Datasheet

LM4950
SNAS174E –JULY 2003–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
HIGH VOLTAGE BOOMER WITH INCREASED OUTPUT POWER
Unlike previous 5V Boomer amplifiers, the LM4950 is designed 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 8Ω BTL load with no more
than 10% THD+N.
Figure 65. Typical LM4950 BTL Application Circuit
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 65, the LM4950 consists of two operational amplifiers that drive a speaker connected
between their outputs. The value of external input and feedback resistors determine the gain of each amplifier.
Resistors RIN
A
and RF
A
set the closed-loop gain of AMP
A
, whereas two 20kΩ resistors set AMP
B
's gain to -1.
The LM4950 drives a load, such as a speaker, connected between the two amplifier outputs, VOUT
A
and
VOUT
B
. Figure 65 shows that AMP
A
's output serves as AMP
B
'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 AMP
B
and driven differentially (commonly referred to as "bridge mode"). This results in a
differential, or BTL, gain of
A
VD
= 2(R
f
/ R
i
) (1)
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single
amplifier's output and ground. For a given supply voltage, bridge mode has a distinct advantage over the single-
ended configuration: 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 AUDIO
POWER AMPLIFIER DESIGN.
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 amplifiers 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 increases internal IC power
dissipation and may permanently damage loads such as speakers.
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