AN1500A South African Micro-Electronic Systems, AN1500A Datasheet - Page 20

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

16.8 DC-mask for various hook transistor arrangements
17 Shunt- and Ringer Transistors
Q3 is used to shunt excess line current to V
line modulation and short circuits the speech part during pulse dialing (ref. Fig.8).
The transistor must be capable of driving >100mA and have a typ. gain B >=100.
Q4 is used to switch the piezo ringer on and off, it can be any NPN single or darlington transistor capable of driving
100mA at 25V U
recommended types:
BC327-16
BCX 51-16 (SMD)
18 On-hook conditions
In on-hook state the circuit is supplied by a very small current to maintain retention of stored numbers and ringing
melody. The hook transistor is off and the HS pin (#10) is forced to zero by R8 and R9//Q2
down, only a very small current flows from line to maintain V
melody. The DC resistance of the application in this state is >5M (= the value of R1).
20/27
AN1500A
9
8
7
6
5
4
3
2
0
CE
5
.
10
Q3
15
sames
sames
sames
sames
20
I Line [mA]
SS
25
to maintain a constant voltage on LI (#27). It is also used for DTMF
hook transistor options
30
MOSFET with surge protection
single PNP (2SA1210)
bipolar darlington
MOSFET with current protection
35
DD
and thus retention of stored memories and ringing
40
45
BCV27 (SMD- Darlington)
BC517 (Darlington)
50
BC547
Q4
Fig. 9 shows a typical DC
mask graph for the optional
hook
arrangements: MOSFET with
overcurrent
MOSFET
protection, single PNP and
PNP darlington .
The
current in mA and the Y-
scale
across
terminals.
Check with your application’s
PTT require- ments to find
the arrangement that fits best
into the specification.
(see also: pt 15.2.1: setting
for low DC mask)
Fig. 11: DC mask for various
X-scale
B,E
shows
. The IC is powered
the
with
a-
the
shows
pro-tection,
transistor
and
voltage
surge
line
b-