Window Protection (Wp) - Philips TDA8950 Product Data Sheet

NXP Semiconductors

8.3.3 Window Protection (WP)

TDA8950_2
Product data sheet
When OCP is activated, the power transistors are turned off. They are turned on again
during the next switching cycle. If the output current is still greater than the OCP threshold,
they will be immediately switched off again. This switching will continue until C
discharged. The amplifier will then be switched off completely and a restart sequence
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 low.
Switching the amplifier on and off in this way will generate unwanted 'audio holes'. This
can be avoided by increasing the value of C
switch-off. C will also prevent the amplifier switching off due to transient
PROT
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 TDA8950 amplifier is fully protected
against short-circuit conditions while avoiding audio holes.
Table 4. Current limiting behavior during low output impedance conditions at different
values of C
[1]
Type V (V) V
P
TDA8950 29.5
[1] V is the supply voltage on pins VDDP1, VDDP2 and VDDA.
P
[2] OVP can be triggered by supply pumping; see
Window Protection (WP) checks the conditions at the output terminals of the power stage
and is activated:
•
During the start-up sequence, when the TDA8950 is switching from Standby to Mute.
Start-up will be interrupted If a short-circuit is detected between one of the output
terminals and pin VDDP1/VDDP2 or VSSP1/VSSP2. The TDA8950 will wait until the
short-circuit to the supply lines has been removed before resuming start-up. The short
circuit will not generate large currents because the short-circuit check is carried out
before the power stages are enabled.
•
When the amplifier is shut down completely because the OCP circuit has detected a
short circuit to one of the supply lines.
WP will be activated when the amplifier attempts to restart after 100 ms (see
Section 8.3.2). The amplifier will not start-up again until the short circuit to the supply
lines has been removed.
PROT
(mV, p-p) f (Hz) C
I
(pF)
500 20 10
1000 10
20 15
1000 15
1000 220
Section
Rev. 02 — 11 June 2009
2 150 W class-D power amplifier
(up to 220 pF) to delay amplifier
PROT
PWM output stops
PROT
Short Short
(Z = 0 ) (Z
th th
yes yes
yes yes
yes yes
yes no
no no
13.6.
TDA8950
is fully
PROT
Short
= 0.5 ) (Z = 1 )
th
[2]
OVP
no
[2]
OVP
no
no
© NXP B.V. 2009. All rights reserved.
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