AN1500A South African Micro-Electronic Systems, AN1500A Datasheet - Page 19

• Manufacturer Part Number: AN1500A
• Description: Single Chip Telephone Demo Board
• Manufacturer: South African Micro-Electronic Systems

16.6 Current Limiting
Principle: the line current generates a voltage drop across R6. If this voltage drop is >0.7V the Q6 switches on
effectively shutting off Q9. The 2W resistor R4 is used to dissipate the power generated as a result of the current
through and voltage across C-E of Q9.
16.7 Overvoltage protection of the PCB:
Additionally to the protection measures described in pt. 16.4 and 16.5 it is highly advised to install an additional
surge protection device directly at the a- and b- terminals.
The maximum C-E breakdown voltages of the line and driver transistors are:
The clamping voltage of a 150V-varistor can be up to 400V at 5 Amp. clamping current. In other words, it may not
be able to protect the line transistor at very high surge spikes.
19/27
AN1500A
transistor type (line)
BSS 92,BSP92
MPS-A 92
2 SA 1209
2 SA 1210
1000k
R10
TP2
V
sames
sames
sames
sames
15k
R3
CEO
Fig. 9 : Example of current limiting circuit
QMPSA43
Q3
,V
I
shutoff
200V
300V
160V
200V
(BR)DSS
= 0.7V / R6
transistor type (driver)
MPS-A42, KSP42
MPS-A43,KSP43
2N5551
10V
300V
200V
160V
V
CEO