TDA8922B
Philips Semiconductors
2 × 50 W class-D power amplifier
The TDA8922B amplifier can distinguish between a low-ohmic short circuit condition and
other overcurrent conditions like dynamic impedance drops of the used loudspeakers. The
impedance threshold (Zth) depends on the supply voltage used.
Depending on the impedance of the short circuit the amplifier will react as follows:
1. Short-circuit impedance > Zth:
The maximum output current of the amplifier is regulated to 5 A, but the amplifier will
not shut-down its PWM outputs. Effectively this results in a clipping output signal
across the load (behavior is very similar to voltage clipping).
2. Short-circuit impedance < Zth:
The amplifier will limit the maximum output current to 5 A and at the same time the
capacitor on the PROT pin is discharged. When the voltage across this capacitor
drops below an internal threshold voltage the amplifier will shut-down completely and
an internal timer will be started.
A typical value for the capacitor on the PROT pin is 220 pF. After a fixed time of
100 ms the amplifier is switched on again. If the requested output current is still too
high the amplifier will switch-off again. Thus the amplifier will try to switch to the
Operating mode every 100 ms. The average dissipation will be low in this situation
because of this low duty cycle. If the overcurrent condition is removed the amplifier will
remain in Operating mode once restarted.
In this way the TDA8922B amplifier is fully robust against short circuit conditions while at
the same time so-called audio holes as a result of loudspeaker impedance drops are
eliminated.
13.7 Pumping effects
In a typical stereo half-bridge (Single-Ended (SE)) application the TDA8922B class-D
amplifier is supplied by a symmetrical voltage (e.g VDD = +26 V and VSS = −26 V). When
the amplifier is used in a SE configuration, a so-called ‘pumping effect’ can occur. During
one switching interval, energy is taken from one supply (e.g. VDD), while a part of that
energy is delivered back to the other supply line (e.g. VSS) and visa versa. When the
voltage supply source cannot sink energy, the voltage across the output capacitors of that
voltage supply source will increase: the supply voltage is pumped to higher levels. The
voltage increase caused by the pumping effect depends on:
• Speaker impedance
• Supply voltage
• Audio signal frequency
• Value of decoupling capacitors on supply lines
• Source and sink currents of other channels.
The pumping effect should not cause a malfunction of either the audio amplifier and/or the
voltage supply source. For instance, this malfunction can be caused by triggering of the
undervoltage or overvoltage protection or unbalance protection of the amplifier.
Best remedy for pumping effects is to use the TDA8922B in a mono full-bridge application
or in case of stereo half-bridge application adapt the power supply (e.g. increase supply
decoupling capacitors).
9397 750 13357
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Preliminary data sheet
Rev. 01 — 1 October 2004
18 of 32
NXP [ NXP ]
