LM4914 datasheet(9/15 Pages) NSC

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Application Information

BRIDGE (BTL) OR SINGLE-ENDED (SE)

CONFIGURATION EXPLANATION

As shown in Figure 2, the LM4914 consists of one input

multiplexer (MUX) and two power amplifiers designed to

drive loads that have a minimum impedance of 4

Ω. In mono

BTL mode, AMP1 and AMP2 drive a speaker connected

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 20k

Ω resistors. In

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

internal 62.5k

Ω feedback resistors to set each amplifier’s

gain. The LM4914 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 LM4914’s outputs, one

driven by AMP1 and the other driven by AMP2.

When the LM4914 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

V (BTL) = -2(AV(SE))

V (SE) = -2(62.5k

Ω)/R

V (BTL) = -125k

Ω /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. 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.

DMAX-SE =(VDD)

2 /2

π2R

Single-Ended

(2)

However, a direct consequence of the increased power de-

livered to the load by a bridge amplifier is higher internal

power dissipation for the same conditions. The LM4914 has

two operational amplifiers driving a mono bridge load. The

maximum internal power dissipation operating in the bridge

mode is twice that of a single-ended amplifier. From Equa-

tion (3), assuming a 5V power supply and an 8

Ω load, the

maximum BTL-mode power dissipation is 158mW.

200634C3

FIGURE 2. Typical Audio Amplifier Application Circuit

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