NXP Semiconductors TDA8932B - Manual (Page 9)

TDA8932B_4 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 04 — 18 December 2008 9 of 48
NXP Semiconductors
TDA8932B
Class-D audio amplifier
8.4.1 Thermal Foldback (TF)
If the junction temperature of the TDA8932B exceeds the threshold level (T
j
> 140
°
C) the
gain of the amplifier is decreased gradually to a level where the combination of dissipation
(P) and the thermal resistance from junction to ambient [R
th(j-a)
] results in a junction
temperature around the threshold level.
This means that the device will not completely switch off, but remains operational at lower
output power levels. Especially with music output signals this feature enables high peak
output power while still operating without any external heat sink other than the
printed-circuit board area.
If the junction temperature still increases due to external causes, the OTP shuts down the
amplifier completely.
8.4.2 OverTemperature Protection (OTP)
If the junction temperature T
j
> 155
°
C, then the power stage will shut down immediately.
8.4.3 OverCurrent Protection (OCP)
When the loudspeaker terminals are short-circuited or if one of the demodulated outputs
of the amplifier is short-circuited to one of the supply lines, this will be detected by the
OCP.
If the output current exceeds the maximum output current (I
O(ocp)
> 4 A), this current will
be limited by the amplifier to 4 A while the amplifier outputs remain switching (the amplifier
is NOT shutdown completely). This is called current limiting.
The amplifier can distinguish between an impedance drop of the loudspeaker and a
low-ohmic short-circuit across the load or to one of the supply lines. This impedance
threshold depends on the supply voltage used:
• In case of a short-circuit across the load, the audio amplifier is switched off completely
and after approximately 100 ms it will try to restart again. If the short-circuit condition
is still present after this time, this cycle will be repeated. The average dissipation will
be low because of this low duty cycle.
• In case of a short to one of the supply lines, this will trigger the OCP and the amplifier
will be shut down. During restart the window protection will be activated. As a result
the amplifier will not start until 100 ms after the short to the supply lines is removed.
• In case of impedance drop (e.g. due to dynamic behavior of the loudspeaker) the
same protection will be activated. The maximum output current is again limited to 4 A,
but the amplifier will NOT switch off completely (thus preventing audio holes from
occurring). The result will be a clipping output signal without any artifacts.
8.4.4 Window Protection (WP)
The WP checks the PWM output voltage before switching from Sleep mode to Mute mode
(outputs switching) and is activated:
• During the start-up sequence, when pin POWERUP is switched from Sleep mode to
Mute mode. In the event of a short-circuit at one of the output terminals to V
DDP1
,
V
SSP1
, V
DDP2
or V
SSP2
the start-up procedure is interrupted and the TDA8932B waits
for open-circuit outputs. Because the check is done before enabling the power stages,
no large currents will flow in the event of a short-circuit.