PAM8403DR datasheet(9/12 Pages) PAM

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Maximum Gain

Mute Operation

As shown in block diagram (page 2), the

PAM8403 has two internal amplifier stages. The

first stage's gain is externally con figurable, while

the second stage's is internally fixed. The

closed-loop gain of the first stage is set by

selecting the ratio of R to R while the second

stage's gain is fixed at 2x.The output of amplifier

1 serves as the input to amplifier 2, thus the two

amplifiers

p roduce

s ignals

identical

magnitude, but different in phase by 180°.

Consequently, the differential gain for the IC is

A =20*log [2*(R /R )]

The PAM8403 sets maximum R =142k

minimum R =18k , so the maximum closed-gain

is 24dB.

The

pin is an input for controlling the

output state of the PAM8403. A logic low on this

pin disables the outputs, and a logic high on this

pin enables the outputs. This pin may be used as

a quick disable or enable of the outputs without a

volume fade. Quiescent current is listed in the

electrical characteristic table. The

pin can

be left floating due to the internal pull-up.

MUTE

Shutdown operation

Power supply decoupling

Input Capacitor (C )

Analog Reference Bypass Capacitor (C

)

In order to reduce power consumption while not

in use, the PAM8403 contains shutdown circuitry

to turn off the amplifier's bias circuitry. This

shutdown feature turns the amplifier off when

logic low is applied to the

pin. By switching

the

pin connected to GND, the PAM8403

supply current draw will be minimized in idle

mode. The

pin can be left floating due to

the internal pull-up.

The PAM8403 is a high performance CMOS

audio amplifier that requires adequate power

supply decoupling to ensure the output THD and

PSRR a s l ow as possible. Power s upply

decoupling affects low frequency response.

Optimum decoupling is achieved by using two

capacitors of different types targeting to different

types of noise on the power supply leads. For

higher frequency transients, spikes, or digital

hash on the line, a good low equivalent-series-

resistance (ESR) ceramic capacitor, typically

1.0μF, works best, placing it as close as possible

to the device V

terminal. For filtering lower-

frequency noise signals, a large capacitor of

20μF (ceramic)

or greater is recommended,

placing it near the audio power amplifier.

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 attenu ation. But in

many cases the speakers used in portable

systems, whether internal or external, have little

ability to reproduce signals below 100Hz to

150Hz. Thus, using a large input capacitor may

not increase actual system perfor mance. In this

case, input capacitor (C ) and input resistance

(R ) of the amplifier form a high-pass filter with

the corner frequency determined by equation

below,

In addition to system cost and size, click and pop

perfor mance is affected by the size of the input

coupling capacitor, C . A larger input coupling

capacitor requires more charge to reach its

quiescent DC voltage (nominally 1/2 V ). This

charge comes from the internal circuit via the

feedback and is apt to create pops upon device

enable. Thus, by minimizing the capacitor size

based on necessary low frequency response,

turn-on pops can be minimized.

The Analog Reference Bypass Capacitor (C

) is

the most critical capacitor and serves several

important functions. During start-up or recovery

from shutdown mode, C

determines the rate at

which the amplifier starts up. The second

function is to reduce noise caused by the power

supply coupling into the output drive signal. This

noise is from the internal analog reference to the

amplifier, which appears as degraded PSRR and

THD+N.

A ceramic bypass capacitor (C

) with values of

0.47μF to 1.0μF is recommended for the best

THD and noise performance. Increasing the

bypass capacitor reduces clicking and popping

noise from power on/off and entering and leaving

shutdown.

SHDN

BYP

Application Information

PAM8403

Filterless 3W Class-D Stereo Audio Amplifier

2πR C

09/2008 Rev 1.3

Power Analog Microelectronics Inc

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