TDA8922C
NXP Semiconductors
2 × 75 W class-D power amplifier
• Short-circuit impedance < Zth: the amplifier limits the maximum output current to IORM
and at the same time discharges the capacitor on pin PROT. When CPROT is fully
discharged, the amplifier shuts down completely and an internal timer is started.
The value of the protection capacitor (CPROT) connected to pin PROT can be between
10 pF and 220 pF (typically 47 pF). While OCP is activated, an internal current source is
enabled that will discharge CPROT
.
When OCP is activated, the active power transistor is turned off and the other power
transistor is turned on to reduce the current (CPROT is partially discharged). Normal
operation is resumed at the next switching cycle (CPROT is recharged). CPROT 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 CPROT, 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 CPROT (up to 220 pF) to delay amplifier
switch-off. CPROT 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 TDA8922C amplifier is fully protected
against short-circuit conditions while avoiding audio holes.
Table 4.
Type
Current limiting behavior during low output impedance conditions at different
values of CPROT
[1]
VDD/VSS VI (mV, p-p) f (Hz) CPROT (pF) PWM output stops
(V)
Short
Short
Short
(Zth = 0 Ω) (Zth = 0.5 Ω) (Zth = 1 Ω)
TDA8922C +29.5/
500
20
1000 10
20 15
10
yes
yes
yes
yes
no
yes
yes
yes
no
yes
yes
yes
no
−29.5
1000 15
1000 220
no
no
[1] Tested using three samples and an external clock.
8.3.3 Window Protection (WP)
Window Protection (WP) checks the conditions at the output terminals of the power stage
and is activated:
• During the start-up sequence, when the TDA8922C is switching from Standby to
Mute.
TDA8922C_1
© NXP B.V. 2009. All rights reserved.
Product data sheet
Rev. 01 — 7 September 2009
10 of 40
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