CIRCUIT DESCRIPTION

INPUT STAGE

The input stage is a differential type preamplifier

stage with two independent inputs and two out-

puts in phase opposition.

It is designed for particular linearity characteristics

in order to have output amplitude large enough

(1VPP) yet maintaining low distortion.

The voltage gain of the stage is 6 dB.The possi-

bility to use the differential input allows the sys-

tem immunity to common-mode noise in case of

long wire connections (fig. 17 )

MUTE

The mute circuit (TTL compatible) acts at pream-

plifier level and disables the inputs without chang-

ing the DC voltage values. In such a way the op-

eration is fully popless.The use of a RC network

produces a soft reduction of the audio signal pro-

viding the best effect from the acoustic point of

view (fig 18).

The mute circuit is also activated during turn-

on/turn-off operations when the voltage at stand-

by pin is lower than about 2 volt

TURN-ON

The TDA7256 is fully popless at turn-on thanks to

a delay circuit which keeps the output low during

the capacitors charge transient.

The delay-time is given by the following formula:

TURN-OFF

The ground compatible structures and the choice

of a soft turn-off circuit ensure a fully popless op-

eration.

OUTPUT STAGE

It is a power stage designed in a way of being

able to drive loads up to 2 ohm in bridge configu-

ration without bootstrap capacitors (22 W with

SVR

The noises coming from the car environment are

essentially inside the bandwith from 300 Hz to 6

KHz.

The ripple rejection circuit which utilizes also the

gain capacitors C11,C9 ensures in this frequency

range a rejection typ. of 60dB.

SHORT CIRCUIT PROTECTION

The short circuit protection circuits intervene in

the following cases:

- s.c. between one output and ground

- s.c. between one output and +Vs

- s.c. between the outputs

In the first two cases they stop the current in both

the final stages,allowing also loudspeaker protec-

tion.In the last case the current is limited, thus

avoiding the load point to reach the SOA of the

output transistors.

STAND-BY

In stand-by condition the current generators are

disabled:the current drops to a very low value

(few

µA). Also this function is fully popless.

Fig. 19 shows the silent transients of turn-on and

turn-off operations through both the mute and the

stand-by pins.

Figure 18: Soft Muting

(

C10

C9

1

)

Figure 17: Balanced -Unbalanced Input

TDA7256

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