Systems and methods for sensing current through a low-side field effect transistor

Patent metadata

Field	Value
Publication number	US-9735773-B2
Application number	US-201414264522-A
Country	US
Kind code	B2
Filing date	Apr 29, 2014
Priority date	Apr 29, 2014
Publication date	Aug 15, 2017
Grant date	Aug 15, 2017

Abstract

Systems and techniques detecting a reverse current are disclosed. An apparatus comprises a switching circuit coupled to a load and a reference node. The switching circuit may be capable of conducting a reverse current from the reference node to the load when a voltage at the load is lower than a voltage at the reference node. A voltage source has a first terminal coupled to the load, a second terminal configured to follow a voltage at the load, and produces a voltage proportional to a voltage drop across the switching circuit. A comparator circuit is coupled to compare a voltage at the second terminal of the voltage source to the voltage at the reference node and configured to indicate when the reverse current has a magnitude greater than a predetermined threshold.

First claim

What is claimed is:

1. An apparatus comprising:
a switching circuit coupled to a load and a reference node, the switching circuit capable of conducting a reverse current from the reference node to the load when a voltage at the load is lower than a voltage at the reference node;
a voltage source having a first terminal coupled to the load, a second terminal configured to follow a voltage at the load, and produce a voltage indicative of a voltage drop across the switching circuit; and
a comparator circuit coupled to compare a voltage at the second terminal of the voltage source to the voltage at the reference node and configured to indicate when the reverse current has a magnitude greater than a predetermined threshold.

2. The apparatus of claim 1 wherein the switching circuit comprises a transistor.

3. The apparatus of claim 2 wherein the transistor is a FET.

4. The apparatus of claim 2 wherein the voltage drop across the switching circuit is a voltage drop across a body diode of the transistor.

5. The apparatus of claim 2 wherein the transistor is a low-side transistor in motor driver circuit.

6. The apparatus of 1 wherein the voltage source comprises a sense transistor and a current source coupled to a source terminal of the sense transistor.

7. The apparatus of claim 6 wherein the current source is a digital-to-analog converter or is controlled by a digital-to-analog converter.

8. The apparatus of claim 6 wherein the produced voltage comprises a voltage drop across a body diode of the sense transistor.

9. The apparatus of claim 6 wherein a voltage at the source terminal of the transistor follows the voltage at the load.

10. The apparatus of claim 6 wherein a current conducted through the sense transistor is less than the reverse current.

11. The apparatus of claim 10 wherein the current conducted through the sense transistor is between about 1000 and about 1400 times less than the reverse current.

12. The apparatus of claim 1 wherein the produced voltage and the voltage drop across the switching circuit have substantially the same magnitude.

13. The apparatus of claim 1 wherein the comparator circuit is configured to indicate that the reverse current is flowing when the voltage at the second terminal of the voltage source is less than the voltage at the reference node.

14. The apparatus of claim 1 wherein the voltage drop across the switching circuit is a conduction threshold of the switching circuit, and the switching circuit is configured to conduct the reverse current when the voltage at the load is less than the voltage at the reference node minus the conduction threshold of the switching circuit when the switching circuit is in a non-conducting state.

15. The apparatus of claim 1 comprising at least one additional switching circuit and at least one additional voltage source.

16. The apparatus of claim 1 wherein the reference node is a ground node.

17. The apparatus of claim 1 wherein the voltage at the first terminal of the voltage source and the voltage at the second terminal of the voltage source both follow the voltage at the load.

18. The apparatus of claim 1 wherein the voltage is proportional to a voltage drop across the switching circuit.

19. A motor driver circuit comprising:
a low-side transistor coupled between a load and a reference node, the low-side transistor capable of conducting a reverse current from the reference node to the load when a voltage at the load is lower than a voltage at the reference node;
a voltage source having a first terminal coupled to the load, a second terminal configured to follow a voltage at the load, and produce a voltage indicative of a voltage drop across a body diode of the low-side transistor; and
a comparator circuit coupled to compare a voltage at the second terminal of the voltage source to the voltage at the reference node and configured to indicate when the reverse current is flowing.

20. The motor driver circuit of claim 19 wherein the load comprises a motor.

21. The motor driver circuit of claim 19 wherein the voltage source comprises a sense transistor and a current source coupled to a source terminal of the sense transistor to provide a current through the sense transistor, and the produced voltage comprises a voltage drop across a body diode of the sense transistor.

22. The motor driver circuit of claim 21 wherein the low-side transistor and the sense transistor are formed on a same substrate.

23. The motor driver circuit of claim 22 wherein the sense transistor is smaller than the low-side transistor, and the current through the second transistor is smaller than the reverse current through the low-side transistor.

24. The motor driver circuit of claim 23 wherein the current through the second transistor is between about 1000 and about 1400 times smaller than the reverse current through the low-side transistor.

25. The motor driver circuit of claim 20 wherein the current source comprises a digital-to-analog converter.

26. A motor driver circuit comprising:
a motor; and
a switched bridge for driving the motor, the switched bridge comprising:
a low-side transistor having a gate terminal coupled to a control circuit, a drain terminal coupled to at least one coil of the motor, and a source terminal coupled to a reference node;
a sense transistor having a gate terminal coupled to the gate terminal of the low-side transistor, a drain terminal coupled to the at least one coil of the motor, and a source terminal;
a current source coupled to the source terminal of the sense transistor to produce a voltage across a body diode of the sense transistor and to allow a voltage at the source terminal of the sense transistor to follow a voltage at the at least one coil of the motor, wherein the source terminal of the sense transistor is configured to produce a voltage indicative of a voltage drop across a body diode of the low-side transistor; and
a comparator circuit coupled to compare the voltage at the source terminal of the sense transistor to a voltage at the reference node and to indicate when a predetermined reverse current is flowing from the reference node to the at least one coil of the motor through the low-side transistor by indicating when the voltage at the source terminal of the sense transistor is less than the voltage at the reference node.

27. The motor driver circuit of claim 26 wherein the low-side transistor is a FET.

28. The motor driver circuit of claim 27 wherein the sense transistor is a FET that is smaller than the low-side transistor by a predetermined scale factor.

29. The motor driver of claim 26 wherein the switched bridge is an H-bridge or a half-bridge.

30. A method comprising:
generating, by a voltage source, a voltage that is indicative of a voltage drop across a switching circuit that is coupled to a load and a reference node;
allowing at least one terminal of the voltage source to follow a voltage at the load; and
indicating whether a predetermined reverse current is flowing through the switching circuit by comparing a voltage at the one terminal of the voltage source to a voltage at the reference node.

31. The method of claim 30 wherein generating the voltage comprises generating a voltage that is a substantially equal to the voltage drop across the switching circuit.

Assignees

• Allegro Microsystems Llc

Inventors

• Mcintosh James
• Christie Robert D
• Bryce Douglas
• Wilson Walter Morrison Stewart

Classifications

• H02P31/00

  Arrangements for regulating or controlling electric motors not provided for in groups H02P1/00 - H02P5/00, H02P7/00 or H02P21/00 - H02P29/00 · CPC title

• H03K17/6871 Primary

  the output circuit comprising more than one controlled field-effect transistor · CPC title

• H02P8/12

  Control or stabilisation of current · CPC title

• A61B17/1626

  Control means; Display units · CPC title

• B26B19/00

  Clippers or shavers operating with a plurality of cutting edges, e.g. hair clippers, dry shavers · CPC title