Application Information

BRIDGE (BTL) OR SINGLE-ENDED (SE)

CONFIGURATION EXPLANATION

As shown in Figure 2, the LM4913 consists of two input

multiplexers (MUX) and two power amplifiers designed to

drive loads that have a minimum impedance of 4 ohms. In

mono BTL mode, AMP1 and AMP2 drive a speaker con-

nected between their outputs. In stereo SE mode, AMP1 and

AMP2 each drive a SE load such as stereo headphones.

In mono BTL mode, R1 works with one of AMP1’s internal

62.5kW feedback resistors to set this amplifier’s gain. AMP2

operates unity gain, set by two internal 20kW resistors. In

stereo SE modes, R2 and R3 work with AMP1’s and AMP2’s

internal 62.5kW feedback resistors to set each amplifier’s

gain. The LM4913 drives a BTL load, such as a speaker,

connected between AMP1’s and AMP2’s outputs. Two SE

loads can also be connected to the LM4913’s outputs, one

driven by AMP1 and the other driven by AMP2.

When the LM4913 operates in BTL mode, AMP1’s output

serves as AMP2’s input through AMP2’s input MUX. This

results in AMP1 and AMP2 producing signals identical in

magnitude, but 180˚ out of phase. Taking advantage of this

phase difference, a load placed between ROUT/M+ and

LOUT/M- is driven differentially (commonly referred to as

"bridge mode"). This results in a differential, or BTL, gain of

A

A

Ω)/R

i

A

Ω /R

i

(1)

Bridge mode amplifiers are different from single-ended am-

plifiers that drive loads connected between a single amplifi-

er’s output and ground. At any 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, capacitively

coupled amplifier under the same conditions. This increase

in attainable output power assumes that an 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 Ampli-

fier Design section.

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.

P

Single-Ended

(2)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is higher internal

20061799

FIGURE 2. Typical Audio Amplifier Application Circuit

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