SWITCHED GATE CURRENT DRIVER

Abstract

Embodiments of the present invention are directed to current driver circuits and methods for driving a load. The current driver circuit includes a first transistor (Q1) and a second transistor (Q2) each having a gate, a drain and a source. The drain of the second transistor (Q2) forms the output of the current driver circuit. The first and second transistors (Q1) and (Q2) function as a current mirror when the gates of the first and second transistors (Q1) and (Q2) are selectively connected together. The current driver circuit also includes a current source, a load mimic circuit, and a control loop (e.g., including an op-amp), which are configured to cause the voltage at the drain of the first transistor (Q1) to substantially equal the voltage at the output of the current driver circuit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates conventional symbols and nomenclature for PMOS and NMOS transistors.

FIG. 2 shows simplified equations governing the relationships between the applied voltages and the supplied currents for PMOS and NMOS transistors.

FIG. 3 describes exemplary sizes of NMOS and PMOS transistors.

FIG. 4 is an exemplary graph showing the relationship between laser current and laser optical power output for a laser diode.

FIG. 5 is an exemplary graph showing the relationship between laser current and laser voltage for a laser diode.

FIG. 6 is a circuit diagram including a current mirror current driver, according to the prior art.

FIGS. 7-10 are graphs that are useful for illustrating the conduction characteristics of different sized MOS devices, with different values for V_ON. In FIG. 7 the MOS device is small (beta=0.05), and V_ON ranges from 0 to 5.5V. In FIG. 8 the MOS device is large, and V_ON ranges from 0 to 1.1V. FIG. 9 corresponds to the MOS device of FIG. 7, but with the load line of a 2.9 ohm resistor to ground. FIG. 10 corresponds to the MOS device of FIG. 8, but with the load line of a 2.9 ohm resistor to ground.

FIG. 11 is a circuit diagram including a cascode switched current driver, according to the prior art.

FIG. 12 is a circuit diagram including a current mode switched gate current driver, according to an embodiment of the present invention.

FIG. 13A is a circuit diagram including a resistive mode switched gate current driver, according to an embodiment of the present invention.

FIG. 13B is a circuit diagram including a resistive mode switched gate current driver, according to an alternative embodiment of the present invention.

FIG. 14A is a circuit diagram including a multiple set point switched gate current driver, according to an embodiment of the present invention.

FIG. 14B is a circuit diagram including a multiple set point switched gate current driver, according to an embodiment of the present invention.

Claims

1. A current driver circuit for driving a load (LD), comprising: first and second transistors (Q1) and (Q2) that function as a current mirror when gates of the first and second transistors (Q1) and (Q2) are selectively connected together, with a drain of the first transistor (Q1) forming an input of the current mirror, a drain of the second transistor (Q2) forming an output of the current mirror, and the drain of the second transistor (Q2) also forming the output of the current driver circuit;a current source (Icontrol) the provides a control current;a load mimic circuit (1308) through which the control current flows, and that provides a voltage approximately equal the voltage at the output of the current driver circuit, when an output current (Iout) provided at the output of the current driver circuit is driving the load (LD); anda control loop that uses the load mimic circuit (1308) to cause the voltage at the drain of the first transistor (Q1) to approximately equal the voltage at the output of the current driver circuit, when the output current (Iout) provided at the output of the current driver circuit is driving the load (LD).

2. The current driver circuit of claim 1, wherein: each transistor includes a channel length and a channel width;the channel lengths of the first and second transistors (Q1) and (Q2) are substantially the same;the channel width of the first transistor (Q1) is less than the channel width of the second transistor (Q2), thereby resulting in a source-drain current of the first transistor (Q1) being less than a source drain current of the second transistor (Q2).

3. A current driver circuit for driving a load (LD) connected between an output of the current driver circuit and a first voltage rail (e.g., GND), comprising: a first transistor (Q1) and a second transistor (Q2) each having a gate, a drain and a source, the sources of the first and second transistors (Q1) and (Q2) connected to a second voltage rail (e.g., VDD), and the drain of the second transistor (Q2) forming the output of the current driver circuit, wherein the first and second transistors (Q1) and (Q2) function as a current mirror when the gates of the first and second transistors (Q1) and (Q2) are selectively connected together;a current source (Icontrol) and a load mimic circuit (1308) connected in series between the first voltage rail (e.g., GND) and the second voltage rail (e.g., VDD); andan op-amp (U) including first and second inputs and an output;wherein the load mimic circuit (1308) includes a first terminal, a second terminal, and a voltage source (B1) connected in series with a resistor (R) between the first and second terminals;wherein a current provided by the current source (Icontrol), which flows through the load mimic circuit, causes a voltage between the first and second terminals of the load mimic circuit (1308) to approximately equal the voltage at the output of the current driver circuit, when an output current (Iout) of the current driver circuit is driving the load (LD); andwherein the op-amp (U) and the load mimic circuit (1308) are configured to cause the voltage at the drain of the transistor (Q1) to approximately equal the voltage at the output of the current driver circuit, when the output current (Iout) provided at the output of the current driver circuit is driving the load (LD).

4. The current driver circuit of claim 3, wherein: one of the inputs of the op-amp (U) is connected to a node between the current source (Icontrol) and the load mimic circuit (1308), so that a voltage at the one of the inputs of the op-amp (U) is substantially equal to the voltage across the load (LD);the other input of the op-amp (U) is connected to the drain of the first transistor (Q1); andthe output of the op-amp (U) is connected to the gate of the first transistor (Q1).

5. The current driver circuit of claim 4, further comprising: a second resistor (R2) connected between one of the inputs of the op-amp (U) and a node between the voltage source (B1) and the resistor (R) of the load mimic circuit (1308).

6. The current driver circuit of claim 3, wherein: each transistor includes a channel length and a channel width;the channel lengths of the first and second transistors (Q1) and (Q2) are substantially the same;the channel width of the first transistor (Q1) is less than the channel width of the second transistor (Q2), thereby resulting in a source-drain current of the first transistor (Q1) being less than a source drain current of the second transistor (Q2).

7. The current driver circuit of claim 3, further comprising: a third transistor (Q6) that selectively connects the gates of the first and second transistors (Q1) and (Q2) together; anda fourth transistor (Q5) that selectively connects the gate of the second transistor (Q2) to the second voltage rail (e.g., VDD) when the third transistor (Q6) is not connecting the gates of the first and second transistors (Q1) and (Q2) together.

8. The current driver circuit of claim 7, wherein: the first transistor (Q1), the third transistor (Q6), the current source (Icontrol), the op-amp (U), the second resistor (R2) and the load mimic circuit (1308) make up a first gate voltage circuit that selectively provides a first voltage to the gate of the second transistor (Q2); andthe current driver circuit outputs a first current when the first voltage is provided to the gate of the second transistor (Q2).

9. The current driver circuit of claim 8, further comprising: a second gate voltage circuit that selectively provides a second voltage to the gate of the second transistor (Q2), where the second voltage is different than the first voltage;wherein the current driver circuit outputs a second current when the second voltage is provided to the gate of the second transistor (Q2), wherein the second current is different than the first current.

10. The current driver circuit of claim 9, wherein the first and second gate voltage circuits share at least a portion of the same load mimic circuit (1308).

11. The current driver circuit of claim 9, wherein the first and second gate voltage circuits share the same voltage source (B1).

12. The current driver circuit of claim 3, wherein the first voltage rail is ground.

13. A current driver circuit for driving a load (LD), comprising: a transistor (Q2) having a gate, a drain and a source, with the drain of the transistor (Q2) forming the output of the current driver circuit; anda plurality of gate voltage circuits, each of which selectively provides a different voltage to the gate of the transistor (Q2);wherein a magnitude of an output current produced at the output of the current driver circuit is dependent upon which of the plurality of gate voltage circuits is selected.

14. The current driver circuit of claim 13, wherein: each gate voltage circuit includes a further transistor (Q1, Q21, etc.), a the current source, an the op-amp, a resistor and a load mimic circuit (1308);when one of the gate voltage circuits is selected, the further transistor of the selected gate voltage circuit functions together with the transistor (Q2) as a current mirror; andthe magnitude of the output current produced at the output of the selected current driver circuit is dependent at least in part upon a channel width of the further transistor (Q1, Q21, etc.) and the current source of the selected gate voltage circuit.

15. The current driver circuit of claim 14, wherein each gate voltage circuit includes another transistor (Q6, Q26, etc.) that selectively connects the gate of the transistor (Q2) to the gate of the further transistor (Q1, Q21, etc.) of the selected one of the gate voltage circuits.

16. The current driver circuit of claim 15, wherein the another transistor (Q6, Q26, etc.) of each gate voltage circuit has a source-drain path connected between the gate of the transistor (Q2) and the gate of the further transistor (Q1, Q21, etc.), wherein one of the gate voltage circuits is selected by turning on the another transistor (Q6, Q26, etc.) of the gate voltage circuit.

17. The current driver circuit of claim 13, wherein the load mimic circuit includes a voltage source in series with a further resistor.

18. A method for driving a load (LD), comprising: providing a transistor (Q2) having a gate, a drain and a source, with the drain of the transistor (Q2) forming the output of a current driver circuit; andproviding a plurality of gate voltage circuits, each of which selectively provides a different voltage to the gate of the transistor (Q2); andselecting one the gate voltage circuits at a time;wherein a magnitude of an output current produced at the output of the current driver circuit is dependent upon which of the gate voltage circuits is selected.

19. The method of claim 18, wherein each gate voltage circuit includes a further transistor (Q1, Q21, etc.) that, when the gate voltage circuit is selected, functions together with the transistor (Q2) as a current mirror; and further comprising: using a load mimic circuit and an op-amp to cause a voltage at the drain of the further transistor (Q1, Q21, etc.), of the selected gate voltage circuit, to substantially equal a voltage at the output of the current driver circuit.

20. The method of claim 19, wherein each gate voltage circuit includes another transistor (Q6, Q26, etc.) that selectively connects the gate of the transistor (Q2) to the gate of the further transistor (Q1, Q21, etc.) of the selected one of the gate voltage circuits.

21. The method of claim 20, wherein the another transistor (Q6, Q26, etc.) of each gate voltage circuit has a source-drain path connected between the gate of the transistor (Q2) and the gate of the further transistor (Q1, Q21, etc.), wherein one of the gate voltage circuits is selected by turning on the another transistor (Q6, Q26, etc.) of the gate voltage circuit.

22. The method of claim 18, further comprising: changing which gate voltage circuit is selected in response to a control signal.