Method of limiting output current from an interface drive circuit
AssigneeDelco Electronics Corporation
InventorsScott B. Kesler, Mark W. Gose
Life of this patent
Estimating the term…
Abstract
A circuit limits the output drive current from an interface drive circuit comprising drive transistors. The circuit makes dual use of an external series supply resistor as a current sensing element. The increased voltage drop across the external resistor due to excessive current draw through the drive transistors is coupled to a control transistor, which reduces base drive current to the drive transistors in response to the increased voltage drop.
Claims4
A circuit comprising:
- a source of electric power;
- an external resistor;
- and
- an integrated circuit comprising
- (a) a first voltage supply line having a first voltage level, VS1, coupled to the source of electric power,
- (b) a second voltage supply line having a second voltage level, VS2, coupled to the source of electric power by the external resistor, the second voltage level, VS2, varying in response to a resistor current flowing through the external resistor,
- (c) an integrated circuit common ground,
- (d) a drive transistor connected to the second voltage supply line and receiving power therefrom, the drive transistor having an output current controlled by a base drive current supplied to a base of the drive transistor,
- (e) a control circuit coupled between the first voltage supply line and the integrated circuit ground, and
- (f) means within the control circuit for limiting the base drive current in response to a variation in the resistor current and resulting variation in the second voltage level VS2, said means limiting the base drive current by reducing the base drive current when VS1 - VS2 is equal to a predetermined value, V.sub.lim, wherein
- the control circuit comprises a control transistor having a collector connected to the first voltage supply line, an emitter coupled to the base of the drive transistor and a control transistor base coupled to a current source, the control transistor operable to reduce the base drive current when VS1 - VS2 equals V.sub.lim.
The apparatus set forth in claim 10, wherein the control transistor has a negative temperature coefficient and the current source has a positive temperature coefficient.
A circuit comprising:
- a source of electric power;
- an external resistor;
- and
- an integrated circuit comprising
- (a) a first voltage supply line having a first voltage level, VS1, coupled to the source of electric power,
- (b) a second voltage supply line having a second voltage level, VS2, coupled to the source of electric power by the external resistor, the second voltage level, VS2, varying in response to a resistor current flowing through the external resistor,
- (c) an integrated circuit common ground,
- (d) a drive transistor connected to the second voltage supply line and receiving power therefrom, the drive transistor having an output current controlled by a base drive current supplied to a base of the drive transistor,
- (e) a control circuit coupled between the first voltage supply line and the integrated circuit ground,
- (f) means within the control circuit for limiting the base drive current in response to a variation in the resistor current and resulting variation in the second voltage level VS2, said means limiting the base drive current by reducing the base drive current when VS1-VS2 is equal to a predetermined value, V.sub.lim ;
- (g) additional drive transistors;
- and
- (h) a set of isolation diodes coupling the control circuit to the drive transistors, one isolation diode for each drive transistor.
Dependencies are inferred from the claim text.