TDF8591TH datasheet(17/34 Pages) NXP

17 / 34 page

TDF8591TH_1

Product data sheet

Rev. 01 — 5 March 2008

17 of 34

NXP Semiconductors

TDF8591TH

× 100 W SE (4 Ω) or 1 × 310 W BTL (4 Ω) class-D ampliﬁer

12.5 Heat sink requirements

In some applications it may be necessary to connect an external heat sink to the

TDF8591TH. 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 (P) is determined by the efﬁciency (

η) of the TDF8591TH. The

efﬁciency measured as a function of output power is given in Figure 30 and 31. The power

dissipation can be derived as function of output power (see Figure 32 and 33).

Example of a heatsink calculation for the 4

Ω BTL application with ±18 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

⁄

10 of the peak power

• The peak RMS output power level is 110 W (0.5 % THD level)

• The average power is 0.1

× 110 W = 11 W

• The dissipated power at an output power of 11 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 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) = 9 K/W

12.6 Pumping effects

When the TDF8591TH 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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