LM4950
7.5W Mono-BTL or 3.1W Stereo Audio Power Amplifier

General Description

The LM4950 is a dual audio power amplifier primarily de-
signed for demanding applications in flat panel monitors and
TV's. It is capable of delivering 3.1 watts per channel to a 4

single-ended load with less than 1% THD+N or 7.5 watts
mono BTL to an 8

load, with less than 10% THD+N from a

12V

power supply.

Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. The LM4950 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 LM4950 features a low-power consumption active-low
shutdown mode. Additionally, the LM4950 features an inter-
nal thermal shutdown protection mechanism along with short
circuit protection.

The LM4950 contains advanced pop & click circuitry that
eliminates noises which would otherwise occur during
turn-on and turn-off transitions.

The LM4950 is a unity-gain stable and can be configured by
external gain-setting resistors.

Key Specifications

Quiscent Power Supply Current

16mA (typ)

OUT

(SE)

= 12V, R

= 4

, 1% THD+N

3.1W (typ)

OUT

(BTL)

= 12V, R

= 8

, 10% THD+N

7.5W (typ)

Shutdown current

40µA (typ)

Features

Pop & click circuitry eliminates noise during turn-on and
turn-off transitions

Low current, active-low shutdown mode

Low quiescent current

Stereo 3.1W output, R

= 4

Mono 7.5W BTL output, R

= 8

Short circuit protection

Unity-gain stable

External gain configuration capability

Applications

Flat Panel Monitors

Flat panel TV's

Computer Sound Cards

Typical Application

Boomer

is a registered trademark of National Semiconductor Corporation.

20047078

FIGURE 1. Typical Bridge-Tied-Load (BTL) Audio Amplifier Application Circuit

August 2004

LM4950

7.5W

Mono-BTL

or
3.1W

Stereo

Audio

Power

Amplifier

DS200470

www.national.com

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

+ 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

(TS)

4°C/W

(TS) (Note 3)

20°C/W

(TA)

4°C/W

(TA) (Note 3)

20°C/W

Operating Ratings

Temperature Range

MIN

MAX

-40°C

85°C

Supply Voltage

9.6V

16V

Electrical Characteristics V

= 12V

(Notes 1, 2)

The following specifications apply for V

= 12V, A

= 0dB (SE) or 6dB (BTL) unless otherwise specified. Limits apply for T

25°C.

Symbol

Parameter

Conditions

LM4950

Units

(Limits)

Typical

(Note 6)

Limit

(Notes 7, 8)

Quiescent Power Supply Current

= 0V, I

= 0A, No Load

mA (max)

Shutdown Current

SHUTDOWN

= GND (Note 9)

µA (max)

Offset Voltage

= 0V, RL = 8

mV (max)

SDIH

Shutdown Voltage Input High

2.0

V (min)

V (max)

SDIL

Shutdown Voltage Input Low

0.4

V (max)

Wake-up Time

= 10µF

440

TSD

Thermal Shutdown Temperature

170

150

190

°C (min)

°C (max)

Output Power

f = 1kHz

= 4

SE, Single Channel,

THD+N = 1%

= 8

BTL, THD+N = 10%

3.1

7.5

3.0

W (min)

THD+N

Total Harmomic Distortion + Noise

= 2.5Wrms; f = 1kHz;

= 4

0.05

= 2.5Wrms; A

= 10; f = 1kHz;

= 4

, SE

0.14

Output Noise

A-Weighted Filter, V

= 0V,

Input Referred

µV

TALK

Channel Separation

= 1kHz, P

= 1W, SE Mode

= 8

= 4

PSRR

Power Supply Rejection Ratio

RIPPLE

= 200mV

p-p

, f = 1kHz,

= 8

, BTL

dB (min)

Output Current Limit

= 0V, R

= 500m

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 T

JMAX

, and the ambient temperature, T

. The maximum

allowable power dissipation is P

DMAX

= (T

JMAX

- T

) /

or the given in Absolute Maximum Ratings, whichever is lower. For the LM4950 typical application (shown

in Figure 1) with V

= 12V, R

= 4

stereo operation the total power dissipation is 3.65W.

= 20°C/W for both TO263 and TO220 packages mounted to 16in

heatsink surface area.

Note 4: Human body model, 100pF discharged through a 1.5 k

resistor.

Note 5: Machine Model, 220pF240pF discharged through all pins.

LM4950

www.national.com

Electrical Characteristics V

= 12V

(Notes 1, 2) (Continued)

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.

External Components Description

Refer to (Figure 1.)

Components

Functional Description

1. R

This is the inverting input resistance that, along with R

, sets the closed-loop gain. Input resistance R

and input capacitance C

form a high pass filter. The filter's cutoff frequency is f

= 1/(2

2. C

This is the input coupling capacitor. It blocks DC voltage at the amplifier's inverting input. C

and R

create a highpass filter. The filter's cutoff frequency is f

= 1/(2

). Refer to the SELECTING

EXTERNAL COMPONENTS, for an explanation of determining C

's value.

3. R

This is the feedback resistance that, along with R

, sets closed-loop gain.

4. C

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.

20047072

FIGURE 2. Typical Stereo Single-Ended (SE) Audio Amplifier Application Circuit

LM4950

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Typical Performance Characteristics

(Continued)

Output Power vs Power Supply Voltage

200470C5

= 4

, SE operation,

both channels driven and loaded (average shown),

at (from top to bottom at 12V): THD+N = 10%,

THD+N = 1%

200470C4

= 8

, SE operation, f

= 1kHz,

both channels driven and loaded (average shown),

at (from top to bottom at 12V): THD+N = 10%,

THD+N = 1%

Power Supply Rejection vs Frequency

200470B9

= 12V, R

= 8

, BTL operation,

RIPPLE

= 200mV

p-p

, at (from top to bottom at 60Hz):

BYPASS

= 1µF, C

BYPASS

= 4.7µF, C

BYPASS

= 10µF,

200470B8

= 12V, R

= 8

, SE operation,

RIPPLE

= 200mV

p-p

, at (from top to bottom at 60Hz):

BYPASS

= 1µF, C

BYPASS

= 4.7µF, C

BYPASS

= 10µF,

Power Supply Rejection vs Frequency

200470D7

= 12V, R

= 8

, BTL operation, V

RIPPLE

= 200mV

p-p

= 20, at (from top to bottom at 60Hz):

BYPASS

= 1µF, C

BYPASS

= 4.7µF, C

BYPASS

= 10µF

200470D8

= 12V, R

= 8

, SE operation, V

RIPPLE

= 200mV

p-p

= 10, at (from top to bottom at 60Hz):

BYPASS

= 1µF, C

BYPASS

= 4.7µF, C

BYPASS

= 10µF

LM4950

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Typical Performance Characteristics

(Continued)

Total Power Dissipation vs Load Dissipation

20047084

= 12V, BTL operation, f

= 1kHz,

at (from top to bottom at 3W):

= 8

, R

= 16

20047081

= 12V, SE operation, f

= 1kHz,

at (from top to bottom at 1W):

= 4

, R

= 8

Output Power vs Load Resistance

20047094

= 12V, BTL operation, f

= 1kHz,

at (from top to bottom at 15

THD+N = 10%, THD+N = 1%

20047091

= 12V, SE operation, f

= 1kHz,

both channels driven and loaded, at (from top to bottom

at 15

THD+N = 10%, THD+N = 1%

Channel-to-Channel Crosstalk vs Frequency

20047098

= 12V, R

= 4

, P

OUT

= 1W, SE operation,

at (from top to bottom at 1kHz): V

INB

driven,

OUTA

measured; V

INA

driven, V

OUTB

measured

200470A3

= 12V, R

= 8

, P

OUT

= 1W, SE operation,

at (from top to bottom at 1kHz): V

INB

driven,

OUTA

measured; V

INA

driven, V

OUTB

measured

LM4950

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Typical Performance Characteristics

(Continued)

Output Power vs Load Resistance

20047095

= 9.6V, BTL operation, f

= 1kHz,

at (from top to bottom at 15

THD+N = 10%, THD+N = 1%

20047092

= 9.6V, SE operation, f

= 1kHz,

both channels driven and loaded, at (from top to bottom

at 15

THD+N = 10%, THD+N = 1%

Channel-to Channel Crosstalk vs Frequency

20047096

= 9.6V, R

= 4

, P

OUT

= 1W, SE operation,

at (from top to bottom at 1kHz): V

INB

driven, V

OUTA

measured; V

INA

driven, V

OUTB

measured

200470A4

= 9.6V, R

= 8

, P

OUT

= 1W, SE operation,

at (from top to bottom at 1kHz): V

INB

driven, V

OUTA

measured; V

INA

driven, V

OUTB

measured

THD+N vs Output Power

200470C0

= 15V, R

= 8

BTL operation, f

= 1kHz

200470C2

= 15V, R

= 4

SE operation, f

= 1kHz

both channels driven and loaded (average shown)

LM4950

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Typical Performance Characteristics

(Continued)

THD+N vs Output Power

200470D3

= 16V, R

= 8

BTL operation, f

= 1kHz, BTLA

= 20

200470C9

= 16V, R

= 4

, A

= 10

SE operation, f

= 1kHz,

both channels driven and loaded (average shown)

Channel-to-Channel Crosstalk vs Frequency

20047097

= 16V, R

= 4

, P

OUT

= 1W, SE operation

at (from top to bottom at 1kHz): V

INB

driven, V

OUTA

measured;

INA

driven, V

OUTB

measured

200470A2

= 16V, R

= 8

, P

OUT

= 1W, SE operation

at (from top to bottom at 1kHz): V

INB

driven, V

OUTA

measured;

INA

driven, V

OUTB

measured

Power Supply Current vs Power Supply Voltage

200470A6

= 8

, BTL operation

= 0V, R

SOURCE

= 50

200470A7

= 4

, SE operation

= 0V, R

SOURCE

= 50

LM4950

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Typical Performance Characteristics

(Continued)

Clipping Voltage vs Power Supply Voltage

20047088

= 8

, BTL operation, f

= 1kHz

at (from top to bottom at 12V):

positive signal swing, negative signal swing

20047086

= 16

, BTL operation, f

= 1kHz

at (from to bottom at 12V):

positive signal swing, negative signal swing

Clipping Voltage vs Power Supply Voltage

20047089

= 4

, SE operation, f

= 1kHz

both channels driven and loaded, at (from top to bottom

at 13V):

negative signal swing, positive signal swing

20047087

= 8

, SE operation, f

= 1kHz

both channels driven and loaded, at (from to bottom at

13V):

negative signal swing, positive signal swing

Power Dissipation vs Ambient Temperature

200470E6

= 12V, R

= 8

(BTL), f

= 1kHz,

(from to bottom at 80°C): 16in

copper plane heatsink

area,

8in

copper plane heatsink area

200470E4

= 12V, R

= 8

(SE), f

= 1kHz,

(from to bottom at 120°C): 16in

copper plane heatsink

area,

8in

copper plane heatsink area

LM4950

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Application Information

HIGH VOLTAGE BOOMER WITH INCREASED OUTPUT
POWER

Unlike previous 5V Boomer

amplifiers, the LM4950 is de-

signed to operate over a power supply voltages range of
9.6V to 16V. Operating on a 12V power supply, the LM4950
will deliver 7.5W into an 8

BTL load with no more than 10%

THD+N.

BRIDGE CONFIGURATION EXPLANATION

As shown in Figure 3, the LM4950 consists of two opera-
tional amplifiers that drive a speaker connected between
their outputs. The value of external input and feedback re-
sistors determine the gain of each amplifier. Resistors RIN

and RF

set the closed-loop gain of AMP

, whereas two

20k

resistors set AMP

's gain to -1. The LM4950 drives a

load, such as a speaker, connected between the two ampli-
fier outputs, VOUT

and VOUT

. Figure 3 shows that

AMP

's output serves as AMP

's input. This results in both

amplifiers producing signals identical in magnitude, but 180°
out of phase. Taking advantage of this phase difference, a
load is placed between AMP

and AMP

and driven differ-

entially (commonly referred to as "bridge mode"). This re-
sults in a differential, or BTL, gain of

= 2(R

/ R

)

(1)

Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifi-
er's output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended con-
figuration: its differential 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 conditions. This increase in attainable output
power assumes that the amplifier is not current limited and
that the output signal is not clipped. To ensure minimum
output signal clipping when choosing an amplifier's closed-
loop gain, refer to the AUDIO POWER AMPLIFIER DESIGN
section.

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 am-
plifier'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

Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. Equation (2)
states the maximum power dissipation point for a single-
ended amplifier operating at a given supply voltage and
driving a specified output load.

20047078

FIGURE 3. Typical LM4950 BTL Application Circuit

LM4950

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Application Information

(Continued)

DMAX-SE

= (V

)

/ (2

Single Ended

(2)

The LM4950's dissipation is twice the value given by Equa-
tion (2) when driving two SE loads. For a 12V supply and two
8

SE loads, the LM4950's dissipation is 1.82W.

The LM4950's dissipation when driving a BTL load is given
by Equation (3). For a 12V supply and a single 8

BTL load,

the dissipation is 3.65W.

DMAX-MONOBTL

= 4(V

)

/ 2

Bridge Mode (3)

The maximum power dissipation point given by Equation (3)
must not exceed the power dissipation given by Equation
(4):

DMAX

' = (T

JMAX

- T

) /

(4)

The LM4950's T

JMAX

= 150°C. In the TS package, the

LM4950's

is 20°C/W when the metal tab is soldered to a

copper plane of at least 16in

. 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

, 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

DMAX

' results in Equation (5). This equation gives the maxi-

mum ambient temperature that still allows maximum stereo
power dissipation without violating the LM4950's maximum
junction temperature.

= T

JMAX

- P

DMAX-MONOBTL

(5)

For a typical application with a 12V power supply and a BTL
8

load, the maximum ambient temperature that allows

maximum stereo power dissipation without exceeding the
maximum junction temperature is approximately 77°C for the
TS package.

JMAX

= P

DMAX-MONOBTL

+ T

(6)

Equation (6) gives the maximum junction temperature
T

JMAX

. If the result violates the LM4950'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.

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 4

(SE operation)

or 8

(BTL operation) do not fall below 3 or 6, respec-

tively. If these measures are insufficient, a heat sink can be
added to reduce

. 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
LM4950'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 LM4950'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 LM4950 features an active-low micro-power shutdown
mode. When active, the LM4950'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 100k

pull-up resistor between the

SHUTDOWN pin and V

and a second 100k

resistor in

parallel with the SPST switch connected between the SHUT-
DOWN pin and GND. The two resistors form a voltage
divider that ensures that the voltage applied to the SHUT-
DOWN pin does not exceed V

/2. Select normal amplifier

operation by opening the switch. Closing the switch applies
GND to the SHUTDOWN pin, activating micro-power shut-
down. The switch and resistor guarantee that the SHUT-
DOWN pin will not float. This prevents unwanted state
changes. In a system with a microprocessor or a microcon-
troller, use a digital output to apply the active-state voltage to
the SHUTDOWN pin. Again, ensure that the microcontroller
or microprocessor logic-high signal does not exceed the
LM4950's V

/2 SHUTDOWN signal limit.

LM4950

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Application Information

(Continued)

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

) and the input

capacitor (C

) produce a high pass filter cutoff frequency

that is found using Equation (7).

= 1/2

(7)

As an example when using a speaker with a low frequency
limit of 50Hz, C

, using Equation (7) is 0.159µF. The 0.39µF

INA

shown in Figure 3 allows the LM4950 to drive high

efficiency, full range speaker whose response extends below
30Hz.

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 LM4950 settles to quiescent operation, its value
is critical when minimizing turn-on pops. The slower the
LM4950's outputs ramp to their quiescent DC voltage (nomi-
nally V

/2), the smaller the turn-on pop. Choosing C

BYPASS

equal to 10µF along with a small value of C

(in the range of

0.1µF to 0.39µF), produces a click-less and pop-less shut-
down function. As discussed above, choosing C

no larger

than necessary for the desired bandwidth helps minimize
clicks and pops.

OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE

The LM4950 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

/2 voltage present at the BYPASS pin ramps to its

final value, the LM4950's internal amplifiers are configured
as unity gain buffers and are disconnected from the AMP

and AMP

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

/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

(µF)

(ms)

1.0

120

2.2

120

4.7

200

440

In order eliminate "clicks and pops", all capacitors must be
discharged before turn-on. Rapidly switching V

may not

allow the capacitors to fully discharge, which may cause
"clicks and pops".

There is a relationship between the value of C

and

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

and R

+ R

to a value less than the turn-on time for a

given value of C

BYPASS

as shown in the table above.

DRIVING PIEZO-ELECTRIC SPEAKER TRANSDUCERS

The LM4950 is able to drive capacitive piezo-electric trans-
ducer loads that are less than equal to 200nF. Stable opera-
tion is assured by placing 33pF capacitors in parallel with the
20k

feedback resistors. The additional capacitors are

shown in Figure 4.

When driving piezo-electric tranducers, sound quality and
accoustic power is entirely dependent upon a transducer's
frequency response and efficiency. In this application, power
dissipated by the LM4950 is very low, typically less than
250mW when driving a 200nF piezo-electric transduce
(V

= 12V).

LM4950

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Application Information

(Continued)

AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 4W into an 8

BTL

The following are the desired operational parameters:

Power Output

RMS

Load Impedance

Input Level

0.3V

RMS

(max)

Input Impedance

20k

Bandwidth

50Hz20kHz

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).

(8)

= V

OUTPEAK

+ V

ODTOP

+ V

ODBOT

(9)

The Output Power vs. Power Supply Voltage graph for an 8

load indicates a minimum supply voltage of 10.2V. The com-
monly used 12V supply voltage easily meets this. The addi-
tional voltage creates the benefit of headroom, allowing the
LM4950 to produce peak output power in excess of 4W
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 LM4950's
power dissipation requirements, the minimum differential
gain needed to achieve 4W dissipation in an 8

BTL load is

found using Equation (10).

(10)

Thus, a minimum gain of 18.9 allows the LM4950's to reach
full output swing and maintain low noise and THD+N perfor-
mance. For this example, let A

V-BTL

= 19. The amplifier's

overall BTL gain is set using the input (RIN

) and feedback

(R) resistors of the first amplifier in the series BTL configu-
ration. Additionaly, A

V-BTL

is twice the gain set by the first

amplifier's R

and R

. With the desired input impedance set

at 20k

, the feedback resistor is found using Equation (11).

/ R

= A

V-BTL

/ 2

(11)

The value of R

is 190k

(choose 191k, the closest value).

The nominal output power is 4W.

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

= 50Hz / 5 = 10Hz

(12)

20047079

FIGURE 4. Piezo-electric Transducer Capacitance

200nF

LM4950

www.national.com

Application Information

(Continued)

and an

= 20kHz x 5 = 100kHz

(13)

As mentioned in the SELECTING EXTERNAL COMPO-
NENTS section, R

INA

and C

INA

create a highpass filter that

sets the amplifier's lower bandpass frequency limit. Find the
coupling capacitor's value using Equation (14).

= 1 / 2

(14)

The result is

1 / (2

x20kx10Hz) = 0.795µF

Use a 0.82µF capacitor, the closest standard value.

The product of the desired high frequency cutoff (100kHz in
this example) and the differential gain A

, determines the

upper passband response limit. With A

= 7 and f

100kHz, the closed-loop gain bandwidth product (GBWP) is
700kHz. This is less than the LM4950'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.

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,
SE-configured LM4950 and associated external compo-
nents. Figure 8 through Figure 10 show the recommended
two-layer PC board layout that is optimized for the TO263-
packaged, BTL-configured LM4950 and associated external
components. These circuits are designed for use with an
external 12V supply and 4

(min)(SE) or 8(min)(BTL)

speakers.

These circuit boards are easy to use. Apply 12V and ground
to the board's V

and GND pads, respectively. Connect a

speaker between the board's OUT

and OUT

outputs.

Demonstration Board Layout

20047063

FIGURE 5. Recommended TS SE PCB Layout:

Top Silkscreen

LM4950

www.national.com

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.

BANNED SUBSTANCE COMPLIANCE

National Semiconductor certifies that the products and packing materials 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.

National Semiconductor
Americas Customer
Support Center
Email: new.feedback@nsc.com
Tel: 1-800-272-9959

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Europe Customer Support Center

Fax: +49 (0) 180-530 85 86

Email: europe.support@nsc.com

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English

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Support Center
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Japan Customer Support Center
Fax: 81-3-5639-7507
Email: jpn.feedback@nsc.com
Tel: 81-3-5639-7560

www.national.com

LM4950

7.5W

Mono-BTL

or
3.1W

Stereo

Audio

Power

Amplifier

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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Document Outline

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