SAN3020
When driving external loads by extracting power from the available line current, care must be taken not to affect
both DC and AC impedance. The extracted voltage should be independent from line current (assuming a line
current >20mA).
The application shown will shut off the external load when going on-hook and power-up the load when going off-
hook.
The current driving capability depends on the available line current, higher line currents allow higher driving
currents. At minimum line current (20mA), the maximum load current is
≈15mA
(see pt.6 for further details).
6 Hardware configuration
Only 4 external components , shown in the dotted area (1 general purpose PNP transistor:B
min
=250, 1 general
purpose diode , 1 resistor and 1 pol. capacitor) must be added to the standard SA253x application to get a high
efficiency power extraction for external loads with the features described in pt.5 above.
When updating an existing circuit, the connection from pin #25 (= CS, SA253x) to the shunt transistor’s base (in
most applications = Q3) must be opened and a diode (D5) installed. All further components are simply added to the
circuit.
7 Circuit description
The shunt transistor (Q3) is regulating the voltage at LI ( #27) to
≈
4.5V. Consequently, the voltage at CS ( #25) is
V
LI
- V
BE
. When adding D5, the voltage drop between LI and CS is Vconst = 2x V
BE
. This constant voltage provides
the base current of the power extraction transistor Q5 via R22.
The maximum load current also depends on the gain (B= I
C
/ I
B
) of the transistor used. The transistor used should
have a B of
≥250
. Transistors with lower gain may also be used, but the maximum load current may be lower.
7.1 Minimum output voltage with high signals on line:
As shown in the diagram (pt. 9), the output voltage at high load currents will drop with high transmit or receive
signals on line, since the load is not supplied during the negative half-wave of the line signal. The diagram shows
the V
out
/I
Load
curves with no AC signal at line and with a constant AC line signal of 1V
peak
.
In practical use, an AC signal of 2V
pp
is not present continuously, so the effective output voltage will be higher. It is
also possible to buffer the voltage drop during line signal bursts by increasing the value of capacitor C19.
However, the voltage decrease with high signal bursts on line should be taken into consideration when the load
voltage must not drop below a certain value.
8 Connecting loads at V
DD
Small loads can also be connected at V
DD
directly, as long as the following rules are observed:
1. When on-hook, the load must only draw <1µA , because it is connected in parallel to the V
DD
-cap, which
maintains memory data retention in on-hook state, charged by a 5MΩ resistor . A higher load current would
discharge this capacitor and erase the memory contents.
2. The load will also be supplied in ringing mode.
3. When off-hook, the load current can be <5mA, provided it does not exceed this current at any time.
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sames
SAMES [ SAMES ]
