AN1500A South African Micro-Electronic Systems, AN1500A Datasheet - Page 18

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

16.3 Bipolar darlington transistor
Schematic
16.4 VMOS-FET with surge protection
Schematic
Principle: the source voltage is limited to 10V (D6; see schematic) in off-hook state . High voltage peaks induced
at line will generate a high positive voltage drop across R3, lifting the gate voltage over the source voltage and thus
cause the (P-channel) Transistor to shut off.
16.5 VMOS-FET with overcurrent protection
Schematic
Principle: the line current generates a voltage drop across R3. If this voltage drop is >0.7V, C-E of Q5 is on and
shuts Q1 off by short circuiting its Gate/Source voltage.
The shutoff current is calculated by
18/27
AN1500A
(surge protection = R3 =20 may also be used with bipolar hook transistor options)
(overcurrent protection =R3,Q5 may also be used with bipolar hook transistor options)
sames
sames
sames
sames
I
shutoff
low cost
powerless switching
low on-resistance
low DC-mask
excellent surge protection
gate resistor (R5) does not affect
AC impedance
powerless switching
low on-resistance
lowest DC-mask
adjustable overcurrent protection
gate resistor (R5) does not affect
AC impedance
= 0.7V / R3 ;
+
+
+
installed = 0.7V/3.9 = 180mA
high DC mask at low currents
may be tight to spec limits for
some PTTs
2 transistors
10k
AC impedance
MOS handling required
higher cost than bipolars
few vendors
MOS handling required
higher cost than bipolars
few vendors
extra (low cost) transistor
required
base resistor (R4) affects
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