TDA8922B
Philips Semiconductors
2 × 50 W class-D power amplifier
13.5 Heatsink requirements
In some applications it may be necessary to connect an external heatsink to the
TDA8922B. Limiting factor is the 150 °C maximum junction temperature Tj(max) which
cannot be exceeded. The expression below shows the relationship between the maximum
allowable power dissipation and the total thermal resistance from junction to ambient:
T
– T
amb
j(max)
R
=
-------------------------------------
th( j – a)
P
diss
Pdiss is determined by the efficiency (η) of the TDA8922B. The efficiency measured in the
TDA8922B as a function of output power is given in Figure 19. The power dissipation can
be derived as function of output power Figure 18.
The derating curves (given for several values of the Rth(j-a)) are illustrated in Figure 8. A
maximum junction temperature Tj = 150 °C is taken into account. From Figure 8 the
maximum allowable power dissipation for a given heatsink size can be derived or the
required heatsink size can be determined at a required dissipation level.
mbl469
30
P
diss
(W)
(1)
20
(2)
10
(3)
(4)
(5)
0
0
20
40
60
80
T
100
(°C)
amb
(1) Rth(j-a) = 5 K/W.
(2) Rth(j-a) = 10 K/W.
(3) Rth(j-a) = 15 K/W.
(4) Rth(j-a) = 20 K/W.
(5) Rth(j-a) = 35 K/W.
Fig 8. Derating curves for power dissipation as a function of maximum ambient
temperature.
13.6 Output current limiting
To guarantee the robustness of the class-D amplifier the maximum output current which
can be delivered by the output stage is limited. An advanced OverCurrent Protection
(OCP) is included for each output power switch.
When the current flowing through any of the power switches exceeds the defined internal
threshold of 5 A (e.g. in case of a short-circuit to the supply lines or a short-circuit across
the load) the maximum output current of the amplifier will be regulated to 5 A.
9397 750 13357
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Preliminary data sheet
Rev. 01 — 1 October 2004
17 of 32
NXP [ NXP ]
