Philips Semiconductors
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TDA1562Q application note
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© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Application note
Rev. 01.02 — 05 May 2006
23 of 62
-10º C) are expected it is advisable to use lifter capacitors with an extended
temperature range at low temperatures.
Since the lift capacitors will be charged up to a voltage nearly equal to the supply
voltage, the voltage rating of these capacitors should be equal to that of the supply
capacitors, so in an automotive application that would be 16V.
•
The Schottky diodes at pins 3 and 15:
The two Schottky diodes at pins 3 and 5 are necessary when it is expected that the
supply voltage will dip more than 1 V during loud music fragments. When this
happens, it is possible that the amplifier will temporarily mute. The Schottky diodes
will prevent that the negative terminals of the lift capacitors are pulled below
substrate level more than 0.4V, thus preventing this phenomenon.
Also during engine start situations the Schottky diodes prevent that the negative
terminals of the lift capacitors are pulled below substrate level, which may cause
plops.
•
The diagnostic LED:
The TDA1562 has a diagnostic output pin. This pin is an open collector output, so it
should be connected to an external voltage through a pull-up resistor. When the
diagnostic pin is activated the voltage at pin 8 (diagnostic) will be pulled down.
In the standard application PCB the diagnostic pin is connected to the supply voltage
through a 1k
Ω
resistor and a LED, so that when the diagnostic is activated, the LED
will light up.
•
The mode select circuit:
The mode select pin (pin 4) is connected to the supply through a switch and an RC
circuit which ensures that the voltage at the mode select pin rises gradually, ensuring
a plop-free switch-on.
•
The EMI filtering capacitors (C-emi) and Boucherot (Zobel) networks
In many applications small capacitors (1-10nF) to ground are are placed near the
loudspeaker connector. The purpose of these capacitors is to filter out any HF
interference coming into the amplifier.
The TDA1562Q is very sensitive to capacitive loads to ground, so a small capacitor
from one of the outputs to ground could cause instability.
The risk of instability exists when the value of the EMI capacitors exceeds 100pF.
When EMI filtering capacitors are used which have a higher capacitance than 100pF
stability can only be maintained by placing Boucherot (Zobel) networks close to the
output pins of the amplifier. Best results have been obtained with a combination of a
47nF capacitor and a 10
Ω
resistor.
5.2 Dimensioning of the Class H application
Since the amplifier relies on the energy stored in the lift capacitors for operation in class
H mode, the amount of energy stored in these capacitors is crucial for proper class H
operation.
Proper class H operation requires that the amplifier is able to replenish the amount of
energy that is delivered into the speaker during operation. When the amount of energy
which is drawn from the lift capacitors is higher than the amount of energy the charge
circuitry can deliver to the capacitors, the lift capacitors will be drained and the amplifier
will deliver no more output power than a conventional BTL automotive amplifier.
