TDA7374B
then an LC filter must be inserted between the
supply and pins 3 and 13, in order to assure that
the pulses at pins 3 and 13 will be held within the
limits shown.
Offset Control
The quiescent output voltage must be as close as
possible to its nominal value, so that less undis-
torted power would be available.
For this reason an input bias current compensa-
tion is implemented to riduce the voltage drop
across the input resistors, which appears ampli-
fied at the outputs.
A suggestedLC network is shown in fig. 15.
With this network, a train of pulses with amplitude
up to 120V and width of 2ms can be applied at
point A. This type of protection is ON when the
supply voltage (pulse or DC) exceeds 18V. For
this reason the maximum operating supply volt-
age is 18V.
Gain Internally Fixed to 26dB
Advantagesof this design choice are in terms of:
componentsand space saving
output noise, supply voltage rejection and dis-
tortion optimization.
Figure 15
Silent Turn On/Off and Muting/Stand-by Func-
tion
The stand-bycan be easily activated by means of
a CMOS level applied to pin 7 through a RC filter.
Under stand-by condition the device is turned off
completely (supply current= 1 µA TYP ; output at-
tenuation=90 dB TYP).
Polarity Inversion
Figure 16
Every ON/OFF operation is virtually pop free.
Furthermore, at turn-on the device stays in muting
condition for a time determined by the value as-
signed to the SVR capacitor (T= Csvr * 7,000).
While in muting the device outputs becomes in-
sensitive to any kinds of signal that may be pre-
sent at the input terminals. In other words every
transient coming from previous stages produces
no unpleasantacoustic effect to the speakers.
Another situation under which the device is totally
muted is whenever the supply voltage drops
lower than 7V. This is helpful to pop suppression
during the turn-off by battery switch.
BUILT-IN PROTECTION SYSTEMS
Full Protection of Device and Loudspeakers
Against AC/DC Short Circuits (to Gnd, to Vs,
across the Speakers).
Reliable and safe operation in presence of all
kinds of short circuit involving the outputs is as-
sured by a built-in protection system that operates
in the following way:
High current (up to 10A) can be handled by the
device with no damage for a longer period than
the blow-out time of a quick 2A fuse (normally
connected in series with the supply). This fea-
tures is added to avoid destruction,if during fitting
to the car, a mistake on the connection of the
supply is made.
In case of overload, a SCR is activated as soon
as the current flowing through the output transis-
tors overcomes a preset threshold value depend-
ing on the chip temperature. The SCR causes an
interruption of the supply current of the power
transistor.
Open Ground
When the radio is in the ON condition and the
ground is accidentally opened, a standard audio
amplifier will be damaged. On the TDA7374B pro-
tection diodes are included to avoid any damage.
Load Dump Voltage Surge
The TDA 7374 has a circuit which enables it to
withstand a voltage pulse train on pins 3 and 13,
of the type shown in fig. 16.
Inductive Load
A protection diode is provided to allow use of the
TDA7374B with inductive loads.
If the supply voltage peaks to more than 50V,
8/13
STMICROELECTRONICS [ ST ]
