TDA7294S_05 Datasheet, PDF (9/16 Pages) STMicroelectronics – 100V - 100W DMOS AUDIO AMPLIFIER WITH MUTE/ST-BY

TDA7294S
In addition to the overload protection described above, the device features a thermal shutdown circuit
which initially puts the device into a muting state (@ T_j = 150°C) and then into stand-by (@ T_j = 160°C).
Full protection against electrostatic discharges on every pin is included.
4.3 Other Features
The device is provided with both stand-by and mute functions, independently driven by two CMOS logic
compatible input pins.
The circuits dedicated to the switching on and off of the amplifier have been carefully optimized to avoid
any kind of uncontrolled audible transient at the output.
The sequence that we recommend during the ON/OFF transients is shown by Figure 7.
The application of figure 5 shows the possibility of using only one command for both st-by and mute func-
tions. On both the pins, the maximum applicable range corresponds to the operating supply voltage.
5 Application Information
5.1 HIGH-EFFICIENCY
Constraints of implementing high power solutions are the power dissipation and the size of the power sup-
ply. These are both due to the low efficiency of conventional AB class amplifier approaches.
Here below (figure 9) is described a circuit proposal for a high efficiency amplifier which can be adopted
for both HI-FI and CAR-RADIO applications.
The TDA7294S is a monolithic MOS power amplifier which can be operated at 90V supply voltage (100V
with no signal applied) while delivering output currents up to ±6.5 A.
This allows the use of this device as a very high power amplifier (up to 100W as peak power with
T.H.D.=10 % and Rl = 4 Ohm); the only drawback is the power dissipation, hardly manageable in the
above power range.
The typical junction-to-case thermal resistance of the TDA7294S is 1 °C/W (max= 1.5 °C/W). To avoid
that, in worst case conditions, the chip temperature exceedes 150°C, the thermal resistance of the heat-
sink must be 0.038 °C/W (@ max ambient temperature of 50 °C).
As the above value is pratically unreachable; a high efficiency system is needed in those cases where the
continuous RMS output power is higher than 50-60 W.
The TDA7294S was designed to work also in higher efficiency way.
For this reason there are four power supply pins: two intended for the signal part and two for the power
part.
T1 and T2 are two power transistors that only operate when the output power reaches a certain threshold
(e.g. 20 W). If the output power increases, these transistors are switched on during the portion of the signal
where more output voltage swing is needed, thus "bootstrapping" the power supply pins (#13 and #15).
The current generators formed by T4, T7, zener diodes Z1, Z2 and resistors R7,R8 define the minimum
drop across the power MOS transistors of the TDA7294S. L1, L2, L3 and the snubbers C9, R1 and C10,
R2 stabilize the loops formed by the "bootstrap" circuits and the output stage of the TDA7294S.
By considering again a maximum average output power (music signal) of 20W, in case of the high effi-
ciency application, the thermal resistance value needed from the heatsink is 2.2°C/W (V_s = ±45V and Rl=
8Ohm).
All components (TDA7294S and power transistors T1 and T2) can be placed on a 1.5°C/W heatsink, with
the power darlingtons electrically insulated from the heatsink.
Since the total power dissipation is less than that of a usual class AB amplifier, additional cost savings can
be obtained while optimizing the power supply, even with a high heatsink .
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