Philips TFA9843AJ 13.3 Thermal behavior and heatsink calculation

TFA9843AJ_1 © Koninklijke Philips Electronics N.V. 2006. All rights reserved.
Preliminary data sheet Rev. 01 — 28 April 2006 13 of 19
Philips Semiconductors TFA9843AJ
20 W stereo power amplifier with volume control
13.3 Thermal behavior and heatsink calculation
The measured maximum thermal resistance of the IC package, Rth(j-mb), is 2.0 K/W.
A calculation for the heatsink can be made, with the following parameters:
Tamb(max) =60°C (example)
VCC = 18 V and RL=4Ω (SE)
Tj(max) = 150 °C (specification)
Rth(tot) is the total thermal resistance between the junction and the ambient including the
heatsink. This can be calculated using the maximum temperature increase divided by the
power dissipation:
Rth(tot) =(T
j(max) −Tamb(max))/P
At VCC = 18 V and RL=4Ω (2 ×SE) the measured worst case sine wave dissipation is
8.4 W; see Figure 8. For Tj(max) = 150 °C the temperature raise, caused by the power
dissipation, is: 150 −60=90°C:
P×Rth(tot) =90°C
Rth(tot) = 90/8.4 K/W = 10.7 K/W
Rth(h-a) =R
th(tot) −Rth(j-mb) = 10.7 K/W −2.0 K/W = 8.7 K/W
This calculation is for an application at worst case (stereo) sine wave output signals. In
practice music signals will be applied, which decreases the maximum power dissipation to
approximately half of the sine wave power dissipation; see Section 8.2.2. This allows the
use of a smaller heatsink:
P×Rth(tot) =90°C
Rth(tot) = 90/4.2 K/W = 21.4 K/W
Rth(h-a) =R
th(tot) −Rth(j-mb) = 21.4 K/W −2.0 K/W = 19.4 K/W