Philips TDA1562Q Application Note page 42

Philips Semiconductors
Driving the amplifier hard at low frequencies will eventually result in the lift capacitors
being drained, so that the capacitors will no longer be able to sustain the high supply
voltage.
Fig. 29 shows what happens when the amplifier is driven so hard that the lift capacitors
can no longer sustain the high supply voltage. In this figure it is visible that the lift voltage
at both lift capacitors decreases. As a result of that the total output voltage swing also
decreases. Since the lift capacitors can no longer be recharged sufficiently, the lift
voltage will eventually be reduced to 0V.
Figures 30 to 32 show the currents in the + terminal of the lift capacitors.
In figure 30 the amplifier is lifting moderately at a signal frequency of 1kHz. The charging
of the capacitor takes place while the other channel is lifting. In this case, at an output
power of 40W the charge current (I charge) does not exceed 2.5A.
As soon as the output starts lifting, current is drawn from the lift capacitor, resulting in a
negative current (I audio) at the +terminal of the lift capacitor. The energy stored in the lift
capacitor is sufficient to sustain the voltage across the lift capacitor throughout the lifting
process.
At an output power of 55W (figure 31) the amplifier is clipping, the charge current
increases to values up to 4A and the current drawn from the lift capacitor increases too.
The lift capacitor is still capable of storing enough energy to sustain the signal during the
complete sinewave.
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Application note
Clift1 +
Fig 29. Waveforms when the lift capacitors are drained (f=40Hz, 8200µF capacitors)
Rev. 01.02 — 05 May 2006
Clift2 +
Vload
AN<nnnnn>
TDA1562Q application note
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
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