PBL 385 73
PowersuppliesDCOandV
+C.
Hook
(See fig.18)
switch
1
PBL 385 73 generates its own DC
supply V dependent of line current with
+Line
PBL 385 73
+C
an internal shunt regulator. This regulator
senses the line voltage V via R and line
L
3
1.2V
current via R in order to set the correct
6
1-10M
+
V
sothecircuitcangeneratetherequired
+C
DCcharacteristicforagivenlineresistance
and the line feeding data of the
0-470Ω
7
R
-
Line
exchange. A decoupling capacitor is
needed betweenpins+Cand -L. TheV
VDC0
DC0
15k
+C
supply changes its voltage linearly with the
line current. It can be used to feed an
electret microphone. Caution must be ta-
ken though not to drain too much current
out of this output because it will affect the
internal quick start circuit by locking itself
into active state. (max. permissible current
drain 700µA)
+
4.7-47
µF
6V
11
15k
-Line
Care has to be taken when deci-
Figure 18. DC - supply for external load.
ding the resistance value of R3. See fig.6.
All resistances that are applied from +Line
to ground (-Line) will be in parallel, forming
the real impedance towards the line. This
will sometimes result in, that the ohmic
value of R3 is increased in order to comply
totheimpedancespecificationtowardsthe
line. The speech circuit sinks ≈ 1mA into
pin4,whichmeansthattheworkingvoltage
for the speech function V+ will decrease
withincreasingR3, thusstarvingintheend
the circuit of its working voltage . This
dependencyisoftenfalselytakenasasign
ofthatthecircuitdoesnotworkdowntothe
lowlinecurrentspecified,butinfactitisthe
working voltage at pin 4 that has become
too low. It is obvious that this problem is
also connected into what kind of DC-
characteristic is set. See fig. 8.
The circuit has further a temperature
and line current compensated DC supply
DC0. DC0 isavoltagesupplyforsupplying
diallers, can be used for memory back up
because it does not leak any current back
into the circuit. Typical voltage 2.4V down
to line voltage of 4.1V, in case the line
voltageislowerthan4.1Vcalculate;actual
line voltage minus 1.9V. In order to pre-
vent noise entering the line, a series
resistor and a reservoir capaciotor is
recommendedinforthisoutput.Theoutput
current given in the specification is 2 mA..
R18
4-8M
Hook
switch
1
VDD
13
R1
200Ω
C8
PBL 385 73
1µF
1µF
9
CMOS
DIALLER
MIC.
AT
AM
AR
10
+
MUTE
C9
R2
200Ω
1N4007
1N4007
14
DC-supply
+
4
7
11
6
12
5
8
3
2
DTMF
Telephone
line
C6
47nF
GND
R7
910Ω
R11
6.2k
1N4007
1N4007
R17
100Ω
C3
100nF
R4
18k
R10
62k
R14
10Ω
R3
910Ω
1
4
7
2
5
8
0
3
R8
560Ω
6
9
#
15V
R6
75Ω
R13
11k
+
C7
47µF
R5
22k
R16
R15
+
C1
47µF
R9
11k
C2
15nF
C5
100nF
*
+
C4
100µF
R12
1k
Figure 19. Typical standard DTMF dialling telephone application.
11
ERICSSON [ ERICSSON ]
