Switch mode power converter current sensing apparatus and method

1. A sensing apparatus comprising:

a switch mode converter circuit including a power transistor having a first power terminal connectable to a source of input voltage, a second power terminal, and a gate terminal configured to receive a pulse width modulated (PWM) signal;

a first field effect transistor connected between the first power terminal of the power transistor and a first circuit node, and comprising a first gate terminal coupled to a constant voltage source;

a second field effect transistor connected between the first circuit node and the second power terminal of the power transistor, the second field effect transistor comprising a second gate terminal coupled to the gate terminal of the power transistor, the first and second field effect transistors forming a series circuit branch in parallel with the power transistor; and

a sensing circuit coupled with the first field effect transistor, the sensing circuit configured to provide an output signal indicative of a current flowing in the power transistor based at least partially on a voltage across the first field effect transistor, the sensing circuit comprising a sense field effect transistor coupled between the first power terminal of the power transistor and a second circuit node, the sense field effect transistor having a gate terminal coupled to the first gate terminal of the first field effect transistor, and the gate terminal coupled to the constant voltage source.

2. A sensing apparatus, comprising:

a switch mode converter circuit including a power transistor having first and second power terminals and a gate terminal;

a first field effect transistor connected between a first terminal of the power transistor and a first circuit node, and comprising a first gate terminal coupled to a constant voltage source;

a second field effect transistor connected between the first circuit node and a second terminal of the power transistor, the second field effect transistor comprising a second gate terminal coupled to the gate terminal of the power transistor, the first and second field effect transistors forming a series circuit branch in parallel with the power transistor; and

a sensing circuit coupled with the first field effect transistor, the sensing circuit configured to an output signal indicative of a current flowing in the power transistor based at least partially on a voltage across the first field effect transistor, the sensing circuit comprising a sense field effect transistor coupled between the first terminal of the power transistor and a second circuit node, the sense field effect transistor having a gate terminal connected to the first gate terminal of the first field effect transistor and coupled to the constant voltage source;

a capacitance coupled between the first circuit node and the circuit ground,

wherein the sensing circuit further comprises:

an amplifier circuit, comprising:

an op amp with a first input coupled with the second circuit node and a second input coupled with the first circuit node, and

an output field effect transistor with a gate terminal connected to an output of the op amp, a source terminal connected to the second circuit node, and a drain terminal providing a current output at an amplifier output indicative of the current flowing in the power transistor; and

at least one resistance coupled between the amplifier output and a circuit ground to receive the current output from the amplifier circuit.

3. The sensing apparatus of claim 2 , comprising:

a first filter resistance connected between the first circuit node and the second input of the op amp;

a first filter capacitance connected between the second input of the op amp and the first terminal of the power transistor;

a second filter resistance connected between the second circuit node and the first input of the op amp; and

a second filter capacitance connected between the first input of the op amp and the first terminal of the power transistor.

4. The sensing apparatus of claim 2 , wherein the power transistor, the first field effect transistor, the second field effect transistor, and the sense field effect transistor are individually formed using a corresponding integer number of matched transistor units formed in a semiconductor body of an integrated circuit.

5. The sensing apparatus of claim 4 , wherein a ratio of a number of matched transistor units forming the power transistor to a number of matched transistor units forming the first field effect transistor is greater than 5.

6. The sensing apparatus of claim 2 , wherein the power transistor and the first field effect transistor are individually formed using a corresponding integer number of matched transistor units formed in a semiconductor body of an integrated circuit, and wherein a ratio of a number of matched transistor units forming the power transistor to a number of matched transistor units forming the first field effect transistor is greater than 5.

7. The sensing apparatus of claim 2 , wherein the power transistor, the first field effect transistor, and the second field effect transistor are individually formed using a corresponding integer number of matched transistor units formed in a semiconductor body of an integrated circuit.

8. The sensing apparatus of claim 7 , wherein a ratio of a number of matched transistor units forming the power transistor to a number of matched transistor units forming the first field effect transistor is greater than 5.

9. The sensing apparatus of claim 7 , wherein the switch mode converter includes a buck converter, wherein the power transistor is a high-side power FET with the first terminal coupled with a converter input and the second terminal coupled with a switch node of the switch mode converter, the integrated circuit further comprising a low-side power FET or rectifier with a first power terminal coupled with the switch node and a second power terminal coupled with the circuit ground.

10. The sensing apparatus of claim 7 , wherein the switch mode converter includes a boost converter, wherein the power transistor is a high-side power FET with the first terminal coupled with a boost converter inductor and the second terminal coupled with a boost converter output.

11. A method for sensing current flowing in a power transistor of a switch mode converter, the method comprising:

sensing a voltage across a first field effect transistor connected in series with a second field effect transistor and having a first node there between and forming a circuit branch in parallel with the power transistor, the first field effect transistor having a gate terminal driven by constant voltage, the power transistor being connected to input voltage controlled by a PWM (pulse width modulated) signal;

providing an output signal at least partially according to the sensed voltage across the first field effect transistor to indicate the current flowing in the power transistor; and

further comprising controlling a gate terminal of a sense field effect transistor connected between the first terminal of the power transistor and a second circuit node, the sense field effect transistor having a gate terminal connected to the first gate terminal of the first field effect transistor and driven by the constant voltage;

concurrently turning on the power transistor and a second field effect transistor connected in a series circuit branch with the first field effect transistor according to the pulse width modulation signal from a driver circuit of the switch mode converter; and

sensing the voltage across the first field effect transistor.

12. The method of claim 11 , comprising controlling a gate terminal of an output field effect transistor using an op amp at least partially according to the sensed voltage across the first field effect transistor to provide a current output indicative of the current flowing in the power transistor.

13. The method of claim 11 , comprising controlling a gate terminal of an output field effect transistor using an op amp at least partially according to the sensed voltage across the first field effect transistor to provide a current output indicative of the current flowing in the power transistor.

14. An integrated circuit, comprising:

a switch mode converter circuit including a power transistor having a first power terminal connectable to a source of input voltage, a second power terminal and a gate terminal configured to receive a pulse width modulated (PWM) signal;

a first field effect transistor connected between the first power terminal of the power transistor and a first circuit node, and comprising a first gate terminal coupled to a constant voltage source;

a second field effect transistor connected between the first circuit node and the second power terminal of the power transistor, the second field effect transistor comprising a second gate terminal coupled to the gate terminal of the power transistor, the first and second field effect transistors forming a series circuit branch in parallel with the power transistor; and

a sensing circuit coupled with the first field effect transistor, the sensing circuit configured to provide an output signal indicative of a current flowing in the power transistor based at least partially on a voltage across the first field effect transistor; and

a sense field effect transistor coupled between the first power terminal of the power transistor and a second circuit node, the sense field effect transistor having a gate terminal coupled to the first gate terminal of the first field effect transistor, and the gate terminal coupled to the constant voltage source.

15. The integrated circuit of claim 14 , further comprising:

an amplifier circuit, comprising:

an op amp with a first input coupled with the second circuit node and a second input coupled with the first circuit node, and

an output field effect transistor with a gate terminal connected to an output of the op amp, a source terminal connected to the second circuit node, and a drain terminal providing a current output at an amplifier output indicative of the current flowing in the power transistor; and

at least one resistance coupled between the amplifier output and a circuit ground to receive the current output from the amplifier circuit.

16. An integrated circuit, comprising:

a semiconductor body;

a switch mode converter circuit including a power transistor having first and second power terminals and a gate terminal;

a first field effect transistor connected between the first power terminal of the power transistor and a first circuit node, and comprising a first gate terminal coupled to a constant voltage source;

a second field effect transistor connected between the first circuit node and the second power terminal of the power transistor, the second field effect transistor comprising a second gate terminal coupled to the gate terminal of the power transistor, the first and second field effect transistors forming a series circuit branch in parallel with the power transistor; and

a sensing circuit providing an output signal indicative of a current flowing in the power transistor, the sensing circuit comprising:

a sense field effect transistor coupled between the first power terminal of the power transistor and a second circuit node, the sense field effect transistor having a gate terminal coupled to the first gate terminal of the first field effect transistor;

an op amp with a first input coupled with the second circuit node and a second input coupled with the first circuit node; and

an output field effect transistor with a gate terminal coupled to an output of the op amp, a source terminal coupled to the second circuit node, and a drain terminal configured to provide the output signal indicative of the current flowing in the power transistor; and

at least one resistance coupled between the output field effect transistor and a circuit ground terminal.