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CURRENT LIMITING CIRCUIT

Author(s): Motorola TDB 

David Soo 

Steve McKinnis 

IP.com number: IPCOM000005703D  

Original Publication Date: October 1, 1988 

IP.com Electronic Publication: October 29, 2001 

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0M MOTOROLA Technical Developments Volume 8 October 1988

CURRENT LIMITING CIRCUIT

by David Soo and Steve McKinnis

A power MOSFET current limiting circuit for monitoring the current flowing through a current sensing

field effect transistor (SENSFET), and turning off the SENSFET if the current reaches a selected maximummagnitude is shown in the figure. A SENSFET M4, is connected to drive a load L, with a gate drive circuit pro-viding the gate voltage to turn the SENSFET on and off. A portion of the load current flowing through M4 isfed back to a monitoring circuit. Since the portion of current being fed back is indicative of the full load current,it can be used to determine when the load current has exceeded a maximum allowable magnitude.

Field effect transistors Ml, M2, and M3 provide a source current for the current monitoring circuit. Bipolartransistor Q2 has an emitter area that is N times the emitter area of bipolar transistor Ql. Field effect transistorM2 is matched to field effect transistor M3 so that the current flowing in Ql is equal to the current flowingin Q2. Because the emitter area of 02 is greater than that of Ql, Ql will have a greater current density and adelta V, develops between the emitters of Ql and Q2. As a result, a reference current equal to delta V,/R is

established by the current flowing through Q2.

The portion of the load current that is fed back to the current monitoring circuit is added to the reference

current. When the portion of the load current reaches a predetermined magnitude, Ql will turn off and Q3 willturn on. The effect of 03 turning on is to pull the gate of M4 low, thereby limiting the load current. Capac itorCt and resistor RI provide stability to the feedback path.

M4

I

I

0 Motorola, Inc. 1988 14