2002 Jan 14
27
Philips Semiconductors
Preliminary specification
4
×
44 W into 4
Ω
or 4
×
75 W into 2
Ω
quad BTL car radio power amplifier
TDA8591J
14.1
Special attention for SMD input capacitors
When SMD capacitors are used as input capacitors, low
frequency noise can occur due to stress on the PCB. The
SMD capacitors can operate like small microphones with
sensitivity of
1
⁄
f
. Special attention should be paid to this
issue when selecting SMD capacitors at the four inputs
(MKT capacitors are recommended).
14.2
Capacitors on outputs
The TDA8591J is optimized for a capacitor of 22 nF from
each output to ground for RF immunity and ESD. These
capacitors can be replaced by the capacitors on the
connector block.
14.3
EMC precautions
The TDA8591J has an all N-type DMOS output stage. The
main advantage of having the same type of power
transistors in the output stage is symmetrical behaviour for
positive and negative signals (sound quality).
A charge pump (DC to DC converter with capacitors only)
is used to generate a voltage above the battery voltage to
drive the high-side power. The clock frequency of the
charge pump (2.9 MHz) is chosen above the AM
frequency band. To prevent possible crosstalk in the FM
frequency band, a SIL pad can be used between the rear
of the TDA8591J and the heatsink. This SIL pad is an
electrical isolator and thermal conductor. It is advisable to
connect the power supply lines of the TDA8591J directly to
the power supply on the printed circuit board of the radio,
so that a one-point earth bonding with the tuner supply is
achieved.
The external capacitor of the charge pump (connected to
pin CP) filters and buffers the voltage generated internally.
The loop area of the capacitor connected to pins CP and
PGND2 should be kept as small as possible. For optimum
performance the capacitor used should have a good
frequency performance, for example an SMD ceramic
capacitor. See Figs 35 and 36 for a good PCB layout.
14.4
Offset detection
As shown in Fig.34, to obtain the DC offset information, an
output from each bridge is summed and filtered through
external 220 k
Ω
resistors and a 1
µ
F capacitor at
pin OFFCAP. The low frequency roll-off can be chosen
with the resistor/capacitor combination. Because of the
random phase of the DC offset voltage, the capacitor on
pin OFFCAP should not be a conventional electrolytic
capacitor as leakage current in this capacitor would cause
a shift in low frequency roll-off because of no pre-biasing.
If the offset detection is not used, pin OFFCAP can be
connected to ground, the external components (resistors
of 220 k
Ω
and 2 k
Ω
and the capacitor of 1
µ
F) are not
needed and the circuit is as shown in Fig.33.
14.5
Channel selection
The following recommendation for a four channel
application is given on the basis of the results of the
channel separation measurements and the dissipation
spread within the package:
Front-left = OUT1
Rear-left = OUT2
Rear-right = OUT3
Front-right = OUT4.
14.6
Detection of short-circuits
Table 3
Detection of short-circuits in standby, mute and operating modes.
AMPLIFIER MODE
SHORT-CIRCUIT ACROSS LOAD
SHORT-CIRCUIT TO SUPPLY
OR GROUND
Standby
no diagnosis
no diagnosis
Mute (no output signal)
the value of short-circuit that activates
diagnosis and protection depends on
the output offset voltage
no diagnosis and no active protection if
short-circuit >100
Ω
Operating (output signal present)
diagnosis and active protection if
short-circuit <0.4
Ω
no diagnosis and no active protection if
short-circuit >100
Ω
