PBL38541 Ericsson, PBL38541 Datasheet - Page 11

• Manufacturer Part Number: PBL38541
• Description: Universal Speech Circuit
• Manufacturer: Ericsson

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.
Power supplies DC1, DC2,
V +C and V PA
supply V +C dependent of line current with
an internal shunt regulator. This regulator
senses the line voltage V L via R 3 and line
current via R 6 in order to set the correct
V +C so the circuit can generate the required
DC characteristic for a given line resistance
R Line and the line feeding data of the
exchange. A decoupling capacitor is
needed between pins +C and -L. The V +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
DTMF input.
The circuit provides a specific input at pin
Figure 18. DC - supply system for external loads.
+
PBL 38541 generates its own DC
V
R
R
I
L
exh.
Line
Feed
V
L
+Line
-Line
(See fig.18)
a. Supply arrangement for a handsfree system power amplifier. For ex. PBL 388 13
b. Supply arrangement for a call monitor cicuit.
V
PA
Hook switch
a.
+
V
Mon.
b.
+
V
+
R3
+C
internal quick start circuit by locking itself
into active state. (max. permissible current
drain 700 A)
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
into pin 4, which means that the working
voltage for the speech function V+ will
decrease with increasing R3, thus starving
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 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.
ture and line current compensated DC
supplies DC1 and DC2. DC2 is a voltage
supply for supplying 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 voltage is lower than 4.1V
calculate ; actual line voltage minus 1.9V.
In order to prevent noise entering the line,
a series resistor and a reservoir capaciotor
C1
4
The circuit has further two tempera-
2
R6
Care has to be taken when deci-
ƒ
Tx
+
-
Lim
+
V
3
1
Ref.
1.2V
PBL 385 41
+
-
15k
15k
14
1mA
6V
3.7V
is recommended in for this output.The
output current is given to be 2 mA in the
specification.
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. The maximum current that can be set
and drawn by maintaining the voltage level
is 2.0 mA (up to 6 mA can be set if the line
current so allows). This supply is ment to
be used to supply microphones, Ericsson´s
handsfree IC’s switching parts (see
applications), opto couplers etc. and in
payphones its auxiliary functions.
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
telephone ( PBL3881, 38813 ). These
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,
it can be supplied from V PA or like in alt. b
in fig. 18.
6V
7
9
8
DC1 is a 3.7 V (typ.) supply whose,
The fourth DC-supply V PA has an
0-470
R17
+
1-10M
4.7-47
F
PBL 385 41
+
4.7-47
F
V
V
DC1
DC2
11