NXP Semiconductors
TDF8590TH
2 × 80 W SE (4 Ω) or 1 × 160 W BTL (8 Ω) class-D amplifier
12.5 Heat sink requirements
In some applications it may be necessary to connect an external heat sink to the
TDF8590TH. The thermal foldback activates on Tj = 140 °C. The expression below shows
the relationship between the maximum power dissipation before activation of the thermal
foldback and the total thermal resistance from junction to ambient:
Rth( j–a)
=
-T----j---–----T----a---m----b
P
Ω
The power dissipation is determined by the efficiency η of the TDF8590TH. The efficiency
measured as a function of output power is given in Figure 23. The power dissipation can
be derived as function of output power (see Figure 24).
Example of a heatsink calculation for the 8 Ω BTL application with ±27 V supply:
• An audio signal with a crest factor of 10 (the ratio between peak power and average
power is 10 dB), this means that the average output power is 1/10th of the peak power
• The peak RMS output power level is 130 W (0.5 % THD level)
• The average power is 0.1 × 130 W = 13 W
• The dissipated power at an output power of 13 W is approximately 5 W
• The total Rth(j-a) = (140 − 85) / 5 = 11 K/W, if the maximum expected Tamb = 85 °C
• The total thermal resistance Rth(j-a) = Rth(j-c) + Rth(c-h) + Rth(h-a)
• Rth(j-c) = 1.1 K/W, Rth(c-h) = 0.5 K/W to 1 K/W (dependent on mounting), so Rth(h-a)
would then be: 11 − (1.1 + 1) = 8.9 K/W
12.6 Pumping effects
When the TDF8590TH is used in a SE configuration, a so-called pumping effect can
occur. During one switching interval, energy is taken from one supply (e.g. VDDA1), while a
part of that energy is delivered back to the other supply line (e.g. VSSA1) and visa versa.
When the voltage supply source cannot sink energy, the voltage across the output
capacitors of that voltage supply source will increase: 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 decoupling capacitors on supply lines
• Source and sink currents of other channels
TDF8590TH_2
Product data sheet
Rev. 02 — 23 April 2007
© NXP B.V. 2007. All rights reserved.
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