Selection of Input Capacitor Size

Large input capacitors are both expensive and space hungry

for portable designs. Clearly, a certain sized capacitor is

needed to couple in low frequencies without severe attenua-

tion. But in many cases the speakers used in portable sys-

tems, whether internal or external, have little ability to repro-

duce signals below 150 Hz. In this case using a large input

capacitor may not increase system performance.

Besides minimizing the input capacitor size, careful consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, C

turn-on pops since it determines how fast the LM4865 turns

on. The slower the LM4865’s outputs ramp to their quiescent

DC voltage (nominally 1/2 V

Choosing C

(in the range of 0.1 µF to 0.39 µF), should produce a click-

less and popless shutdown function.Pick C

sible as to minimize clicks and pops.

Click And Pop Circuitry

The LM4865 contains circuitry to minimize turn-on and shut-

down transients or “clicks and pops”. In this case, turn-on re-

fers to either power supply turn-on or the device coming out

of shutdown mode. When the device is turning on, the ampli-

fiers are internally configured as unity gain buffers. An inter-

nal current source ramps up the voltage of the bypass pin.

Both the inputs 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

soon as the bypass node is stable, the device will become

fully operational, where the gain is set by the external volt-

age on the DC Vol/SD pin.

Although the bypass pin current source cannot be modified,

the size of C

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

value of C

ever, the tradeoff for using a larger bypass capacitor is an in-

crease in turn-on time for this device. There is a linear rela-

tionship between the size of C

are some typical turn-on times for a given C

C

B

T

ON

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 switching of VDD may

cause the

″clicks and pops″ to be not easily controlled. In a

single-ended configuration, the output coupling capacitor, C

O

, is of particular concern. This capacitor discharges through

internal 20 k

Ω resistors. Depending on the size of C

time constant can be relatively large. To reduce transients in

single-ended mode, an external 1 k

Ω–5 kΩ resistor can be

placed in parallel with the internal 20 k

Ω resistor. The

tradeoff for using this resistor is an increase in quiescent cur-

rent.

DS101025-34

FIGURE 2. Headphone Circuit

www.national.com

7