Supply Current vs Supply Voltage

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20142127

Application Information

ELIMINATING THE OUTPUT COUPLING CAPACITOR

The LM4953 features a low noise inverting charge pump that

generates an internal negative supply voltage. This allows

the outputs of the LM4953 to be biased about GND instead

of a nominal DC voltage, like traditional headphone amplifi-

ers. Because there is no DC component, the large DC

blocking capacitors (typically 220µF) are not necessary. The

coupling capacitors are replaced by two, small ceramic

charge pump capacitors, saving board space and cost.

Eliminating the output coupling capacitors also improves low

frequency response. In traditional headphone amplifiers, the

headphone impedance and the output capacitor form a high

pass filter that not only blocks the DC component of the

output, but also attenuates low frequencies, impacting the

bass response. Because the LM4953 does not require the

output coupling capacitors, the low frequency response of

the device is not degraded by external components.

In addition to eliminating the output coupling capacitors, the

ground referenced output nearly doubles the available dy-

namic range of the LM4953 when compared to a traditional

headphone amplifier operating from the same supply volt-

age.

BRIDGE CONFIGURATION EXPLANATION

The Audio Amplifier portion of the LM4953has two internal

amplifiers allowing different amplifier configurations. The first

amplifier’s gain is externally configurable, whereas the sec-

ond amplifier is internally fixed in a unity-gain, inverting

configuration. The closed-loop gain of the first amplifier is set

by selecting the ratio of Rf to Ri while the second amplifier’s

gain is fixed by the two internal 20k

Ω resistors. Figure 1

shows that the output of amplifier one serves as the input to

amplifier two. This results in both amplifiers producing sig-

nals identical in magnitude, but out of phase by 180˚. Con-

sequently, the differential gain for the Audio Amplifier is

A

By driving the load differentially through outputs OUT A and

OUT B, an amplifier configuration commonly referred to as

“bridged mode” is established. Bridged mode operation is

different from the classic single-ended amplifier configura-

tion where one side of the load is connected to ground.

A bridge amplifier design has a few distinct advantages over

the single-ended configuration. It provides differential drive

to the load, thus doubling the output swing for a specified

supply voltage. Four times the output power is possible as

compared to a single-ended amplifier under the same con-

ditions. This increase in attainable output power assumes

that the amplifier is not current limited or clipped. In order to

choose an amplifier’s closed-loop gain without causing ex-

cessive clipping, please refer to the Audio Power Amplifier

Design section.

The bridge configuration also creates a second advantage

over single-ended amplifiers. Since the differential outputs,

OUT A and OUT B, are biased at half-supply, no net DC

voltage exists across the load. This eliminates the need for

an output coupling capacitor which is required in a single

supply, single-ended amplifier configuration. Without an out-

put coupling capacitor, the half-supply bias across the load

would result in both increased internal IC power dissipation

and also possible loudspeaker damage.

OUTPUT TRANSIENT (’CLICK AND POPS’)

ELIMINATED

The LM4953 contains advanced circuitry that virtually elimi-

nates output transients (’clicks and pops’). This circuitry

prevents all traces of transients when the supply voltage is

first applied or when the part resumes operation after coming

out of shutdown mode.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a

successful design. Equation 1 states the maximum power

dissipation point for a single-ended amplifier operating at a

given supply voltage and driving a specified output load.

P

(1)

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