LM4873MT datasheet(13/19 Pages) NSC

13 / 19 page

Application Information (Continued)

CLICK AND POP CIRCUITRY

The LM4873 contains circuitry to minimize turn-on transients

or “clicks and pops”. In this case, turn-on refers to either

power supply turn-on or the device coming out of shutdown

mode. When the device is turning on, the amplifiers are inter-

nally configured as unity gain buffers. An internal current

source ramps up the voltage of the bypass pin. Both the in-

puts and outputs ideally track the voltage at the bypass pin.

The device will remain in buffer mode until the bypass pin

has reached its half supply voltage, 1/2 V

DD. As soon as the

bypass node is stable, the device will become fully opera-

tional, where the gain is set by the external resistors.

Although the bypass pin current source cannot be modified,

the size of C

B can be changed to alter the device turn-on

time and the amount of “clicks and pops”. By increasing

amount of turn-on pop can be reduced. However, the

tradeoff for using a larger bypass capacitor is an increase in

turn-on time for this device. There is a linear relationship be-

tween the size of C

B and the turn-on time. Here are some

typical turn-on times for a given C

0.01 µF

20 ms

0.1 µF

200 ms

0.22 µF

420 ms

0.47 µF

840 ms

1.0 µF

2 Sec

In order eliminate “clicks and pops”, all capacitors must be

discharged before turn-on. Rapid on/off switching of the de-

vice or the shutdown function may cause the “click and pop”

circuitry to not operate fully, resulting in increased “click and

pop” noise. In a single-ended configuration, the output cou-

pling capacitor, C

O, is of particular concern. This capacitor

discharges through the internal 20 k

Ω resistors. Depending

on the size of C

O, the time constant can be relatively large.

To reduce transients in single-ended mode, an external

Ω–5 kΩ resistor can be placed in parallel with the internal

20 k

Ω resistor. The tradeoff for using this resistor is an in-

crease in quiescent current.

The value of C

I will also reflect turn-on pops. Clearly, a cer-

tain size for C

I is needed to couple in low frequencies without

excessive attenuation. But in many cases, the speakers

used in portable systems, whether integral or external, have

little ability to reproduce signals below 100 Hz to 150 Hz. In

this case, using a large input and output capacitor may not

increase system performance. In most cases, choosing a

small value of C

I in the range of 0.1 µF to 0.33 µF), along

with C

B equal to 1.0 µF should produce a virtually clickless

and popless turn-on. In cases where C

is larger than

0.33 µF, it may be advantageous to increase the value of C

Again, it should be understood that increasing the value of

B will reduce the “clicks and pops” at the expense of a

longer device turn-on time.

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