Browse Prior Art Database

FOLDBACK SHUTDOWN FOR VOLTAGE REGULATORS

IP.com Disclosure Number: IPCOM000008146D
Original Publication Date: 1997-Jun-01
Included in the Prior Art Database: 2002-May-22
Document File: 2 page(s) / 96K

Publishing Venue

Motorola

Related People

Petr Kadanka: AUTHOR [+2]

Abstract

The circuit described below and illustrated in the accompanying schematic (Figure 1) provides very effective foldback shutdown protection against accidental short circuits on the output of a voltage regulator or other circuit. It is described for the example of a voltage regulator. Foldback shutdown means that a short circuit on the output causes the output current to drop to a level below the satura- tion level of the regulator, i.e., it automatically turns itself off.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 50% of the total text.

Page 1 of 2

6% MOZOROLA Technical Developments

FOLDBACK SHUTDOWN FOR VOLTAGE REGULATORS

Petr Kadanka and Antonin Rozsypal

  The circuit described below and illustrated in the accompanying schematic (Figure 1) provides very effective foldback shutdown protection against accidental short circuits on the output of a voltage regulator or other circuit. It is described for the example of a voltage regulator. Foldback shutdown means that a short circuit on the output causes the output current to drop to a level below the satura- tion level of the regulator, i.e., it automatically turns itself off.

  Referring to Figure 1, the emitter of Qx is tied to the output (OUT) pin of the circuit. If there is no short circuit on the OUT pin, e.g., to GNU, then the output voltage of the regulator appears on the emitter of Qx. while the base voltage of Qx is approximately one Vbe. In this case, the voltage on the emitter of Qx is higher than the base voltage of Qx and transistor Qx is not active. There is a small voltage drop across Rx but this is ordinarily not sufficient to activate Qx into its reverse mode.

  If there is a short circuit on the OUT pin, then the voltage on the emitter of Qx drops. It is general- ly not zero because of the current flowing through the short, which is usually limited by a current lim- iting portion of the regulator (not shown). Part of the current flowing from the emitter of QS begins to flow through resistor Rx and into the collector of Qx. The voltage drop across Rx increases the voltage on the base of Qx so that Qx draws more current and cuts off transistor Q9 which feeds the base of Qout. In this way the foldback shutdown is achieved. When the short circuit across the OUT pm disappears, the voltage on the emitter of transistor Qx rises and Qx becomes inactive and the foldback shut- down terminates. There is negligible voltage drop on Rx if there is no short circuit on the output. So, operation is automatic.

  As an example, with Rl equal to about 60 kn and Rx equal to about 500 R and with a 200 mA current limit, the output curren...