TDA7256 Datasheet, PDF (7/10 Pages) STMicroelectronics – 30W BRIDGE CAR RADIO AMPLIFIER

TDA7256
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 )
Figure 17: Balanced -Unbalanced Input
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:
To
=
800
C10
+
600
(C9
+
C11)
(
C10
C9 + C11
+
1)
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
RL=4 ohm , 30W with RL=2 ohm).
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
Figure 18: Soft Muting
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.
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