TDF8590TH datasheet(17/30 Pages) NXP

17 / 30 page

TDF8590TH_2

Product data sheet

Rev. 02 — 23 April 2007

17 of 30

NXP Semiconductors

TDF8590TH

× 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:

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

th j a

–

()

amb

–

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