Philips TDA8950 User Manual download pdf (Page 19)

Preliminary data sheet Rev. 01 — 9 September 2008 19 of 39

NXP Semiconductors

TDA8950

2 × 150 W class-D power ampliﬁer

13.5 Noise

Noise should be measured using a high order low-pass ﬁlter with a cut-off frequency of

20 kHz. The standard audio band pass ﬁlters, used in audio analyzers, do not suppress

the residue of the carrier frequency sufﬁciently to ensure a reliable measurement of the

audible noise. Noise measurements should preferably be carried out using AES 17

(‘brickwall’) ﬁlters or an audio precision AUX 0025 ﬁlter, which was designed speciﬁcally

for measuring class-D switching ampliﬁers.

13.6 Heatsink requirements

In many applications it may be necessary to connect an external heatsink to the

TDA8950.

Equation 5 shows the relationship between the maximum power dissipation, before

activation of the TFB, and the total thermal resistance from junction to ambient

(5)

Power dissipation (P

diss

) is determined by the efﬁciency of the TDA8950. The efﬁciency

measured as a function of output power is given in Figure 21. The power dissipation can

be derived as a function of output power (Figure 20).

In the following example, a heatsink calculation is made for an 8 Ω BTL application with a

±35 V supply:

(1) R

th(j-a)

= 5 K/W.

(2) R

th(j-a)

= 10 K/W.

(3) R

th(j-a)

= 15 K/W.

(4) R

th(j-a)

= 20 K/W.

(5) R

th(j-a)

= 35 K/W.

Fig 9. De-rating curves for power dissipation as a function of maximum ambient

temperature

th j a–()

amb

–

diss

------------------------

(W)

amb

(°C)

(1)

(2)

(3)

(4)

(5)

0 20 10040 60 80

mbl469

1 2 ... 14 15 16 17 18 19 20 21 22 23 24 ... 38 39

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