TDA8922TH datasheet(17/36 Pages) PHILIPS

17 / 36 page

2003 Mar 20

Philips Semiconductors

Objective speciﬁcation

× 25 W class-D power ampliﬁer

TDA8922

16.5

Heatsink requirements

In some applications it may be necessary to connect an

external heatsink to the TDA8922. The determining factor

is the 150

°C maximum junction temperature Tj(max) which

cannot be exceeded. The expression below shows the

relationship between the maximum allowable power

dissipation and the total thermal resistance from junction

to ambient:

Pdiss is determined by the efficiency (η) of the TDA8922.

The efficiency measured in the TDA8922 as a function of

output power is given in Fig.19. The power dissipation can

be derived as function of output power (see Fig.18).

The derating curves (given for several values of the Rth(j-a))

are illustrated in Fig.9. A maximum junction temperature

Tj = 150 °C is taken into account. From Fig.9 the maximum

allowable power dissipation for a given heatsink size can

be derived or the required heatsink size can be determined

at a required dissipation level.

Example 1:

Po =2 × 25 W into 8 Ω

Tj(max) = 150 °C

Tamb =60 °C

Pdiss(tot) = 4.2 W (from Fig.18)

The required Rth(j-a) = 21.4 K/W can be calculated.

The Rth(j-a) of the TDA8922 in free air is 35 K/W; the Rth(j-c)

of the TDA8922 is 1.3 K/W, thus a heatsink of 20.1 K/W is

required for this example.

In actual applications, other factors such as the average

power dissipation with music source (as opposed to a

continuous sine wave) will determine the size of the

heatsink required.

Example 2:

Po =2 × 25 W into 4 Ω

Tj(max) = 150 °C

Tamb =60 °C

Pdiss(tot) = 5.5 W (from Fig.18)

The required Rth(j-a) = 16.4 K/W.

The Rth(j-a) of the TDA8922 in free air is 35 K/W; the Rth(j-c)

of the TDA8922 is 1.3 K/W, thus a heatsink of 15.1 K/W is

required for this example.

16.6

Output current limiting

To guarantee the robustness of the class-D amplifier the

maximum output current which can be delivered by the

output stage is limited. An overcurrent protection is

included for each output power switch. When the current

flowing through any of the power switches exceeds a

defined internal threshold (e.g. in case of a short-circuit to

the supply lines or a short-circuit across the load), the

amplifier will shut down immediately and an internal timer

will be started. After a fixed time (e.g. 100 ms) the amplifier

is switched on again. If the requested output current is still

too high the amplifier will switch-off again. Thus the

amplifier will try to switch to the operating mode every

100 ms. The average dissipation will be low in this

situation because of this low duty cycle. If the overcurrent

condition is removed the amplifier will remain operating.

Because the duty cycle is low the amplifier will be switched

off for a relatively long period of time which will be noticed

as a so-called audio-hole; an audible interruption in the

output signal.

th(j-a)

j(max)

amb

–

diss

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

handbook, halfpage

Pdiss

(W)

100

Tamb (°C)

(1)

(2)

(3)

(4)

(5)

MBL469

Fig.9

Derating curves for power dissipation as a

function of maximum ambient temperature.

(1) Rth(j-a) = 5 K/W.

(2) Rth(j-a) = 10 K/W.

(3) Rth(j-a) = 15 K/W.

(4) Rth(j-a) = 20 K/W.

(5) Rth(j-a) = 35 K/W.

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