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# Accurate Current Limiting for Series Regulator

IP.com Disclosure Number: IPCOM000084581D
Original Publication Date: 1975-Dec-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 2 page(s) / 36K

IBM

## Abstract

Shown above in Figs. 1 and 2 are two current-limiting circuits for a series regulator. As hereinafter described, both these circuits precisely define the current limit of the regulator.

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Accurate Current Limiting for Series Regulator

Shown above in Figs. 1 and 2 are two current-limiting circuits for a series regulator. As hereinafter described, both these circuits precisely define the current limit of the regulator.

In Fig. 1 the unregulated bias voltage is applied at nodes 10 and 12, while a control signal is applied to the base of control transistor Q1. The load is represented by resistor RL1. Resistors R1 and R3 are bias resistors. Transistor Q2 with its associated zener diode Z1 and resistor R2 perform a current-limiting function.

In normal operation the voltage across the load resistor RL1 is controlled by the control signal applied to transistor Q1. Precise limiting of current through transistor Q1 during transient conditions is accomplished by transistor Q2 stealing current from the base of transistor Q1. When not current-limiting, transistor Q2 is biased off. However, as current through transistor Q1 goes up, the voltage at the base of transistor Q2 goes up until transistor Q2 turns on and limits the current through transistor Q1. The precise point at which transistor Q2 turns on is given by the following expressions: V(BEQ1) + (I(limit))(R2) = V(Z1) + V(BEQ2). For V(BEQ1) = V(BEQ2) I(limit) = V(Z1)/R2.

Accordingly, the current limit for transistor Q1 may be precisely controlled by selecting zener diode Z1 and a precision resistor R2.

Fig. 2 shows an alternative embodiment to accomplish precise current limiting if transistors Q1 and Q2 of...