11

PBL 385 41

Power supplies DC1, DC2,

V+C and VPA

(See fig.18)

PBL 38541 generates its own DC

supply V+C dependent of line current with

an internal shunt regulator. This regulator

senses the line voltage VL via R3 and line

current via R6 in order to set the correct

V+C so the circuit can generate the required

DC characteristic for a given line resistance

RLine 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

+

1

Hook switch

ƒ

Lim

+

V

Tx

+

-

3

2

4

+Line

+

+

-Line

C1

R3

R6

µF

0-470

Ω

1-10M

Ref.

1.2V

µF

+

a.

b.

a. Supply arrangement for a handsfree system power amplifier. For ex. PBL 388 13

b. Supply arrangement for a call monitor cicuit.

+

-

15k

15k

8

9

7

14

6V

6V

PBL 385 41

R17

3.7V

internal quick start circuit by locking itself

into active state. (max. permissible current

drain 700

µA)

Care has to be taken when deci-

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

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.

The circuit has further two tempera-

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

is recommended in for this output.The

output current is given to be 2 mA in the

specification.

DC1 is a 3.7 V (typ.) supply whose,

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.

The fourth DC-supply VPA 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

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 VPA or like in alt. b

in fig. 18.

Figure 18. DC - supply system for external loads.

DTMF input.

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.