# Patent

## Title
Low voltage CMOS driver for inductive loads

## Abstract

A low voltage Complementary Metal Oxide Semiconductor (CMOS) driver circuit for inductive loads is described. The circuit includes at least two first transistor devices, each first transistor device having a drain coupled to an inductive load, a respective second transistor device corresponding to each first transistor device having a drain coupled to a gate of the corresponding first transistor device, and a respective third transistor device corresponding to each first transistor device having a drain coupled to a source of the corresponding first transistor device. If a second transistor device corresponding to a first transistor device of the two first transistor devices is turned off, then a third transistor device corresponding to the remaining first transistor device is turned off, a gate of the first transistor device is floating and a drain-to-gate voltage at the first transistor device is reduced to below a predetermined supply voltage applied at a source of the respective second transistor device.

## Background
TECHNICAL FIELD
The invention relates generally to the field of analog electronic devices, and more particularly, to a low voltage Complementary Metal Oxide Semiconductor (CMOS) driver circuit for inductive loads.
BACKGROUND
FIG. 1A is a circuit diagram illustrating a prior art CMOS driver for inductive loads. As illustrated in FIG. 1A, a Metal Oxide Semiconductor (MOS) transistor device M1 110 has a gate coupled to an input signal 105 and a drain coupled to an inductive load L 120.
In the circuit shown in FIG. 1A, and as illustrated in the graph of FIG. 1B, at a predetermined time t0, when the input signal 105 goes to zero and the MOS transistor device 110 is turned off, the voltage at the output VOUT and the drain-to-gate voltage VDG at the transistor device M1 110 increase abruptly above the supply voltage VDD, thus reducing the reliability of the device. Among the various contemplated solutions to improve the reliability of the device by keeping the drain-to-gate voltage VDG always lower than the supply voltage VDD, FIG. 2A shows a circuit diagram, wherein two Metal Oxide Semiconductor (MOS) transistor devices M1 210 and M2 220 are coupled in series, with the drain of transistor device M2 220 coupled to an inductive load 250. The gate of transistor device M2 220 is always coupled to the supply voltage VDD, while the gate of transistor device M1 210 receives an input signal IN1 201. FIG. 2A illustrates a half bridge circuit, thus corresponding transistor devices M3 230 and M4 240 are coupled in series such that the gate of transistor device 240 receives an inverse input signal −IN2 202 and the gate of transistor device M3 230 is grounded. In the circuit shown in FIG. 2A and, as illustrated in the graph of FIG. 2B, when the input signal −IN2 goes high and the transistor device M4 240 is turned off, the output voltage VOUT goes below ground and, thus, the drain-to-gate voltage VDG2 at the transistor device M2 220 is greater than the supply voltage VDD, thus reducing the reliability of the device. Similarly, when the input signal IN1 goes low and the transistor M1 210 is turned off, the output voltage goes above VDD and, thus, the drain-to-gate voltage at the transistor device M3 230 is greater than the supply voltage VDD, again affecting the reliability of the device.
SUMMARY
A low voltage Complementary Metal Oxide Semiconductor (CMOS) driver circuit for inductive loads is described. The circuit includes at least two first transistor devices, each first transistor device having a drain coupled to an inductive load, a respective second transistor device corresponding to each first transistor device having a drain coupled to a gate of the corresponding first transistor device, and a respective third transistor device corresponding to each first transistor device having a drain coupled to a source of the corresponding first transistor device. If a second transistor device corresponding to a first transistor device of the two first transistor devices is turned off, then a third transistor device corresponding to the remaining first transistor device is turned off, a gate of the first transistor device is floating and a drain-to-gate voltage at the first transistor device is reduced to below a predetermined supply voltage applied at a source of the respective second transistor device.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a circuit diagram illustrating a prior art CMOS driver for inductive loads;
FIG. 1B is a graph illustrating the input signals, output signals, and drain-to-gate voltage parameters of the circuit shown in FIG. 1A as a function of a time parameter;
FIG. 2A is a circuit diagram illustrating an alternate prior art CMOS driver;
FIG. 2B is a graph illustrating the input signals, output signals, and drain-to-gate voltage parameters of the circuit shown in FIG. 2A as a function of a time parameter;
FIG. 3A is a circuit diagram illustrating a low voltage CMOS driver circuit for inductive loads, according to one embodiment of the invention;
FIG. 3B is a graph illustrating the input signals, output signals, and drain-to-gate voltage parameters of the circuit shown in FIG. 3A as a function of a time parameter;
FIG. 4 is a circuit diagram illustrating a low voltage CMOS driver circuit for inductive loads, according to an alternate embodiment of the invention;
FIG. 5A is a circuit diagram illustrating a low voltage CMOS driver circuit for inductive loads, according to another alternate embodiment of the invention;
FIG. 5B is a graph illustrating the input signals, output signals, and drain-to-gate voltage parameters of the circuit shown in FIG. 5A as a function of a time parameter.



DETAILED DESCRIPTION
FIG. 3A is a circuit diagram illustrating a low voltage CMOS driver circuit 300 for inductive loads, according to one embodiment of the invention. As illustrated in FIG. 3A, in one embodiment, Metal Oxide Semiconductor (MOS) transistor devices M1 310 and M2 320 are coupled in series, with the drain of transistor device M2 320 coupled to an inductive load 370. The gate of transistor device M2 320 is coupled to the drain of a MOS transistor device M5 350, which receives an input signal −IN3 303 at its gate and has a supply voltage VDD applied at its source.
Corresponding transistor devices M3 330 and M4 340 are coupled in series, such that the gate of transistor device M3 330 is coupled to the drain of a MOS transistor device M6 360, which receives an input signal IN4 304 at its gate and has the source grounded. The gate of transistor device M4 340 receives an input signal −IN2 302.
The gate of transistor device M1 310 receives an input signal IN1 301. Transistor device M2 320 works as a shield for the drain of transistor device M1 310. As illustrated in the graph of FIG. 3B, when the input signal IN4 304 goes to zero and transistor device M6 360 is turned off, then the input signal IN1 goes to zero at a time t0 and transistor device M1 310 is turned off, the gate of transistor device M3 330 is thus floating, and a drain-to-gate capacitance of M3 is pushing the gate voltage VG3 higher than ground, thus reducing the drain-to-gate voltage VDG3 at the transistor device M3 330. In a similar embodiment, when the input signal −IN3 303 goes high, transistor device M5 350 is turned off, then the input signal −IN2 goes high and transistor device M4 340 is turned off, the gate of transistor device M2 320 is thus floating, and a drain-to-gate capacitance of M2 is pushing the gate voltage of M2 320 lower than VDD, thus reducing the drain-to-gate voltage at the transistor device M2 320.
FIG. 4 is a circuit diagram illustrating a low voltage CMOS driver circuit for inductive loads, according to an alternate embodiment of the invention. As illustrated in FIG. 4, MOS transistor devices M1 410 and M2 420 are coupled in series, with the drain of transistor device M2 420 coupled to an inductive load L 480. The gate of transistor device M2 420 is coupled to the drain of a MOS transistor device M5 450, which receives an input signal −IN2 402 at its gate and has the supply voltage VDD applied at its source. The gate of transistor device M1 410 is coupled to a buffer device 470, which receives an input signal −IN1 401. Corresponding transistor devices M3 430 and M4 440 are coupled in series, such that the gate of transistor device M3 430 is coupled to the drain of a MOS transistor device M6 460, which receives the input signal −IN1 401 at its gate and has the source grounded. The gate of transistor device M4 440 is coupled to a second buffer device 475, which receives the input signal −IN2 402. In one embodiment described in connection with FIG. 3A, the presence of the buffer device 470 introduces a delay and guarantees that the transistor device M6 460 turns off prior to the transistor device M1 410. Similarly, the presence of the buffer device 475 introduces a delay and guarantees that the transistor device M5 450 turns off prior to the transistor device M4 440.
FIG. 5A is a circuit diagram illustrating a low voltage CMOS driver circuit for inductive loads, according to another alternate embodiment of the invention. As illustrated in FIG. 5A, MOS transistor devices M1 510 and M2 520 are coupled in series, with the drain of transistor device M2 520 coupled to an inductive load L 599. The gate of transistor device M2 520 is coupled to respective drains of MOS transistor devices M5 550 and M7 570. The gate of transistor device M5 550 receives an input signal −IN1 501. A supply voltage VDD is applied at the source of the transistor device M5 550. The gate of transistor device M7 570 receives an input signal IN2 502. The source of the transistor device M7 570 is coupled to the ground. Corresponding transistor devices M3 530 and M4 540 are coupled in series, with the drain of transistor device M3 530 coupled to the inductive load L 599. The gate of transistor device M3 530 is coupled to respective drains of MOS transistor devices M6 560 and M8 580. The gate of transistor device M8 580 receives the input signal −IN1 501. A supply voltage VDD is applied at the source of the transistor device M8 580. The gate of transistor device M6 560 receives the input signal IN2 502. The source of the transistor device M6 560 is coupled to the ground. An inverter device 595 is coupled to the gate of the transistor device M4 540, the inverter device 595 receiving the input signal IN2 502.
FIG. 5B is a graph illustrating the input signals, output signals, and drain-to-gate voltage parameters of the circuit shown in FIG. 5A as a function of a time parameter. In one embodiment, when the input signal −IN1 is low, transistor devices M1 510, M2 520, M5 550, and M8 580 are on, and the gate voltage VG3 at the transistor device M3 530 is equal to the supply voltage value VDD. When the input signal −IN1 goes high, then the transistor device M1 510 is turned off, the output voltage VOUT increases above the supply voltage VDD, but the gate of the transistor device M3 530 is already at the supply voltage value VDD, and, thus, the drain-to-gate voltage VDG3 at the transistor device M3 530 falls to zero and is, thus, maintained below VDD.
Similarly, when the input signal IN2 is high, transistor devices M3 530, M4 540, M6 560, and M7 570 are on, and the gate voltage at the transistor device M2 520 is grounded. When the input signal IN2 goes low, then the transistor device M4 540 is turned off, the output voltage VOUT goes below ground, but the gate of the transistor device M2 520 is already coupled to the ground, and, thus, the drain-to-gate voltage at the transistor device M2 520 falls to zero and is, thus, maintained below VDD.
It is to be understood that embodiments of this invention may be used as or to support software programs executed upon some form of processing core (such as the CPU of a computer) or otherwise implemented or realized upon or within a machine or computer readable medium. A machine readable medium includes any mechanism for storing or transmitting information in a form readable by a machine, e.g. a computer. For example, a machine readable medium includes read-only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals, e.g. carrier waves, infrared signals, digital signals, etc.; or any other type of media suitable for storing or transmitting information.
In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.

## Claims

1. A circuit comprising:
at least two first transistor devices, each first transistor device having a drain coupled to an inductive load;
a respective second transistor device corresponding to said each first transistor device having a drain coupled to a gate of said each first transistor device; and
a respective third transistor device corresponding to said each first transistor device having a drain coupled to a source of said each first transistor device;
wherein, if a second transistor device corresponding to a first transistor device of said at least two first transistor devices is turned off, then a third transistor device corresponding to a remaining first transistor device of said at least two first transistor devices is turned off, a gate of said first transistor device is floating and a drain-to-gate voltage at said first transistor device is reduced to below a predetermined supply voltage applied at a source of said respective second transistor device.

2. The circuit according to claim 1, wherein said first, second, and third transistor devices are Metal Oxide Semiconductor (MOS) devices.

3. The circuit according to claim 1, wherein each of said second and third transistor devices receives a corresponding input signal at a respective gate, the circuit further comprising a respective buffer device coupled to said gate of said each third transistor device to introduce a delay in said corresponding input signal and to guarantee that said each third transistor device turns off subsequent to said corresponding second transistor device.

4. A method comprising:
coupling a drain of each first transistor device of at least two first transistor device to an inductive load;
coupling a drain of a respective second transistor device corresponding to said each first transistor device to a gate of said each first transistor device; and
coupling a drain of a respective third transistor device corresponding to said each first transistor device to a source of said each first transistor device;
wherein, if a second transistor device corresponding to a first transistor device of said at least two first transistor devices is turned off, then a third transistor device corresponding to a remaining first transistor device of said at least two first transistor devices is turned off, a gate of said first transistor device is floating and a drain-to-gate voltage at said first transistor device is reduced to below a predetermined supply voltage applied at a source of said respective second transistor device.

5. The method according to claim 4, wherein said first, second, and third transistor devices are Metal Oxide Semiconductor (MOS) devices.

6. The method according to claim 4, further comprising:
receiving a corresponding input signal at a respective gate of each of said second and third transistor devices; and
coupling a respective buffer device to said gate of said each third transistor device to introduce a delay in said corresponding input signal and to guarantee that said each third transistor device turns off subsequent to said corresponding second transistor device.