CODECs have outputs of 1V

the Audio Power Amplifier Design section for more infor-

mation on selecting the proper gain.

Input and Output Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value

input and output coupling capacitors (C

A high value capacitor can be expensive and may compro-

mise 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

the LM4809’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. Please refer to the Optimizing Click and

Pop Reduction Performance section for a more detailed

discussion on click and pop performance.

As shown in Figure 1, the input resistor, R

capacitor, C

quency that is found using Equation (3). In addition, the

output load R

high pass filter cutoff frequency defined by Equation (4).

f

πR

(3)

f

πR

(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.

Bypass Capacitor Value Selection

Besides minimizing the input capacitor size, careful consid-

eration should be paid to the value of C

connected to the BYPASS pin. Since C

fast the LM4809 settles to quiescent operation, its value is

critical when minimizing turn-on pops. The slower the

LM4809’s outputs ramp to their quiescent DC voltage (nomi-

nally 1/2 V

equal to 4.7µF along with a small value of C

0.1µF to 0.47µF), produces a click-less and pop-less shut-

down function. As discussed above, choosing C

than necessary for the desired bandwith helps minimize

clicks and pops.

OPTIMIZING CLICK AND POP REDUCTION

PERFORMANCE

The LM4809 contains circuitry that minimizes turn-on and

shutdown transients or “clicks and pop”. For this discussion,

turn-on refers to either applying the power supply voltage or

when the shutdown mode is deactivated. During turn-on, the

LM4809’s internal amplifiers are configured as unity gain

buffers. An internal current source charges up the capacitor

on the BYPASS pin in a controlled, linear manner. The gain

of the internal amplifiers remains unity until the voltage on

the BYPASS pin reaches 1/2 V

the BYPASS pin is stable, the device becomes fully opera-

tional. During device turn-on, a transient (pop) is created

from a voltage difference between the input and output of the

amplifier as the voltage on the BYPASS pin reaches 1/2 V

For this discussion, the input of the amplifier refers to the

node between R

the voltage applied to the BYPASS pin. During turn-on, the

buffer-configured amplifier output charges the input capaci-

tor, C

delays the charging time of C

difference between the input and output that results in a

transient (pop). Higher value capacitors need more time to

reach a quiescent DC voltage (usually 1/2 V

when

charged with a fixed current. Decreasing the value of C

R

-3dB frequency.

Although the BYPASS pin current cannot be modified,

changing the size of C

the magnitude of “clicks and pops”. Increasing the value of

C

presents a tradeoff: as the size of C

time increases. There is a linear relationship between the

size of C

turn-on times for various values of C

C

B

T

ON

0.1µF

80ms

0.22µF

170ms

0.33µF

270ms

0.47µF

370ms

0.68µF

490ms

1.0µF

920ms

2.2µF

1.8sec

3.3µF

2.8sec

4.7µF

3.4sec

10µF

7.7sec

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

discharged before turn-on. Rapidly switching V

allow the capacitors to fully discharge, which may cause

“clicks and pops”. In a single-ended configuration, the output

is coupled to the load by C

high value. C

Ω resistors.

Depending on the size of C

can be relatively large. To reduce transients in single-ended

mode, an external 1k

Ω–5kΩ resistor can be placed in par-

allel with the internal 20k

Ω resistor. The tradeoff for using

this resistor is increased quiescent current.

AUDIO POWER AMPLIFIER DESIGN

Design a Dual 70mW/32

Ω Audio Amplifier

Given:

Power Output

70 mW

Load Impedance

32

Ω

Input Level

1 Vrms (max)

Input Impedance

20k

Ω

Bandwidth

The design begins by specifying the minimum supply voltage

necessary to obtain the specified output power. One way to

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