General structure

STA540SAN

18/26

Doc ID 18306 Rev 1

8

General structure

8.1

High application flexibility

The availability of four independent channels makes it possible to accomplish several kinds

of applications ranging from four-speaker stereo (F/R) to two-speaker bridge solutions.

When working with single-ended conditions, the polarity of the speakers driven by the

inverting amplifier must be reversed with respect to those driven by non-inverting channels.

This is to avoid phase irregularities causing sound alterations especially during the

reproduction of low frequencies.

8.2

Easy single-ended to bridge transition

The change from single-ended to bridge configurations is made simple by means of a short

circuit across the inputs (resulting in no need of additional external components).

8.3

Internally fixed gain

The gain is internally fixed to 20 dB in single-ended mode and 26 dB in bridge mode.

The advantages of this design choice are in terms of:

components and space saving

output noise, supply voltage rejection and distortion optimization.

8.4

Silent turn on/off and muting/standby functions

Standby mode can be easily activated by means of a CMOS logic level applied to pin 7

through a RC filter.

Under standby conditions, the device is turned off completely (supply current = 1 mA typical,

output attenuation = 80 dB minimum).

All on/off operations are virtually pop-free. Furthermore, at turn-on the device stays in mute

condition for a time determined by the value assigned to the SVR capacitor. In mute mode,

the device outputs are insensitive to any kind of signal that may be present at the input

terminals. In other words, any transients coming from previous stages produce no

unpleasant acoustic effects at the speakers.

8.5

Standby driving (pin 7)

Some precautions need to be taken when defining standby driving networks. Pin 7 cannot

be directly driven by a voltage source having a current capability higher than 5 mA. In

practical cases a series resistance must be inserted, giving it the double purpose of limiting

the current at pin 7 and smoothing down the standby on/off transitions. And, when done in

combination with a capacitor, prevents output pop.

A capacitor of at least 100 nF from pin 7 to S_GND, with no resistance in between, is

necessary to ensure correct turn-on.