Browse Prior Art Database

Short Circuit Protect for Emitter Follower

IP.com Disclosure Number: IPCOM000121814D
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 1 page(s) / 56K

Publishing Venue

IBM

Related People

Braden, JJ: AUTHOR [+5]

Abstract

A circuit for limiting the output current of an emitter-follower is disclosed below. The circuit monitors the amount of current in the emitter-follower and limits the amount of current drive to it.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 65% of the total text.

Short Circuit Protect for Emitter Follower

      A circuit for limiting the output current of an
emitter-follower is disclosed below.  The circuit monitors the amount
of current in the emitter-follower and limits the amount of current
drive to it.

      The solution is to limit the maximum amount of current the
emitter-follower can source.  Thus, the emitter-follower will
automatically current limit itself if its output pin is short
circuited to ground.  The maximum short circuit current is picked so
that no damage will be done to the device itself or to the rest of
the chip when there is a continuous short.

      The emitter-follower short circuit protect circuitry is shown
in the figure.  The emitter-follower device is Q0.  In a non-short
condition Q0 is driven by PFET T2 which has its gate connected to
ground by NFET T1.  Thus, T2 acts like a very low series base
impedance to Q0.  Therefore, Q0 will emitter-follow the voltage at
the IN pin.  R1 provides a small bias current for Q0 when there is no
load connected to the OUT pin.

      The current limit circuitry is composed of R0, T0, T1, and T2.
The circuitry is activated when current through R0 increases until
the voltage drop across R0 actively biases T0 on.  T0 will start to
supply current into NET2 in an amount proportional to the voltage
drop across R0.  T1 keeps NET2 pulled close to ground for small
amounts of current from T0.  As the current from T0 increases NET2
will rise in voltage...