2003 Mar 20
17
Philips Semiconductors
Objective specification
2
×
25 W class-D power amplifier
TDA8922
16.5
Heatsink requirements
In some applications it may be necessary to connect an
external heatsink to the TDA8922. The determining factor
is the 150
°
C maximum junction temperature T
j(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:
P
diss
is determined by the efficiency (
η
) of the TDA8922.
The efficiency measured in the TDA8922 as a function of
output power is given in Fig.19. The power dissipation can
be derived as function of output power (see Fig.18).
The derating curves (given for several values of the R
th(j-a)
)
are illustrated in Fig.9. A maximum junction temperature
T
j
= 150
°
C is taken into account. From Fig.9 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.
Example 1:
P
o
= 2
×
25 W into 8
Ω
T
j(max)
= 150
°
C
T
amb
= 60
°
C
P
diss(tot)
= 4.2 W (from Fig.18)
The required R
th(j-a)
= 21.4 K/W can be calculated.
The R
th(j-a)
of the TDA8922 in free air is 35 K/W; the R
th(j-c)
of the TDA8922 is 1.3 K/W, thus a heatsink of 20.1 K/W is
required for this example.
In actual applications, other factors such as the average
power dissipation with music source (as opposed to a
continuous sine wave) will determine the size of the
heatsink required.
Example 2:
P
o
= 2
×
25 W into 4
Ω
T
j(max)
= 150
°
C
T
amb
= 60
°
C
P
diss(tot)
= 5.5 W (from Fig.18)
The required R
th(j-a)
= 16.4 K/W.
The R
th(j-a)
of the TDA8922 in free air is 35 K/W; the R
th(j-c)
of the TDA8922 is 1.3 K/W, thus a heatsink of 15.1 K/W is
required for this example.
16.6
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 overcurrent protection is
included for each output power switch. When the current
flowing through any of the power switches exceeds a
defined internal threshold (e.g. in case of a short-circuit to
the supply lines or a short-circuit across the load), the
amplifier will shut down immediately and an internal timer
will be started. After a fixed time (e.g. 100 ms) the amplifier
is switched on again. If the requested output current is still
too high the amplifier will switch-off again. Thus the
amplifier will try to switch to the operating mode every
100 ms. The average dissipation will be low in this
situation because of this low duty cycle. If the overcurrent
condition is removed the amplifier will remain operating.
Because the duty cycle is low the amplifier will be switched
off for a relatively long period of time which will be noticed
as a so-called audio-hole; an audible interruption in the
output signal.
R
th(j-a)
T
j(max)
T
amb
–
P
diss
-----------------------------------
=
handbook, halfpage
0
Pdiss
(W)
30
20
10
0
20
100
Tamb (
°
C)
40
(1)
(2)
(3)
(4)
(5)
60
80
MBL469
Fig.9
Derating curves for power dissipation as a
function of maximum ambient temperature.
(1) R
th(j-a)
= 5 K/W.
(2) R
th(j-a)
= 10 K/W.
(3) R
th(j-a)
= 15 K/W.
(4) R
th(j-a)
= 20 K/W.
(5) R
th(j-a)
= 35 K/W.
