Note 8) 0.01 5 µA (max)
VOS Output Offset Voltage 5 30 mV (max)
VSDIH Shutdown Voltage Input High 1.2 V (min)
VSDIL Shutdown Voltage Input Low 0.4 V (max)
RPULLDOWN Pull-down Resistor on SD pin 75 45 kΩ (min)
TWU Wake-up Time CB = 1.0µF 25 35 ms (max)
TSD Shutdown time CB = 1.0µF 10 ms (max)
TSD Thermal Shutdown Temperature 170 150
190
°C (min)
°C (max)
POOutput Power THD = 1% (max); f = 1kHz
RL = 8Ω Mono BTL 1.8 1.5 W (min)
THD+N Total Harmonic Distortion + Noise
PO = 600mWRMS; f = 1kHz
AV-BTL = 6dB 0.07 0.5 % (max)
PO = 600mWRMS; f = 1kHz
AV-BTL = 26dB 0.35 %
εOS Output Noise A-Weighted Filter, Ri = Rf = 20kΩ
Input Referred (Note 9) 10 µV
PSRR Power Supply Rejection Ratio VRIPPLE = 200mVp-p, f = 217Hz,
CB = 1.0μF, Input Referred 66 56 dB (min)
Electrical Characteristics VDD = 3.3V (Notes 1, 2)
The following specifications apply for VDD = 3.3V, AV-BTL = 6dB, RL = 8Ω unless otherwise specified. Limits apply for TA = 25°C.
Symbol Parameter Conditions
LM4951A Units
(Limits)
Typical
(Note 6)
Limit
(Note 7)
IDD Quiescent Power Supply Current VIN = 0V, IO = 0A, RL = 8Ω BTL 2.5 4.5 mA (max)
ISD Shutdown Current VSHUTDOWN = GND (Note 8) 0.01 2 µA (max)
VOS Output Offset Voltage 3 30 mV (max)
VSDIH Shutdown Voltage Input High 1.2 V (min)
VSDIL Shutdown Voltage Input Low 0.4 V (max)
TWU Wake-up Time CB = 1.0µF 25 ms
TSD Shutdown time CB = 1.0µF 10 ms (max)
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LM4951A
Symbol Parameter Conditions
LM4951A Units
(Limits)
Typical
(Note 6)
Limit
(Note 7)
POOutput Power THD = 1% (max); f = 1kHz
RL = 8Ω Mono BTL 280 230 mW (min)
THD+N Total Harmonic Distortion + Noise
PO = 100mWRMS = 1kHz
AV-BTL = 6dB 0.07 0.5 % (max)
PO = 100mWRMS; f = 1kHz
AV-BTL = 26dB 0.35 %
εOS Output Noise A-Weighted Filter, Ri = Rf = 20kΩ
Input Referred, (Note 9) 10 µV
PSRR Power Supply Rejection Ratio VRIPPLE = 200mVp-p, f = 217Hz,
CB = 1μF, Input Referred 71 61 dB (min)
Note 1: “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability
and/or performance. Functional operation of the device and/or non-degradation at the s or other conditions beyond those indicated in the Recommended Operating
Conditions is not implied. The Recommended Operating Conditionsindicate conditions at which the device is functional and the device should not be operated
beyond such conditions. All voltages are measured with respect to the ground pin, unless otherwise specified.
Note 2: The Electrical Characteristics tables list guaranteed specifications under the listed Recommended Operating Conditions except as otherwise modified
or specified by the Electrical Characteristics Conditions and/or Notes. Typical specifications are estimations only and are not guaranteed.
Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by TJMAX, θJA, and the ambient temperature, TA. The maximum
allowable power dissipation is PDMAX = (TJMAX - TA) / θJA or the number given in Absolute Maximum Ratings, whichever is lower. For the LM4951A typical application
(shown in Figure 1) with VDD = 7.5V, RL = 8Ω mono-BTL operation the max power dissipation is 1.42W. θJA = 73ºC/W.
Note 4: Human body model, applicable std. JESD22-A114C.
Note 5: Machine model, applicable std. JESD22-A115-A.
Note 6: Typical values represent most likely parametric norms at TA = +25ºC, and at the Recommended Operation Conditions at the time of product
characterization and are not guaranteed.
Note 7: Datasheet min/max specification limits are guaranteed by test or statistical analysis.
Note 8: Shutdown current is measured in a normal room environment. The Shutdown pin should be driven as close as possible to GND for minimum shutdown
current.
Note 9: Noise measurements are dependent on the absolute values of the closed loop gain setting resistors (input and feedback resistors).
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LM4951A
Typical Performance Characteristics
THD+N vs Frequency
VDD = 3.3V, PO = 100mW, AV = 6dB
300578f9
THD+N vs Frequency
VDD = 3.3V, PO = 100mW, AV = 26dB
30057802
THD+N vs Frequency
VDD = 5V, PO = 400mW, AV = 6dB
30057803
THD+N vs Frequency
VDD = 5V, PO = 400mW, AV = 26dB
30057804
THD+N vs Frequency
VDD = 7.5V, PO = 600mW, AV = 6dB
30057805
THD+N vs Frequency
VDD = 7.5V, PO = 600mW, AV = 26dB
300578g0
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LM4951A
THD+N vs Output Power
VDD = 3.3V, f = 1kHz, AV = 6dB
300578g1
THD+N vs Output Power
VDD = 3.3V, f = 1kHz, AV = 26dB
30057808
THD+N vs Output Power
VDD = 5V, f = 1kHz, AV = 6dB
30057809
THD+N vs Output Power
VDD = 5V, f = 1kHz, AV = 26dB
30057810
THD+N vs Output Power
VDD = 7.5V, f = 1kHz, AV = 6dB
30057811
THD+N vs Output Power
VDD = 7.5V, f = 1kHz, AV = 26dB
30057812
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LM4951A
Power Supply Rejection vs Frequency
VDD = 3.3V, AV = 6dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
30057813
Power Supply Rejection vs Frequency
VDD = 3.3V, AV = 26dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
30057814
Power Supply Rejection vs Frequency
VDD = 5V, AV = 6dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
30057815
Power Supply Rejection vs Frequency
VDD = 5V, AV = 26dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
30057816
Power Supply Rejection vs Frequency
VDD = 7.5V, AV = 6dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
30057817
Power Supply Rejection vs Frequency
VDD = 7.5V, AV = 26dB, VRIPPLE = 200mVP-P
Input Terminated into 10Ω
30057818
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LM4951A
Noise Floor
VDD = 3.3V, AV = 6dB, Ri = Rf = 20kΩ
BW < 80kHz, A-weighted
30057819
Noise Floor
VDD = 3V, AV = 26dB, Ri = 20kΩ, Rf = 200kΩ
BW < 80kHz, A-weighted
30057820
Noise Floor
VDD = 5V, AV = 6dB, Ri = Rf = 20kΩ
BW < 80kHz, A-weighted
30057821
Noise Floor
VDD = 5V, AV = 26dB, Ri = 20kΩ, Rf = 200kΩ
BW < 80kHz, A-weighted
30057822
Noise Floor
VDD = 7.5V, AV = 6dB, Ri = Rf = 20kΩ
BW < 80kHz, A-weighted
30057823
Noise Floor
VDD = 7.5V, AV = 26dB, Ri = 20kΩ, Rf = 200kΩ
BW < 80kHz, A-weighted
30057824
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LM4951A
Power Dissipation
vs Output Power
VDD = 3.3V, RL = 8Ω, f = 1kHz
30057825
Power Dissipation
vs Output Power
VDD = 7.5V, RL = 8Ω, f = 1kHz
30057826
Supply Current
vs Supply Voltage
RL = 8Ω, VIN = 0V, Rsource = 50Ω
30057827
Clipping Voltage vs Supply Voltage
RL = 8Ω,
from top to bottom: Negative Voltage Swing; Positive
Voltage Swing
300578e9
Output Power vs Supply Voltage
RL = 8Ω,
from top to bottom: THD+N = 10%, THD+N = 1%
300578f0
Output Power vs Load Resistance
VDD = 3.3V, f = 1kHz
from top to bottom: THD+N = 10%, THD+N = 1%
300578f1
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LM4951A
Output Power vs Load Resistance
VDD = 7.5V, f = 1kHz
from top to bottom: THD+N = 10%, THD+N = 1%
300578f2
Frequency Response vs Input Capacitor Size
RL = 8Ω
from top to bottom: Ci = 1.0µF, Ci = 0.39µF, Ci = 0.039µF
300578f3
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LM4951A
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4951A consists of two opera-
tional amplifiers that drive a speaker connected between their
outputs. The value of input and feedback resistors determine
the gain of each amplifier. External resistors Ri and Rf set the
closed-loop gain of AMPA, whereas two 20kΩ internal resis-
tors set AMPB's gain to -1. Figure 1 shows that AMPA's output
serves as AMPB's input. This results in both amplifiers pro-
ducing signals identical in magnitude, but 180° out of phase.
Taking advantage of this phase difference, a load is placed
between AMPA and AMPB and driven differentially (commonly
referred to as "bridge-tied load"). This results in a differential,
or BTL, gain of:
AVD = 2(Rf / Ri) (V/V) (1)
Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifier's
output and ground. For a given supply voltage, bridge mode
has an advantage over the single-ended configuration: its dif-
ferential output doubles the voltage swing across the load.
Theoretically, this produces four times the output power when
compared to a single-ended amplifier under the same condi-
tions. This increase in attainable output power assumes that
the amplifier is not current limited and that the output signal
is not clipped. Under rare conditions, with unique combina-
tions of high power supply voltage and high closed loop gain
settings, the LM4951A may exhibit low frequency oscillations.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
AMP1's and AMP2's outputs at half-supply. This eliminates
the coupling capacitor that single supply, single-ended am-
plifiers require. Eliminating an output coupling capacitor in a
typical single-ended configuration forces a single-supply
amplifier's half-supply bias voltage across the load. This in-
creases internal IC power dissipation and may permanently
damage loads such as speakers.
POWER DISSIPATION
The LM4951A's dissipation when driving a BTL load is given
by Equation (2). For a 7.5V supply and a single 8Ω BTL load,
the dissipation is 1.42W.
PDMAX-MONOBTL = 4(VDD) 2 / 2π2RL (W) (2)
The maximum power dissipation point given by Equation (2)
must not exceed the power dissipation given by Equation (3):
PDMAX = (TJMAX - TA) / θJA (3)
The LM4951A's TJMAX = 150°C. In the SD package, the
LM4951A's θJA is 73°C/W when the metal tab is soldered to
a copper plane of at least 1in2. This plane can be split between
the top and bottom layers of a two-sided PCB. Connect the
two layers together under the tab with an array of vias. At any
given ambient temperature TA, use Equation (3) to find the
maximum internal power dissipation supported by the IC
packaging. Rearranging Equation (3) and substituting
PDMAX for PDMAX' results in Equation (4). This equation gives
the maximum ambient temperature that still allows maximum
stereo power dissipation without violating the LM4951A's
maximum junction temperature.
TA = TJMAX - PDMAX-MONOBTLθJA (°C) (4)
For a typical application with a 7.5V power supply and a BTL
8Ω load, the maximum ambient temperature that allows max-
imum stereo power dissipation without exceeding the maxi-
mum junction temperature is 46°C for the SD package.
TJMAX = PDMAX-MONOBTLθJA + TA (°C) (5)
Equation (5) gives the maximum junction temperature
TJMAX. If the result violates the LM4951A's maximum junction
temperature of 150°C, reduce the maximum junction temper-
ature by reducing the power supply voltage or increasing the
load resistance. Further allowance should be made for in-
creased ambient temperatures.
The above examples assume that a device is operating
around the maximum power dissipation point. Since internal
power dissipation is a function of output power, higher ambi-
ent temperatures are allowed as output power or duty cycle
decreases.
If the result of Equation (2) is greater than that of Equation (3),
then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. Further, en-
sure that speakers rated at a nominal 8Ω do not fall below
6Ω. If these measures are insufficient, a heat sink can be
added to reduce θJA. The heat sink can be created using ad-
ditional copper area around the package, with connections to
the ground pins, supply pin and amplifier output pins. Refer
to the Typical Performance Characteristics curves for pow-
er dissipation information at lower output power levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is crit-
ical for low noise performance and high power supply rejec-
tion. Applications that employ a voltage regulator typically use
a 10µF in parallel with a 0.1µF filter capacitors to stabilize the
regulator's output, reduce noise on the supply line, and im-
prove the supply's transient response. However, their pres-
ence does not eliminate the need for a local 1.0µF tantalum
bypass capacitance connected between the LM4951A's sup-
ply pins and ground. Do not substitute a ceramic capacitor for
the tantalum. Doing so may cause oscillation. Keep the length
of leads and traces that connect capacitors between the
LM4951A's power supply pin and ground as short as possible.
Connecting a larger capacitor, CBYPASS, between the BY-
PASS pin and ground improves the internal bias voltage's
stability and improves the amplifier's PSRR. The PSRR im-
provements increase as the bypass pin capacitor value in-
creases. Too large, however, increases turn-on time and can
compromise the amplifier's click and pop performance. The
selection of bypass capacitor values, especially CBYPASS, de-
pends on desired PSRR requirements, click and pop perfor-
mance, system cost, and size constraints.
MICRO-POWER SHUTDOWN
The LM4951A features an active-low micro-power shutdown
mode. When active, the LM4951A's micro-power shutdown
feature turns off the amplifier's bias circuitry, reducing the
supply current. The low 0.01µA typical shutdown current is
achieved by applying a voltage to the SHUTDOWN pin that
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LM4951A
is as near to GND as possible. A voltage that is greater than
GND may increase the shutdown current.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Two quantities determine the value of the input coupling ca-
pacitor: the lowest audio frequency that requires amplification
and desired output transient suppression.
As shown in Figure 1, the input resistor (Ri) and the input ca-
pacitor (Ci) create a high-pass filter. The cutoff frequency can
be found using Equation (6).
fc = 1/2πRiCi (Hz) (6)
As an example when using a speaker with a low frequency
limit of 50Hz, Ci, using Equation (6) is 0.159µF with Ri set to
20kΩ. The values for Ci and Ri shown in Figure 1 allow the
LM4951A to drive a high efficiency, full range speaker whose
response extends down to 20Hz.
Selecting Value A For RC
The LM4951A is designed for very fast turn on time. The
CCHG pin allows the input capacitor to charge quickly to im-
prove click/pop performance. RC protects the CCHG pin from
any over/under voltage conditions caused by excessive input
signal or an active input signal when the device is in shut-
down. The recommended value for RC is 1kΩ. If the input
signal is less than VDD+0.3V and greater than -0.3V, and if the
input signal is disabled when in shutdown mode, RC may be
shorted out.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4951A contains circuitry that eliminates turn-on and
shutdown transients ("clicks and pops"). For this discussion,
turn-on refers to either applying the power supply voltage or
when the micro-power shutdown mode is deactivated.
As the VDD/2 voltage present at the BYPASS pin ramps to its
final value, the LM4951A's internal amplifiers are configured
as unity gain buffers. An internal current source charges the
capacitor connected between the BYPASS pin and GND in a
controlled manner. Ideally, the input and outputs track the
voltage applied to the BYPASS pin.
The gain of the internal amplifiers remains unity until the volt-
age on the bypass pin reaches VDD/2. As soon as the voltage
on the bypass pin is stable, there is a delay to prevent unde-
sirable output transients (“click and pops”). After this delay,
the device becomes fully functional.
THERMAL SHUTDOWN AND SHORT CIRCUIT
PROTECTION
The LM4951A has thermal shutdown and short circuit pro-
tection to fully protect the device. The thermal shutdown
circuit is activated when the die temperature exceeds a safe
temperature. The short circuit protection circuitry senses the
output current. When the output current exceeds the thresh-
old under a short condition, a short will be detected and the
output deactivated until the short condition is removed. If the
output current is lower than the threshold then a short will not
be detected and the outputs will not be deactivated. Under
such conditions the die temperature will increase and, if the
condition persist to raise the die temperature to the thermal
shutdown threshold, initiate a thermal shutdown response.
Once the die cools the outputs will become active.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figures 2–4 show the recommended two-layer PC board lay-
out that is optimized for the SD10A. This circuit is designed
for use with an external 7.5V supply 8Ω (min) speakers.
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LM4951A
Demonstration Board Circuit
30057830
FIGURE 2. Demo Board Circuit
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LM4951A
Demonstration Board Layout
30057832
FIGURE 3. Top Silkscreen
300578f7
FIGURE 4. Top Layer
300578f6
FIGURE 5. Bottom Layer
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LM4951A
Bill Of Materials
TABLE 2. Bill Of Materials
Designator Value Tolerance Part Description Comments
RIN1 20kΩ1% 1/8W, 0805 Resistor
R1200kΩ1% 1/8W, 0805 Resistor
RPULLUP 100kΩ1% 1/8W, 0805 Resistor
R21kΩ1% 1/8W, 0805 Resistor
R4, R50Ω 1% 1/8W, 0805 Resistor
CIN1 0.39μF10% Ceramic Capacitor, 25V, Size 1206
CSUPPLY 4.7μF10% 16V Tantalum Capacitor, Size A
CBYPASS 1μF10% 16V Tantalum Capacitor, Size A
C1 Not Used
0.100” 1x2 header, vertical mount Input, Output, Vdd/GND
Shutdown
U1 LM4951A, Mono, 1.8W, Audio Amplifier SDC10A package
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LM4951A
Revision History
Rev Date Description
1.0 08/13/08 Initial release.
1.01 09/05/08 Text edits.
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LM4951A
Physical Dimensions inches (millimeters) unless otherwise noted
Order Number LM4951ASD
NS Package Number SDC10A
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LM4951A
Notes
LM4951A Wide Voltage Range 1.8 Watt Audio Amplifier With Short Circuit Protection
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