sames
SAN3020
APPLICATION NOTE
SINGLE CHIP TELEPHONE POWER
EXTRACTION FOR EXTERNAL LOADS
1 Scope
This application note describes a simple add-on circuit for extracting current to supply external loads. It also
includes hardware description ,schematic and Vout/Iout curves.
2 Key Features
•
•
•
Only few additional low-cost components necessary
Can drive line currents up to ILine-5mA (=15mA for 20mA line current)
No influence in AC impedance or DC mask even when load is short circuited
Table of Contents
1 SCOPE.............................................................................................................................................................................................................1
2 KEY FEATURES ..............................................................................................................................................................................................1
3 OTHER APPLICABLE DOCUMENTS AND PAPERS ......................................................................................................................................1
4 REVISION STATUS .........................................................................................................................................................................................1
5 GENERAL DESCRIPTION ...............................................................................................................................................................................1
6 HARDWARE CONFIGURATION ......................................................................................................................................................................2
7 CIRCUIT DESCRIPTION ..................................................................................................................................................................................2
8 CONNECTING LOADS AT VDD ........................................................................................................................................................................2
9 APPLICATION SCHEMATIC............................................................................................................................................................................3
10 LIABILITY AND COPYRIGHT STATEMENT ..................................................................................................................................................4
3 Other applicable documents and papers
1. Data Sheet SA2531, SA2532
2. Pin-out comparison SA2531/2
4 Revision status
SAN3020 Application Note (this document)
AN3020 Schematic
Rev. B00
5 General description
Note: all further component references apply to the schematic shown in pt.9 of this application note !
SAN3020
PDS038-SA2531/2-002
Rev. B
21-03-00
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 offhook.
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:Bmin=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
VLI - VBE. When adding D5, the voltage drop between LI and CS is Vconst = 2x VBE. 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= IC / IB) 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 Vout/ILoad curves with no AC signal at line and with a constant AC line signal of 1Vpeak.
In practical use, an AC signal of 2Vpp 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 VDD
Small loads can also be connected at VDD 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 VDD -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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9 Application schematic
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10 Liability and Copyright Statement
Disclaimer:
The information contained in this document is confidential and proprietary to South African
Micro-Electronic Systems (Pty) Ltd ("SAMES”) and may not be copied or disclosed to a third party, in whole or in
part, without the express written consent of SAMES. The information contained herein is current as of the date of
publication; however, delivery of this document shall not under any circumstances create any implication that the
information contained herein is correct as of any time subsequent to such date. SAMES does not undertake to
inform any recipient of this document of any changes in the information contained herein, and SAMES expressly
reserves the right to make changes in such information, without notification,even if such changes would render
information contained herein inaccurate or incomplete. SAMES makes no representation or warranty that any
circuit designed by reference to the information contained herein, will function without errors and as intended by the
designer.
South African Micro-Electronic Systems (Pty) Ltd
P O Box 15888,
Lynn East,
0039
Republic of South Africa,
33 Eland Street,
Koedoespoort Industrial Area,
Pretoria,
Republic of South Africa
Tel:
Fax:
Tel:
Fax:
012 333-6021
012 333-3158
Web Site : http://www.sames.co.za
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Int +27 12 333-6021
Int +27 12 333-3158