Philips Semiconductors
AN<nnnnn>
TDA1562Q application note
<12NC>
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Application note
Rev. 01.02 — 05 May 2006
27 of 62
Fig. 6 shows what happens when the amplifier is driven so hard that the lift capacitors
are fully discharged. This situation usually only occurs when the amplifier is driven into
hard clipping with a low frequency signal, because a low frequency signal contains
considerably more energy than a signal with a higher frequency
During the negative half of the sinewave, the charge circuit will attempt to recharge the
lift capacitor, but as long as the voltage across the lift capacitor is below 5V, the charge
current will be limited to 1A to protect the internal charge transistor. As long as the input
signal is not reduced to below 14W, the lift capacitor cannot be recharged sufficiently to
allow proper class H operation.
These pictures show that the TDA1562 can only function properly, with low distortion, as
long as sufficient energy can be stored in the lift capacitors. Two conditions are vital for
this:
1: The amount of energy drawn form the lift capacitors during lifting should preferably not
exceed the energy which the charge circuit can “put back” into the lift capacitors during
charging.
2: The signal should be “dynamic” enough to ensure that the lift capacitors can be
recharged after they have been discharged.
Condition 1 shows that it is important that the amount of energy which can be stored in
the lift capacitors should be sufficient.
When the amplifier should mainly drive low
frequency signals, the lift capacitors should be large enough to store sufficient
energy for the expected signal.
For frequencies down to 20Hz a capacitance of
22000
µ
F is recommended.
Condition 2 shows that driving the amplifier very hard with a signal with a low crest factor
will result in discharging the lift capacitors. Then, because of the low crest factor, the lift
capacitors cannot be recharged, resulting in much more distortion and a lower SPL.
Driving the amplifier into severe clipping with a noise signal with a crest factor of 2:1 will
only result in the lift capacitors being discharged and staying discharged simply because
the charge circuit will not be able to recharge the capacitors.
Testing should be carried
out with a signal with sufficient dynamics to allow the charge circuits to recharge
the lift capacitors in case they are fully discharged. Therefore a noise signal with a
crest factor of 4:1 should be used.
The situation can get even worse when two amplifiers are used to drive a dual voice coil
speaker. When the lift capacitors of one of the amplifiers are discharged, an unbalance
will arise between the two voice coils. Since the two voice coils will work as a kind of
transformer, the amplifier that is still lifting may induce voltage spikes onto the outputs of
the amplifier that is no longer lifting. This may eventually result in damage to the
amplifiers.
