TDF8599 datasheet(36/52 Pages) NXP

36 / 52 page

TDF8599_1

Product data sheet

Rev. 01 — 13 November 2008

36 of 52

NXP Semiconductors

TDF8599

Class-D power ampliﬁer with load diagnostics

14.4 Speaker conﬁguration and impedance

A ﬂat-frequency response (due to a 2nd order Butterworth ﬁlter) is obtained by changing

the low-pass ﬁlter components (LLC, CLC) based on the speaker conﬁguration and

impedance. Table 22 shows the required values.

Remark: When using a 1

Ω load impedance in Parallel mode, the outputs are shorted

after the low-pass ﬁlter switches two 2

Ω ﬁlters in parallel.

14.5 Heat sink requirements

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

TDF8599. Thermal foldback activates at Tj = 145 °C. The expression below shows the

relationship between the maximum power dissipation before activation of thermal foldback

and the total thermal resistance from junction to ambient;

(7)

Pmax is determined by the efﬁciency (η) of the TDF8599. The efﬁciency measured as a

function of output power is given in Figure 39. The power dissipation can be derived as a

function of output power (see Figure 32).

Example 1:

• Vp = 14.4 V

• Po =2 × 25 W into 4 Ω (THD = 10 % continuous)

• Tj(max) = 140 °C

• Tamb =25 °C

• Pmax = 5.8 W (from Figure 39)

• The required Rth(j-a) = 115 °C/5.8W=19K/W

The total thermal resistance Rth(j-a) consists of:

• Rth(j-c) + Rth(c-h) + Rth(h-a)

Where:

• Thermal resistance from junction to case (Rth(j-c)) = 1.1 K/W

• Thermal resistance from case to heat sink (Rth(c-h)) = 0.5 − 1 K/W (depending on

mounting)

• Thermal resistance from heat sink to ambient (Rth(h-a)) would then be

− (1.1+1)=17K/W.

If an audio signal has a crest factor of 10 (the ratio between peak power and average

power = 10 dB) then Tj will be much lower.

Table 22.

Filter components values

Load impedance (

Ω)

LLC (µH)

CLC (µF)

2.5

4.4

2.2

410

th j

–

()

j max

()

amb

–

max

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