PBL 385 41
is recommended in for this output.The
output current is given to be 2 mA in the
specification.
internal quick start circuit by locking itself
intoactivestate. (max. permissiblecurrent
drain 700µA)
DTMF input.
DC1 is a 3.7 V (typ.) supply whose,
Care has to be taken when deci-
The circuit provides a specific input at pin
10 for DTMF dialling signals. The output of
the amplifier is controlled by the mute
signal provided by the dialler. This input
makes it easy to time the DTMF signals
entering the line. Most of the diallers wake
up at pressing of a key and the output will
not be stable imidiately. This unstable
state will be of some length and is limited
in some telephone specifications.
for the design required maximum output
current can be set at the control input pin
7. The set current will flow constant
(necessary to keep the line current cons-
tant) used or not used by a function, so
care has to be taken when setting the
current limit so it won´t be unnecessary
high.Themaximumcurrentthatcanbeset
anddrawnbymaintainingthevoltagelevel
is 2.0 mA (up to 6 mA can be set if the line
current so allows). This supply is ment to
beused tosupplymicrophones,Ericsson´s
handsfree IC’s switching parts (see
applications), opto couplers etc. and in
payphones its auxiliary functions.
ding the resistance value of R3. 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
to the impedance specification towards
the line. The speech circuit sinks ≈ 1mA
into pin 4, 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
Power supplies DC1, DC2,
V
+C
and V
PA
(See fig.18)
The fourth DC-supply V
PBL 38541 generates its own DC in fact it is the working voltage at pin 4 that
supply V dependent of line current with has become too low. It is obvious that this
PA has an
advantage that it does not influence the
circuits DC characteristics even at high
current drain. The supply has a floating
ground reference in the +line in order to
minimize RFI problems and is used to
supply the power amplifier of a handsfree
+C
an internal shunt regulator. This regulator problem is also connected into what kind
senses the line voltage V via R and line of DC-characteristic is set. See fig. 8.
L
3
current via R in order to set the correct
V
The circuit has further two tempera-
6
sothecircuitcangeneratetherequired ture and line current compensated DC
+C
DCcharacteristicforagivenlineresistance supplies DC1 and DC2. DC2 is a voltage
and the line feeding data of the supply for supplying diallers, can be used
These
R
telephone ( PBL3881, 38813 ).
Line
exchange. A decoupling capacitor is for memory back up because it does not
needed betweenpins+Cand -L. TheV leakanycurrentbackintothecircuit.Typical
circuits have a current controlled charging
of the supply capacitor and the control
signal is taken across the resistor R6. In
case a monitor amplifier is required where
the ground reference is hardly necessary,
+C
supply changes its voltage linearly with the voltage 2.4V down to line voltage of 4.1V,
line current. It can be used to feed an in case the line voltage is lower than 4.1V
electret microphone. Caution must be ta- calculate ; actual line voltage minus 1.9V.
ken though not to drain too much current In order to prevent noise entering the line,
out of this output because it will affect the aseriesresistorandareservoircapaciotor
it can be supplied from V
in fig. 18.
or like in alt. b
PA
Hook switch
I
L
+Line
1
R3
RLine
+
PBL 385 41
1-10M
VPA
7
Ref.
1.2V
V
R17
4
V+C
9
+
+
Tx
-
+
V
DC1
VL
3.7V
b.
0-470Ω
-
8
V
DC2
15k
RFeed
6V
6V
Lim
V
Mon.
ƒ
15k
14
a.
+
+
+
3
4.7-47
µF
2
4.7-47
µF
Vexh.
+
+
C1
R6
-Line
a. Supply arrangement for a handsfree system power amplifier. For ex. PBL 388 13
b. Supply arrangement for a call monitor cicuit.
Figure 18. DC - supply system for external loads.
11
ERICSSON [ ERICSSON ]
