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EN 106
9.
FTP1.1E
Protections
Short-circuit Protection (e.g. Diagram A3 for "Left High")
A protection is made against a too high temperature of
transistor 7355 in case of a short-circuit of output FET 7365-1.
Transistor 7340 is sensing the current through transistor 7355
via R3355, and activates the DC-protection line (see below) in
case the current becomes too high. This is the same for all four
amplifier parts.
DC-protection (Diagram A7)
OUT_LH
OUT_LL
VCC_10_POS
3770
3771
A
3780
3781
2760
OUT_RH
OUT_RL
VCC_10_NEG
Figure 9-10 DC Protection
Because of the symmetrical supply, a DC-blocking capacitor,
between the amplifier and the speaker, is not necessary.
However, it is still necessary to protect the speaker for DC
voltages.
The following protections are therefore implemented:
•
Via R3765 and R3775, each stabilised supply voltage line
(via items 7735 and 7745) is checked on deviations.
•
Via R3770/3771/3780/3781, each amplifier output is
checked for DC-voltage.
Via R3765/3775, a virtual earth is imposed on point A. When
one of the supply voltages deviates, a DC voltage will occur on
this point. If point A is positive, T7751 will conduct. If it is
negative, T7761 will conduct.
Both cases will make T7735 conduct, so that the DC-PROT
signal will be made high. This ensures that the power supply is
rapidly trimmed back.
Capacitor C2760 will ensure that only DC-signals at point A will
activate the protection.
Circuit Descriptions, Abbreviation List, and IC Data Sheets
+9V_STBY
5753
2753
3752
7735
3751
3775
3754
7751
7761
3750
DC_PROT
7755
3765
3760
CL16532099_001.eps
200801
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