LM4950TS datasheet(15/25 Pages) NSC

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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

Ω 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

A set the closed-loop gain of AMPA, whereas two

20k

Ω resistors set AMP

B’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

A’s output serves as AMPB’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

A and AMPB and driven differ-

entially (commonly referred to as "bridge mode"). This re-

sults in a differential, or BTL, gain of

VD = 2(Rf / Ri)

(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

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