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BIPOLAR IC CURRENT LIMITER

IP.com Disclosure Number: IPCOM000006135D
Original Publication Date: 1991-Apr-01
Included in the Prior Art Database: 2001-Dec-06
Document File: 1 page(s) / 56K

Publishing Venue

Motorola

Related People

Bill Martin: AUTHOR

Abstract

An IC Current Limiter scheme which eliminates the traditional voltage drop associated with the series output sense resistor is presented. The circuit allows the output voltage to swing to the supply rails, resulting in increased overall circuit efficiency, and operation at lower supply voltages.

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MOlYlROlA INC. Technical Developments Volume 12 April 1991

BIPOLAR IC CURRENT LIMITER

by Bill Martin

   An IC Current Limiter scheme which eliminates the traditional voltage drop associated with the series output sense resistor is presented. The circuit allows the output voltage to swing to the supply rails, resulting in increased overall circuit efficiency, and operation at lower supply voltages.

REFERRING TO THE SCHEMATIC DIAGRAM IN FIGURE 1

  Transistors Ql and Q2 operate as a "reverse current mirror", that is, the larger high current device 01, mirrors the load current through RL to transistor Q2 by the ratio (ICQl)/Xn, where Xn is the emitter area multiplier of transistor Ql. Therefore, ICQ2=(ICQl)/Xn. The collector current flowing in transistor Q2 in turn, develops a voltage across Rsense, which will reach the turn-on threshold of transistor Q3 when the output current through RL reaches or exceeds the desired current limit setting. The value of the sensing resistor required to establish the current limit setting is: Rsense=(VbeQ3)/ICQ2. When the desired current limit setting has been reached, Transistor Q3 turns on Transistor Q4 through

bias resistors R3 and R4. Transistor Q4 removes drive from Transistors Ql and Q2, stabilizing the control loop sense voltage at VbeQ3, and preventing output current through RL in excess of the current limit setting. The control loop consisting of Q2, Rsense, Q3, R3, R4 and Q4 may be compensated by capacitor Cc if found necess...