Charge Pump Capacitor Selection

Use low (<100m

Ω) ESR (equivalent series resistance) ce-

ramic capacitors with an X7R dielectric for best performance.

Low ESR capacitors keep the charge pump output

impedance to a minimum, extending the headroom on the

negative supply. Higher ESR capacitors result in reduced

output power from the audio amplifiers.

Charge pump load regulation and output impedance are af-

fected by the value of the flying capacitor (C

C

charge pump output resistance. The switch-on resistance

dominates the output impedance for capacitor values above

2.2

μF.

The output ripple is affected by the value and ESR of the out-

put capacitor (C

the negative power supply. Lower ESR capacitors minimize

the output ripple and reduce the output impedance of the

charge pump.

The LM48820 charge pump design is optimized for 2.2

μF, low

ESR, ceramic, flying, and output capacitors.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input coupling capacitors (C

value capacitor can be expensive and may compromise

space efficiency in portable designs. In many cases, however,

the speakers used in portable systems, whether internal or

external, have little ability to reproduce signals below 150Hz.

Applications using speakers with this limited frequency re-

sponse reap little improvement by using high value input and

output capacitors.

Besides affecting system cost and size, the input coupling

capacitor has an effect on the LM48820's click and pop per-

formance. The magnitude of the pop is directly proportional

to the input capacitor's size. Thus, pops can be minimized by

selecting an input capacitor value that is no higher than nec-

essary to meet the desired −3dB frequency.

As shown in Figure 1, the internal input resistor, R

input capacitor, C

cutoff frequency that is found using Equation (4).

f

(4)

Also, careful consideration must be taken in selecting a cer-

tain type of capacitor to be used in the system. Different types

of capacitors (tantalum, electrolytic, ceramic) have unique

performance characteristics and may affect overall system

performance.

13

www.national.com