Many electronic devices, as for example DC-DC converters or devices for controlling electric current motors require accurate measurement of a current through an inductor. A very common approach for measuring a current through an inductor, as for example described in U.S. Pat. Nos. 5,731,731 B1 and 5, 420, 777 B1, uses a resistor, which is connected in series to the inductor. However, this conventional solution increases power dissipation and reduces efficiency of the electronic device.
U.S. Pat. No. 6,377,034 B1 describes an approach where sense transistors are used in order to derive a current from currents through power transistors that are equal to the inductor current during respective switching cycles of a buck converter. The circuit is shown in FIG. 1. The circuit 1 is configured to measure current supplied to the inductor 17 by a first transistor 2 and a second transistor 3. The first transistor 2 has a drain coupled to an input voltage conductor 8 (VIN), the gate coupled to the switching signal 4, and the source coupled to the first terminal of an inductor 17. A first sense transistor 7 has a gate and a drain connected to the gate and train, respectively, of the first transistor 2, and a source coupled to a first input of a first amplifier 13 and a drain of a third transistor 12 having a source coupled to a second supply voltage conductor 9, and a gate connected to the gate of a fourth transistor 15. The source of the fourth transistor 15 is connected to the second supply voltage conductor 9 and its drain is connected to an output terminal 18 and a second terminal coupled to the second supply voltage conductor 9. The first transistor 2 and second transistor 3 are included in a DC-DC converter. A second sense transistor 10 has a gate connected to the gate of the second transistor 3 and a drain connected to the drain of the second transistor 3 and to the first terminal of the inductor 17. The source of the second sense transistor 10 is coupled to the output terminal 18. The first and second transistors are operated so that one is on while the other is off, such that the current through the output terminal represents the current through the inductor regardless of which of the first and second transistors supplies the inductor current.
This prior art circuit does not use a series resistor and it allows the inductor current to be precisely determined. However, the circuit is very complex and consumes too much power itself, which adversely affects efficiency of the circuit.