LM4671 datasheet(10/18 Pages) NSC

10 / 18 page

Typical Performance Characteristics (Continued)

Gain vs Supply Voltage

in = 150k

Ω

Supply Current vs Supply Voltage

L = 15µH+8

Ω+15µH

20107319

20107320

Shutdown Current vs Supply Voltage

L = 15µH+8

Ω+15µH

20107321

Application Information

GENERAL AMPLIFIER FUNCTION

The LM4671 features a filterless modulation scheme. The

differential outputs of the device switch at 300kHz from V

to GND. When there is no input signal applied, the two

outputs (V

O1 and VO2) switch with a 50% duty cycle, with

both outputs in phase. Because the outputs of the LM4671

are differential, the two signals cancel each other. This re-

sults in no net voltage across the speaker, thus there is no

load current during an idle state, conserving power.

With an input signal applied, the duty cycle (pulse width) of

the LM4671 outputs changes. For increasing output volt-

ages, the duty cycle of V

O1 increases, while the duty cycle of

O2 decreases. For decreasing output voltages, the con-

verse occurs, the duty cycle of V

O2 increases while the duty

cycle of V

O1 decreases. The difference between the two

pulse widths yields the differential output voltage.

POWER DISSIPATION AND EFFICIENCY

In general terms, efficiency is considered to be the ratio of

useful work output divided by the total energy required to

produce it with the difference being the power dissipated,

typically, in the IC. The key here is “useful” work. For audio

systems, the energy delivered in the audible bands is con-

sidered useful including the distortion products of the input

signal.

Sub-sonic

(DC)

and

super-sonic

components

(>22kHz) are not useful. The difference between the power

flowing from the power supply and the audio band power

being transduced is dissipated in the LM4671 and in the

transducer load. The amount of power dissipation in the

LM4671 is very low. This is because the ON resistance of the

switches used to form the output waveforms is typically less

than 0.25

Ω. This leaves only the transducer load as a po-

tential "sink" for the small excess of input power over audio

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