EDN Access--12.18.97 60-mA current source feeds telecomm lines

-December 18, 1997

December 18, 1997

60-mA current source feeds telecomm lines

Kurk Mathews, Linear Technology Corp, Milpitas, CA

An isolated 60-mA current source provides 120V of compliance for use in telecommunication interfaces (Figure 1). Constant-current line feed provides dc power for downstream circuitry independent of line length and provides inherent short-circuit protection. The key to isolated current sources is providing feedback and overvoltage protection without burning excess power on the transformer secondary. IC2, Q1, and associated circuitry provide these features.

An applied input voltage causes current to flow through R1 and into the load, which generates a voltage at the REF pin of IC2. IC2 adjusts the current to IC1's feedback optocoupler to maintain 2.5V across R1, which accounts for 55 mA of output current. The remaining 5 mA of constant current flows through either IC2 or D1.

Under normal operating conditions, Q1 is off. With no load on the supply, the output rises to approximately 120V. This output voltage forward-biases the base-emitter junction of Q1 and increases the current through the optocoupler, which in turn limits the open-circuit output voltage. R2 and D2 protect the feedback circuitry in the event of a short circuit.

The primary circuitry forms a simple flyback converter. The current-feedback loop's unity-gain crossover frequency is 200 Hz. Q2 provides a regulated 23V to power IC3. Figure 1 also provides details on how to build T1. (DI #2131)

Figure 1
IC2, Q1, and associated components provide isolated feedback and overvoltage protection for this 60-mA constant-current source with 120V of compliance.

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Copyright c 1997 EDN Magazine, EDN Access. EDN is a registered trademark of Reed Properties Inc, used under license. EDN is published by Cahners Publishing Company, a unit of Reed Elsevier Inc.

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