LM4940
6W Stereo Audio Power Amplifier
General Description
The LM4940 is a dual audio power amplifier primarily de-
signed for demanding applications in flat panel monitors and
TV’s. It is capable of delivering 6 watts per channel to a 4Ω
load with less than 10% THD+N while operating on a
14.4V
DC
power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4940 does not require boot-
strap capacitors or snubber circuits. Therefore, it is ideally
suited for display applications requiring high power and mini-
mal size.
The LM4940 features a low-power consumption active-low
shutdown mode. Additionally, the LM4940 features an inter-
nal thermal shutdown protection mechanism along with short
circuit protection.
The LM4940 contains advanced pop & click circuitry that
eliminates noises which would otherwise occur during
turn-on and turn-off transitions.
The LM4940 is a unity-gain stable and can be configured by
external gain-setting resistors.
Key Specifications
jQuiscent Power Supply Current 40mA (max)
jP
OUT
(SE)
V
DD
= 14.4V, R
L
=4Ω, 10% THD+N 6W (typ)
jShutdown current 40µA (typ)
Features
nPop & click circuitry eliminates noise during turn-on and
turn-off transitions
nLow current, active-low shutdown mode
nLow quiescent current
nStereo 6W output, R
L
=4Ω
nShort circuit protection
nUnity-gain stable
nExternal gain configuration capability
Applications
nFlat Panel Monitors
nFlat Panel TV’s
nComputer Sound Cards
Typical Application
Boomer®is a registered trademark of National Semiconductor Corporation.
20075672
FIGURE 1. Typical Stereo Audio Amplifier Application Circuit
April 2005
LM4940 6W Stereo Audio Power Amplifier
© 2005 National Semiconductor Corporation DS200756 www.national.com
Connection Diagram
Plastic Package, TO-263
200756E7
Top View
U = Wafer Fab Code
Z = Assembly Plant Code
XY = Date Code
TT = Die Traceability
Order Number LM4940TS
See NS Package Number TS9A
LM4940
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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 (pin 6, referenced
to GND, pins 4 and 5) 18.0V
Storage Temperature −65˚C to +150˚C
Input Voltage
pins 3 and 7 −0.3V to V
DD
+ 0.3V
pins 1, 2, 8, and 9 −0.3V to 9.5V
Power Dissipation (Note 3) Internally limited
ESD Susceptibility (Note 4) 2000V
ESD Susceptibility (Note 5) 200V
Junction Temperature 150˚C
Thermal Resistance
θ
JC
(TS) 4˚C/W
θ
JA
(TS) (Note 3) 20˚C/W
θ
JC
(TA) 4˚C/W
θ
JA
(TA) (Note 3) 20˚C/W
Operating Ratings
Temperature Range
T
MIN
≤T
A
≤T
MAX
−40˚C ≤T
A
≤85˚C
Supply Voltage 10V ≤V
DD
≤16V
Electrical Characteristics V
DD
= 12V (Notes 1, 2)
The following specifications apply for V
DD
= 12V, A
V
= 10, R
L
=4Ω, f = 1kHz unless otherwise specified. Limits apply for T
A
=
25˚C.
Symbol Parameter Conditions LM4940 Units
(Limits)
Typical
(Note 6)
Limit
(Notes 7, 8)
I
DD
Quiescent Power Supply Current V
IN
= 0V, I
O
= 0A, No Load 16 40 mA (max)
I
SD
Shutdown Current V
SHUTDOWN
= GND (Note 9) 40 100 µA (max)
V
SDIH
Shutdown Voltage Input High 2.0
V
DD
/2
V (min)
V (max)
V
SDIL
Shutdown Voltage Input Low 0.4 V (max)
P
O
Output Power
Single Channel
W (min)
THD+N = 1% 3.1 2.8
THD+N = 10% 4.2
V
DD
= 14.4V, THD+N = 10% 6.0
THD+N Total Harmomic Distortion + Noise P
O
= 1Wrms, A
V
= 10, f = 1kHz 0.15 %
e
OS
Output Noise A-Weighted Filter, V
IN
= 0V,
Input Referred 10 µV
X
TALK
Channel Separation P
O
=1W 70 dB
PSRR Power Supply Rejection Ratio V
RIPPLE
= 200mV
p-p
,f
RIPPLE
=
1kHz 56 dB
Note 1: All voltages are measured with respect to the GND pin, unless otherwise specified.
Note 2: 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 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 P DMAX =(T
JMAX −T
A)/θJA or the given in Absolute Maximum Ratings, whichever is lower. For the LM4940 typical application (shown
in Figure 1) with VDD = 12V, RL=4Ωstereo operation the total power dissipation is 3.65W. θJA = 20˚C/W for both TO263 and TO220 packages mounted to 16in2
heatsink surface area.
Note 4: Human body model, 100pF discharged through a 1.5 kΩresistor.
Note 5: Machine Model, 220pF–240pF discharged through all pins.
Note 6: Typicals are measured at 25˚C and represent the parametric norm.
Note 7: Limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: Datasheet min/max specification limits are guaranteed by design, test, or statistical analysis.
Note 9: 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.
LM4940
www.national.com3
Typical Application
External Components Description Refer to (Figure 1.)
Components Functional Description
1. R
IN
This is the inverting input resistance that, along with R
F
, sets the closed-loop gain. Input
resistance R
IN
and input capacitance C
IN
form a high pass filter. The filter’s cutoff frequency is f
C
=1/(2πR
IN
C
IN
).
2. C
IN
This is the input coupling capacitor. It blocks DC voltage at the amplifier’s inverting input. C
IN
and
R
IN
create a highpass filter. The filter’s cutoff frequency is f
C
=1/(2πR
IN
C
IN
). Refer to the
SELECTING EXTERNAL COMPONENTS section for an explanation of determining C
IN
’s value.
3. R
F
This is the feedback resistance that, along with R
i
, sets closed-loop gain.
4. C
S
The supply bypass capacitor. Refer to the POWER SUPPLY BYPASSING section for information
about properly placing, and selecting the value of, this capacitor.
5. C
BYPASS
This capacitor filters the half-supply voltage present on the BYPASS pin. Refer to the Application
section, SELECTING EXTERNAL COMPONENTS, for information about properly placing, and
selecting the value of, this capacitor.
6. C
OUT
This is the output coupling capacitor. It blocks the nominal V
DD
/2 voltage present at the output
and prevents it from reaching the load. C
OUT
and R
L
form a high pass filter whose cutoff
frequency is f
C
=1/(2πR
L
C
OUT
). Refer to the SELECTING EXTERNAL COMPONENTS section
for an explanation of determining C
OUT
’s value.
20075672
FIGURE 2. Typical Stereo Audio Amplifier Application Circuit
LM4940
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Typical Performance Characteristics
THD+N vs Frequency THD+N vs Frequency
20075699
V
DD
= 12V, R
L
=4Ω, SE operation,
both channels driven and loaded (average shown),
P
OUT
= 1W, A
V
=1
200756A0
V
DD
= 12V, R
L
=4Ω, SE operation,
both channels driven and loaded (average shown),
P
OUT
= 2.5W, A
V
=1
THD+N vs Frequency THD+N vs Output Power
200756A1
V
DD
= 12V, R
L
=8Ω, SE operation,
both channels driven and loaded (average shown),
P
OUT
= 1W, A
V
=1
200756F3
V
DD
= 14.4V, R
L
=4Ω, SE operation, A
V
=1
single channel driven/single channel measured,
f
IN
= 1kHz
THD+N vs Output Power THD+N vs Output Power
200756D9
V
DD
= 12V, R
L
=4Ω, SE operation, A
V
=1
single channel driven/single channel measured,
f
IN
= 1kHz
200756E0
V
DD
= 12V, R
L
=8Ω, SE operation, A
V
=1
single channel driven/single channel measured,
f
IN
= 1kHz
LM4940
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Typical Performance Characteristics (Continued)
THD+N vs Output Power THD+N vs Output Power
200756E1
V
DD
= 12V, R
L
=16Ω, SE operation, A
V
=1
single channel driven/single channel measured,
f
IN
= 1kHz
200756C7
V
DD
= 12V, R
L
=4Ω, SE operation, A
V
=10
single channel driven/single channel measured,
f
IN
= 1kHz
THD+N vs Output Power THD+N vs Output Power
200756C6
V
DD
= 12V, R
L
=8Ω, SE operation, A
V
=10
single channel driven/single channel measured,
f
IN
= 1kHz
20075666
V
DD
= 12V, R
L
=16Ω, SE operation, A
V
=10
single channel driven/single channel measured,
f
IN
= 1kHz
Output Power vs Power Supply Voltage Output Power vs Power Supply Voltage
200756E8
R
L
=4Ω, SE operation, f
IN
= 1kHz,
both channels driven and loaded (average shown),
at (from top to bottom at 12V): THD+N = 10%,
THD+N = 1%
200756E9
R
L
=8Ω, SE operation, f
IN
= 1kHz,
both channels driven and loaded (average shown),
at (from top to bottom at 12V): THD+N = 10%,
THD+N = 1%
LM4940
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Typical Performance Characteristics (Continued)
Output Power vs Power Supply Voltage Power Supply Rejection vs Frequency
20075667
R
L
=16Ω, SE operation, f
IN
= 1kHz,
both channels driven and loaded (average shown),
at (from top to bottom at 12V): THD+N = 10%,
THD+N = 1%
200756B8
V
DD
= 12V, R
L
=8Ω, SE operation,
V
RIPPLE
= 200mV
p-p
, at (from top to bottom at 60Hz):
C
BYPASS
= 1µF, C
BYPASS
= 4.7µF, C
BYPASS
= 10µF,
Power Supply Rejection vs Frequency Total Power Dissipation vs Load Dissipation
200756D8
V
DD
= 12V, R
L
=8Ω, SE operation, V
RIPPLE
= 200mV
p-p
,
A
V
= 10, at (from top to bottom at 60Hz):
C
BYPASS
= 1µF, C
BYPASS
= 4.7µF, C
BYPASS
= 10µF
20075681
V
DD
= 12V, SE operation, f
IN
= 1kHz,
at (from top to bottom at 1W):
R
L
=4Ω,R
L
=8Ω
Output Power vs Load Resistance Channel-to-Channel Crosstalk vs Frequency
20075691
V
DD
= 12V, SE operation, f
IN
= 1kHz,
both channels driven and loaded,
at (from top to bottom at 15Ω):
THD+N = 10%, THD+N = 1%
20075698
V
DD
= 12V, R
L
=4Ω,P
OUT
= 1W, SE operation,
at (from top to bottom at 1kHz): V
INB
driven,
V
OUTA
measured; V
INA
driven, V
OUTB
measured
LM4940
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Typical Performance Characteristics (Continued)
Channel-to-Channel Crosstalk vs Frequency Power Supply Current vs Power Supply Voltage
200756A3
V
DD
= 12V, R
L
=8Ω,P
OUT
= 1W, SE operation,
at (from top to bottom at 1kHz): V
INB
driven,
V
OUTA
measured; V
INA
driven, V
OUTB
measured
200756F0
R
L
=4Ω, SE operation
V
IN
= 0V, R
SOURCE
=50Ω
Clipping Voltage vs Power Supply Voltage Clipping Voltage vs Power Supply Voltage
200756F1
R
L
=4Ω, SE operation, f
IN
= 1kHz
both channels driven and loaded,
at (from top to bottom at 13V):
negative signal swing, positive signal swing
200756F2
R
L
=8Ω, SE operation, f
IN
= 1kHz
both channels driven and loaded, at (from top to bottom
at 13V):
negative signal swing, positive signal swing
Power Dissipation vs Ambient Temperature
200756E4
V
DD
= 12V, R
L
=8Ω(SE), f
IN
= 1kHz,
(from top to bottom at 120˚C): 16in
2
copper plane
heatsink area,
8in
2
copper plane heatsink area
LM4940
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Application Information
HIGH VOLTAGE BOOMER WITH INCREASED OUTPUT
POWER
Unlike previous 5V Boomer®amplifiers, the LM4940 is de-
signed to operate over a power supply voltages range of 10V
to 15V. Operating on a 12V power supply, the LM4940 will
deliver 3.1W per channel into 4Ωloads with no more than
1% THD+N.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended amplifier. Equation (2) states the
maximum power dissipation point for a single-ended ampli-
fier operating at a given supply voltage and driving a speci-
fied output load.
P
DMAX-SE
=(V
DD
)
2
/(2π
2
R
L
): Single Ended (1)
The LM4940’s dissipation is twice the value given by Equa-
tion (2) when driving two SE loads. For a 12V supply and two
8ΩSE loads, the LM4940’s dissipation is 1.82W.
The maximum power dissipation point (twice the value given
by Equation (2)) must not exceed the power dissipation
given by Equation (4):
P
DMAX
’=(T
JMAX
-T
A
)/θ
JA
(2)
The LM4940’s T
JMAX
= 150˚C. In the TS package, the
LM4940’s θ
JA
is 20˚C/W when the metal tab is soldered to a
copper plane of at least 16in
2
. This plane can be split be-
tween the top and bottom layers of a two-sided PCB. Con-
nect the two layers together under the tab with a 5x5 array of
vias. For the TA package, use an external heatsink with a
thermal impedance that is less than 20˚C/W. At any given
ambient temperature T
A
, use Equation (4) to find the maxi-
mum internal power dissipation supported by the IC packag-
ing. Rearranging Equation (4) and substituting P
DMAX
for
P
DMAX
’ results in Equation (5). This equation gives the maxi-
mum ambient temperature that still allows maximum stereo
power dissipation without violating the LM4940’s maximum
junction temperature.
T
A
=T
JMAX
-P
DMAX-SE
θ
JA
(3)
For a typical application with a 12V power supply and two 4Ω
SE loads, the maximum ambient temperature that allows
maximum stereo power dissipation without exceeding the
maximum junction temperature is approximately 113˚C for
the TS package.
T
JMAX
=P
DMAX-SE
θ
JA
+T
A
(4)
Equation (6) gives the maximum junction temperature
T
JMAX
. If the result violates the LM4940’s 150˚C, reduce the
maximum junction temperature by reducing the power sup-
ply voltage or increasing the load resistance. Further allow-
ance should be made for increased 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 am-
bient temperatures are allowed as output power or duty
cycle decreases.
20075672
FIGURE 3. Typical LM4940 Stereo Amplifier Application Circuit
LM4940
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Application Information (Continued)
If the result of Equation (3) is greater than that of Equation
(4), then decrease the supply voltage, increase the load
impedance, or reduce the ambient temperature. Further,
ensure that speakers rated at a nominal 4Ωdo not fall below
3Ω. If these measures are insufficient, a heat sink can be
added to reduce θ
JA
. The heat sink can be created using
additional copper area around the package, with connec-
tions to the ground pins, supply pin and amplifier output pins.
Refer to the Typical Performance Characteristics curves
for power dissipation information at lower output power lev-
els.
POWER SUPPLY VOLTAGE LIMITS
Continuous proper operation is ensured by never exceeding
the voltage applied to any pin, with respect to ground, as
listed in the Absolute Maximum Ratings section.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a voltage regulator typi-
cally use a 10µF in parallel with a 0.1µF filter capacitors to
stabilize the regulator’s output, reduce noise on the supply
line, and improve the supply’s transient response. However,
their presence does not eliminate the need for a local 1.0µF
tantalum bypass capacitance connected between the
LM4940’s supply pins and ground. Do not substitute a ce-
ramic capacitor for the tantalum. Doing so may cause oscil-
lation. Keep the length of leads and traces that connect
capacitors between the LM4940’s power supply pin and
ground as short as possible. Connecting a 10µF capacitor,
C
BYPASS
, between the BYPASS pin and ground improves
the internal bias voltage’s stability and improves the amplifi-
er’s PSRR. The PSRR improvements increase as the by-
pass pin capacitor value increases. Too large, however,
increases turn-on time and can compromise the amplifier’s
click and pop performance. The selection of bypass capaci-
tor values, especially C
BYPASS
, depends on desired PSRR
requirements, click and pop performance (as explained in
the section, SELECTING EXTERNAL COMPONENTS),
system cost, and size constraints.
MICRO-POWER SHUTDOWN
The LM4940 features an active-low micro-power shutdown
mode. When active, the LM4940’s micro-power shutdown
feature turns off the amplifier’s bias circuitry, reducing the
supply current. The low 40µA typical shutdown current is
achieved by applying a voltage to the SHUTDOWN pin that
is as near to GND as possible. A voltage that is greater than
GND may increase the shutdown current.
There are a few methods to control the micro-power shut-
down. These include using a single-pole, single-throw switch
(SPST), a microprocessor, or a microcontroller. When using
a switch, connect a 100kΩpull-up resistor between the
SHUTDOWN pin and V
DD
and the SPST switch between the
SHUTDOWN pin and GND. Select normal amplifier opera-
tion by opening the switch. Closing the switch applies GND
to the SHUTDOWN pin, activating micro-power shutdown.
The switch and resistor guarantee that the SHUTDOWN pin
will not float. This prevents unwanted state changes. In a
system with a microprocessor or a microcontroller, use a
digital output to apply the active-state voltage to the SHUT-
DOWN pin.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Two quantities determine the value of the input coupling
capacitor: the lowest audio frequency that requires amplifi-
cation and desired output transient suppression.
As shown in Figure 3, the input resistor (R
IN
) and the input
capacitor (C
IN
) produce a high pass filter cutoff frequency
that is found using Equation (7).
f
c
= 1/2πR
i
C
i
(5)
As an example when using a speaker with a low frequency
limit of 50Hz, C
i
, using Equation (7) is 0.159µF. The 0.39µF
C
INA
shown in Figure 3allows the LM4940 to drive high
efficiency, full range speaker whose response extends below
30Hz.
Output Coupling Capacitor Value Selection
The capacitors C
OUTA
and C
OUTB
that block the V
DD
/2 out-
put DC bias voltage and couple the output AC signal to the
amplifier loads also determine low frequency response.
These capacitors, combined with their respective loads cre-
ate a highpass filter cutoff frequency. The frequency is also
given by Equation (6).
Using the same conditions as above, with a 4Ωspeaker,
C
OUT
is 820µF (nearest common valve).
Bypass Capacitor Value
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
BYPASS
, the capacitor
connected to the BYPASS pin. Since C
BYPASS
determines
how fast the LM4940 settles to quiescent operation, its value
is critical when minimizing turn-on pops. The slower the
LM4940’s outputs ramp to their quiescent DC voltage (nomi-
nally V
DD
/2), the smaller the turn-on pop. Choosing C
BYPASS
equal to 10µF along with a small value of C
IN
(in the range of
0.1µF to 0.39µF), produces a click-less and pop-less shut-
down function. As discussed above, choosing C
IN
no larger
than necessary for the desired bandwidth helps minimize
clicks and pops.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4940 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 V
DD
/2 voltage present at the BYPASS pin ramps to its
final value, the LM4940’s internal amplifiers are configured
as unity gain buffers and are disconnected from the AMP
A
and AMP
B
pins. An internal current source charges the ca-
pacitor 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 voltage applied to the BY-
PASS pin.
The gain of the internal amplifiers remains unity until the
voltage on the bypass pin reaches V
DD
/2. As soon as the
voltage on the bypass pin is stable, the device becomes fully
operational and the amplifier outputs are reconnected to
their respective output pins. Although the BYPASS pin cur-
rent cannot be modified, changing the size of C
BYPASS
alters
the device’s turn-on time. Here are some typical turn-on
times for various values of C
BYPASS
:
LM4940
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Application Information (Continued)
C
B
(µF) T
ON
(ms)
1.0 120
2.2 120
4.7 200
10 440
In order eliminate "clicks and pops", all capacitors must be
discharged before turn-on. Rapidly switching V
DD
may not
allow the capacitors to fully discharge, which may cause
"clicks and pops".
There is a relationship between the value of C
IN
and
C
BYPASS
that ensures minimum output transient when power
is applied or the shutdown mode is deactivated. Best perfor-
mance is achieved by setting the time constant created by
C
IN
and R
i
+R
f
to a value less than the turn-on time for a
given value of C
BYPASS
as shown in the table above.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 3W into a 4Ωload
The following are the desired operational parameters:
Power Output 3W
RMS
Load Impedance 4Ω
Input Level 0.3V
RMS
(max)
Input Impedance 20kΩ
Bandwidth 100Hz–20kHz ±0.25dB
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
find the minimum supply voltage is to use the Output Power
vs Power Supply Voltage curve in the Typical Performance
Characteristics section. Another way, using Equation (8), is
to calculate the peak output voltage necessary to achieve
the desired output power for a given load impedance. To
account for the amplifier’s dropout voltage, two additional
voltages, based on the Clipping Dropout Voltage vs Power
Supply Voltage in the Typical Performance Characteris-
tics curves, must be added to the result obtained by Equa-
tion (8). The result is Equation (9).
(6)
V
DD
=V
OUTPEAK
+V
ODTOP
+V
ODBOT
(7)
The Output Power vs. Power Supply Voltage graph for an 8Ω
load indicates a minimum supply voltage of 11.8V. The com-
monly used 12V supply voltage easily meets this. The addi-
tional voltage creates the benefit of headroom, allowing the
LM4940 to produce an output power of 3W without clipping
or other audible distortion. The choice of supply voltage must
also not create a situation that violates of maximum power
dissipation as explained above in the Power Dissipation
section. After satisfying the LM4940’s power dissipation re-
quirements, the minimum differential gain needed to achieve
3W dissipation in a 4ΩBTL load is found using Equation
(10).
(8)
Thus, a minimum gain of 11.6 allows the LM4940’s to reach
full output swing and maintain low noise and THD+N perfor-
mance. For this example, let A
V
= 12. The amplifier’s overall
BTL gain is set using the input (RIN
A
) and feedback (R)
resistors of the first amplifier in the series BTL configuration.
Additionaly, A
V-BTL
is twice the gain set by the first amplifier’s
R
IN
and R
f
. With the desired input impedance set at 20kΩ,
the feedback resistor is found using Equation (11).
R
f
/R
IN
=A
V
(9)
The value of R
f
is 240kΩ. The nominal output power is 3W.
The last step in this design example is setting the amplifier’s
-3dB frequency bandwidth. To achieve the desired ±0.25dB
pass band magnitude variation limit, the low frequency re-
sponse must extend to at least one-fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. The gain variation for
both response limits is 0.17dB, well within the ±0.25dB-
desired limit. The results are an
f
L
= 100Hz/5=20Hz (10)
and an
f
L
= 20kHzx5=100kHz (11)
As mentioned in the SELECTING EXTERNAL COMPO-
NENTS section, R
INA
and C
INA
, as well as C
OUT
and R
L
,
create a highpass filter that sets the amplifier’s lower band-
pass frequency limit. Find the coupling capacitor’s value
using Equation (14).
C
IN
=1/2πR
IN
f
L
(12)
The result is
1/(2πx20kΩx20Hz) = 0.398µF = C
IN
and
1/(2πx4Ωx20Hz) = 1989µF = COUT
Use a 0.39µF capacitor for C
IN
and a 2000µF capacitor for
C
OUT
, the closest standard values.
The product of the desired high frequency cutoff (100kHz in
this example) and the differential gain A
V
, determines the
upper passband response limit. With A
V
= 12 and f
H
=
100kHz, the closed-loop gain bandwidth product (GBWP) is
1.2mHz. This is less than the LM4940’s 3.5MHz GBWP. With
this margin, the amplifier can be used in designs that require
more differential gain while avoiding performance restricting
bandwidth limitations.
LM4940
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Application Information (Continued)
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figure 5 through Figure 7 show the recommended two-layer
PC board layout that is optimized for the TO263-packaged
LM4940 and associated external components. This circuit
board is designed for use with an external 12V supply and
4Ω(min) speakers.
This circuit board is easy to use. Apply 12V and ground to
the board’s V
DD
and GND pads, respectively. Connect a
speaker between the board’s OUT
A
and OUT
B
outputs and
their respective GND terminals.
Demonstration Board Layout
20075663
FIGURE 4. Recommended TS PCB Layout:
Top Silkscreen
20075664
FIGURE 5. Recommended TS PCB Layout:
Top Layer
LM4940
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Demonstration Board Layout (Continued)
20075665
FIGURE 6. Recommended TS PCB Layout:
Bottom Layer
LM4940
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Physical Dimensions inches (millimeters) unless otherwise noted
Plastic Package,
Order Number LM4940TS
NS Package Number TS9A
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.
For the most current product information visit us at www.national.com.
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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.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor manufactures products and uses packing materials that meet the provisions of the Customer Products
Stewardship Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain
no ‘‘Banned Substances’’ as defined in CSP-9-111S2.
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Support Center
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LM4940 6W Stereo Audio Power Amplifier
