PBL 385 71
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
intoactivestate. (max. permissiblecurrent
drain 700µA)
to the impedance specification towards
the line. The speech circuit sinks ≈ 1mA
into pin 6, which means that the working
voltage for the speech function V+ will
decreasewithincreasingR3,thusstarving
in the end the circuit of its working voltage
. This dependency is often falsely taken as
a sign of that the circuit does not work
down to the low line current specified, but
in fact it is the working voltage at pin 6 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.
Power supply V
(See fig.18)
+C.
PBL 385 71 generates its own DC
supply V dependent of line current with
+C
an internal shunt regulator. This regulator
senses the line voltage V via R and line
L
3
Care has to be taken when deci-
current via R in order to set the correct
6
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
V
sothecircuitcangeneratetherequired
+C
DCcharacteristicforagivenlineresistance
and the line feeding data of the
R
Line
exchange. A decoupling capacitor is
needed betweenpins+Cand -L. TheV
+C
supply changes its voltage linearly with the
1-5M
Hook
switch
15k
1
15
+
1
µ
F
200Ω
PBL 385 71
VDD
10
47µF
5.1V
100Ω
MIC.
AT
AM
AR
11
+
CMOS
DIALLER
1nF
1µF
1N4007
1N4007
MUTE
200
Ω
16
4,5,
12,13
+
6
14
8
9
3
2
7
DTMF
GND
Teleph
line
47nF
62k
1N4007
1N4007
6.2k
910Ω
18k
100nF
R3
910Ω
10Ω
1
4
7
2
5
8
0
3
560Ω
15V
1W
6
9
#
C2
15nF
75
Ω
11k
22k
+
C1
47µ
R2
R1
11k
F
100nF
*
+
F
100
µ
1k
Figure 18. Typical standard DTMF dialling telephone application.
11
ERICSSON [ ERICSSON ]
