Systems and Methods for Sensing Current Through a Low-Side Field Effect Transistor

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 a produced voltage proportional to 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.

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 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;
• 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.

A method comprising:

• generating, by a voltage source, a voltage that is proportional to 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.