參數(shù)資料
型號(hào)： TDA8924TH/N2,518
廠商： NXP SEMICONDUCTORS
元件分類(lèi)： 音頻／視頻放大
英文描述： 120 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO24
封裝： PLASTIC, SOT-566-3, HSOP-24
文件頁(yè)數(shù)： 8/35頁(yè)
文件大?。?/td> 722K
代理商： TDA8924TH/N2,518
2003 Jul 28
16
Philips Semiconductors
Objective specication
2
× 120 W class-D power amplier
TDA8924
16.5
Heatsink requirements
Although the TDA8924 is a class-D amplifier a heatsink is
required. Reason is that though efficiency is high, the
output power is high as well, resulting in heating up of the
device. The relation between temperatures, dissipation
and thermal behaviour is given below.
Pdiss is determined by the efficiency (η) of the TDA8924.
The efficiency measured in the TDA8924 as a function of
output power is given in Figs. 17 and 18. The power
dissipation can be derived as function of output power; see
Figs. 15 and 16.
The derating curves (given for several values of the Rth(j-a))
are illustrated in Fig.8. A maximum junction temperature
Tj = 150 °C is taken into account. From Fig.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.
Example:
Po =2 × 100 W into 2
Tj(max) = 150 °C
Tamb =60 °C
Pdiss(tot) = 37 W (see Fig.15).
The required Rth(j-a) = 2.43 K/W can be calculated.
The Rth(j-a) of the TDA8924 in free air is 35 K/W; the Rth(j-c)
of the TDA8924 is 1.3 K/W, thus a heatsink of 1.13 K/W is
required for this example.
This example demonstrates that one might end up with
unrealistically low Rth(j-a) figure. It has to be kept in mind
that in actual applications, other factors such as the
average power dissipation with a music source (as
opposed to a continuous sine wave) will determine the size
of the heatsink required.
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
A
–
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.8
Derating curves for power dissipation as a
function of maximum ambient temperature.
(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.
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