TDA8954_1 © NXP B.V. 2009. All rights reserved.
Product data sheet Rev. 01 — 24 December 2009 12 of 46
NXP Semiconductors
TDA8954
2 × 210 W class-D power amplifier
When OCP is activated, the active power transistor is turned off and the other power
transistor is turned on to reduce the current (C
PROT
is partially discharged). Normal
operation is resumed at the next switching cycle (C
PROT
is recharged). C
PROT
is partially
discharge each time OCP is activated during a switching cycle. If the fault condition that
caused OCP to be activated persists long enough to fully discharge C
PROT
, the amplifier
will switch off completely and a restart sequence will be initiated.
After a fixed period of 100 ms, the amplifier will attempt to switch on again, but will fail if
the output current still exceeds the OCP threshold. The amplifier will continue trying to
switch on every 100 ms. The average power dissipation will be low in this situation
because the duty cycle is short.
Switching the amplifier on and off in this way will generate unwanted ‘audio holes’. This
can be avoided by increasing the value of C
PROT
(up to 220 pF) to delay amplifier
switch-off. C
PROT
will also prevent the amplifier switching off due to transient
frequency-dependent impedance drops at the speakers.
The amplifier will switch on, and remain in Operating mode, once the overcurrent
condition has been removed. OCP ensures the TDA8954 amplifier is fully protected
against short-circuit conditions while avoiding audio holes.
[1] OVP can be triggered by supply pumping; see Section 14.6.
Pin DIAG2 pin can be used to:
1. Monitor the OCP status - a pulsed signal at the switching frequency is generated on
DIAG2 when current limiting has been enabled.
2. Monitor the protection status - a pulsed signal with a minimum width of typically
100
ms will be generated on pin DIAG2 to indicate that the amplifier has been
switched off by one of the protection circuits (see
Table 5). This signal is also
generated at start-up before the amplifier output starts switching.
When a short circuit occurs between the load and the supply voltage, the current will
increase rapidly to I
ORM
, when current limiting will be activated. A pulsed signal at the
switching frequency will be transmitted on pin DIAG2 to indicate that OCP is active. If the
short circuit condition persists long enough to cause the OCP circuit to shut down the
amplifier, the DIAG2 signal will be transmitted continuously until the amplifier has started
up again and has commenced switching (see
Figure 9).
Table 4. Current limiting behavior during low output impedance conditions at different
values of C
PROT
Type V
DD
/V
SS
(V) V
I
(mV, p-p) f (Hz) C
PROT
(pF)
PWM output stops
Short
(Z
th
= 0 Ω)
Short
(Z
th
= 0.5 Ω)
Short
(Z
th
= 1 Ω)
TDA8954 +41/−41 500 20 10 yes
[1]
yes
[1]
yes
[1]
1000 10 yes no no
20 15 yes
[1]
yes
[1]
yes
[1]
1000 15 yes no no
1000 220 no no no
