TDA7381_08 Datasheet, PDF (7/13 Pages) STMicroelectronics – 4 x 25W quad bridge car radio amplifier

TDA7381
3
Application hints
Application hints
3.1
Biasing and SVR
As shown by Figure 3, all the TDA7381’s main sections, such as Inputs, Outputs and AC-
GND (pin 16) are internally biased at half supply voltage level (Vs/2), which is derived from
the supply voltage rejection (SVR) block. In this way no current flows through the internal
feedback network.
The AC-GND is common to all the 4 amplifiers and represents the connection point of all the
inverting inputs. Both individual inputs and AC-GND are connected to Vs/2 (SVR) by means
of 100 kΩ resistors.
To ensure proper operation and high supply voltage rejection, it is of fundamental
importance to provide a good impedance matching between INPUTS and AC-GROUND
terminations.
This implies that C₁, C₂, C₃, C₄, C₅ capacitors have to carry the same nominal value and
their tolerance should never exceed ±10 %.
Besides its contribution to the ripple rejection, the SVR capacitor governs the turn ON/OFF
time sequence and, consequently, plays an essential role in the pop optimization during
ON/OFF transients. To conveniently serve both needs, its minimum recommended value
is 10 μF.
3.2
Input stage
The TDA7381’S inputs are ground-compatible and can stand very high input signals
(± 8 Vpk) without any performances degradation. If the standard value for the input
capacitors (0.1 μF) is adopted, the low frequency cut-off will amount to 16 Hz.
3.3
Stand-by and muting
Stand-by and muting facilities are both CMOS-compatible. If unused, a straight connection
to Vs of their respective pins would be admissible.
Conventional low-power transistors can be employed to drive muting and stand-by pins in
absence of true CMOS ports or microprocessors. R-C cells have always to be used in order
to smooth down the transitions for preventing any audible transient noises. Since a DC
current of about 10 μA normally flows out of pin 22, the maximum allowable muting-series
resistance (R₂) is 70 kΩ, which is sufficiently high to permit a muting capacitor reasonably
small (about 1μF). If R₂ is higher than recommended, the involved risk will be that the
voltage at pin 22 may rise to above the 1.5 V threshold voltage and the device will
consequently fail to turn OFF when the mute line is brought down.
About the stand-by, the time constant to be assigned in order to obtain a virtually pop-free
transition has to be slower than 2.5 V/ms.
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