Pumping Effects - Philips TDA8950 Product Data Sheet

NXP Semiconductors

13.6 Pumping effects

TDA8950_2
Product data sheet
In the following example, a heatsink calculation is made for an 8
30 V supply:
The audio signal has a crest factor of 10 (the ratio between peak power and average
power (20 dB)); this means that the average output power is
Thus, the peak RMS output power level is the 0.5 % THD level, i.e. 170 W.
The average power is then
The dissipated power at an output power of 17 W is approximately 7 W.
When the maximum expected ambient temperature is 50 C, the total R
148 50 –
= 14 K/W
-------------------------
7
R = R + R
th(j-a) th(j-c) th(c-h)
R (thermal resistance from junction to case) = 1.1 K/W
th(j-c)
R (thermal resistance from case to heatsink) = 0.5 K/W to 1 K/W (dependent on
th(c-h)
mounting)
So the thermal resistance between heatsink and ambient temperature is:
R (thermal resistance from heatsink to ambient) = 14
th(h-a)
The derating curves for power dissipation (for several R
Figure 9. A maximum junction temperature T
maximum allowable power dissipation for a given heatsink size can be derived, or the
required heatsink size can be determined, at a required power dissipation level; see
Figure 9.
In a typical stereo single-ended configuration, the TDA8950 is supplied by a symmetrical
supply voltage (e.g. V
configuration, a 'pumping effect' can occur. During one switching interval, energy is taken
from one supply (e.g. V
(e.g. V ) and vice versa. When the voltage supply source cannot sink energy, the voltage
SS
across the output capacitors of that voltage supply source increases and the supply
voltage is pumped to higher levels. The voltage increase caused by the pumping effect
depends on:
•
Speaker impedance
•
Supply voltage
•
Audio signal frequency
•
Value of supply line decoupling capacitors
•
Source and sink currents of other channels
Pumping effects should be minimized to prevent the malfunctioning of the audio amplifier
and/or the voltage supply source. Amplifier malfunction due to the pumping effect can
trigger UVP, OVP or UBP.
1
170 W = 17 W.
10
+ R
th(h-a)
= 30 V and V = 30 V). When the amplifier is used in an SE
DD SS
), while a part of that energy is returned to the other supply line
DD
Rev. 02 — 11 June 2009
2 150 W class-D power amplifier
1
10
(1.1 + 1) = 11.9 K/W
values) are illustrated in
th(j-a)
= 150 C is taken into account. The
j
TDA8950
BTL application with a
of the peak power.
becomes
th(j-a)
© NXP B.V. 2009. All rights reserved.
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