Datasheet

LM4953, LM4953SDBD

www.ti.com

SNAS299E –SEPTEMBER 2005–REVISED FEBRUARY 2006

Charge Pump Capacitor Selection

Use low ESR (equivalent series resistance) (<100mΩ) ceramic 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 affected by the value of the flying capacitor (C1). A

larger valued C1 (up to 3.3uF) improves load regulation and minimizes charge pump output resistance. Beyond

3.3uF, the switch-on resistance dominates the output impedance for capacitor values above 2.2uF.

The output ripple is affected by the value and ESR of the output capacitor (C2). Larger capacitors reduce output

ripple on the negative power supply. Lower ESR capacitors minimize the output ripple and reduce the output

impedance of the charge pump.

The LM4953 charge pump design is optimized for 2.2uF, low ESR, ceramic, flying, and output capacitors.

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value input coupling capacitors (C

in Figure 1). A high

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 response reap little improvement by

using high value input and output capacitors.

Besides affecting system cost and size, C

has an effect on the LM4953's click and pop performance. 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 necessary to meet the desired −3dB frequency.

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

and the input capacitor, C

, produce a -3dB high pass filter

cutoff frequency that is found using Equation 4. Conventional headphone amplifiers require output capacitors;

Equation 4 can be used, along with the value of R

, to determine towards the value of output capacitor needed to

produce a –3dB high pass filter cutoff frequency.

i-3dB

= 1 / 2πR

(4)

Also, careful consideration must be taken in selecting a certain 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. (See the section entitled Charge Pump Capacitor Selection.)

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