Feedback-controlled low voltage current sink/source

• US 6,172,556 B1
• Filed: 03/04/1999
• Issued: 01/09/2001
• Est. Priority Date: 03/04/1999
• Status: Expired due to Term

• Current Assignee: Synaptics Incorporated
• Original Assignee: Intersil Corporation (Renesas Electronics Corporation)
• Inventors: Prentice, John S.
• Primary Examiner(s): Tran, Toan
• Assistant Examiner(s): TRA, ANH QUAN
• Application Number:
  US09/261,981
• Time in Patent Office: 677 Days
• Field of Search: 327/540, 327/541, 327/543, 327/538, 323/313, 323/315
• US Class Current: 327/543
• CPC Class Codes: G05F 3/262 using field-effect transistors only
• CPC Class Codes: G05F 3/265 using bipolar transistors only

Abstract

A low voltage MOSFET-configured current sink/source has an output node coupled to the drain of an output MOSFET, that is current mirror coupled with a diode-connected reference current MOSFET. The output MOSFET has its gate resistor-coupled to the gate of the reference current MOSFET and its drain coupled to the source a third V_GS-feedback control MOSFET of a feedback circuit, that includes a further current mirror circuit. This third MOSFET has its gate electrode coupled in common with the drain-gate connection of the reference current MOSFET. The V_GS-feedback control MOSFET is coupled with a further current mirror, the output of which is coupled to the gate-coupling resistor. In response to a drop in the drain-source voltage V_DS of the output MOSFET so that the point that output MOSFET no longer operates in its saturation region, the V_GS-feedback control MOSFET turns on, causing the flow of drain current in the feedback control MOSFET. The further current mirror circuit mirrors this drain current through the gate-coupling resistor, causing a voltage drop across it, so that the value of gate-source voltage applied to the output MOSFET is increased. The effect of this increase in the value of V_GS of the output MOSFET for a reduced value of its drain-source voltage V_DS is to shift the knee or (saturation-linear) transition region of the output (drain-to-source) current I_DS of the output MOSFET to a lower knee point, thereby reducing the amount of headroom voltage required of a given sink/source current at the output terminal.

Claims

1. A method of generating an output current comprising the steps of:

(a) providing a current mirror that contains

a first current-generating device having a first input terminal, a first output terminal and a first control terminal, and

a second current-generating device having a second input terminal, a second output terminal and a second control terminal, and wherein

said first input terminal is coupled to a first voltage supply terminal, said first control terminal is coupled to said second control terminal, said second input terminal is coupled to said first voltage supply terminal, and said second output terminal is coupled to receive a reference current, and said first output terminal provides a prescribed output current in accordance with a first voltage applied across said first input terminal and said first output terminal for a second voltage applied across said first control terminal and said first input terminal; and

(b) controlling the operation of said first current generating device so as to maintain said prescribed output current at said first output terminal in the presence of a change in said first voltage applied across said first input terminal and said first output terminal.

2. A method according to claim 1, wherein step (b) comprises, in the presence of said change in said first voltage applied across said first input terminal and said first output terminal, controllably modifying said second voltage applied across said first control terminal and said first output terminal to a value that is effective to maintain said prescribed output current.

3. A method according to claim 2, wherein

step (a) comprises coupling said first control terminal to said second control terminal through a voltage-dropping circuit element that provides a voltage drop thereacross in response to current flow therethrough, and

step (b) comprises coupling a third current-generating device in circuit with said first and second current-generating devices, and with said voltage-dropping circuit element, and causing said third current-generating device to supply current through said voltage-dropping circuit element, so as to modify said second voltage applied across said first control terminal and said first input terminal to a value that is effective to maintain said prescribed output current.

4. A method according to claim 3, wherein step (b) further comprises coupling a second current mirror circuit to said third current-generating device and to said voltage-dropping circuit element, and causing said second current mirror circuit to supply current through said voltage-dropping circuit element so as to modify said second voltage applied across said first control terminal and said first input terminal to a value that is effective to maintain said prescribed output current.

5. A method according to claim 2, wherein

step (a) comprises coupling said first control terminal to said second control terminal through a voltage-dropping circuit element that provides a voltage drop thereacross in response to current flow therethrough, and

wherein step (b) further comprises coupling a current feedback network in circuit with said first and second current-generating devices, and with said voltage-dropping circuit element, and causing said current feedback network to supply current through said voltage-dropping circuit element and modify said second voltage applied across said first control terminal and said first input terminal to a value that is effective to maintain said prescribed output current.

6. A method according to claim 5, wherein said current feedback network includes a second current mirror circuit containing a third current-generating device, coupled in circuit with said first and second current-generating devices and with said voltage-dropping circuit element, and wherein step (b) comprises, in response to said change in said first voltage applied across said first input terminal and said first output terminal, causing said third current-generating device to generate a current that is mirrored by said second current mirror circuit and applied to said voltage-dropping circuit element, so as to modify said second voltage applied across said first control terminal and said first input terminal to a value that is effective to maintain said prescribed output current.

7. A method according to claim 6, wherein

said first current-generating device comprises a first field effect transistor having a first gate electrode, a first source electrode and a first drain electrode, said second current-generating device comprises a second field effect transistor having a second gate electrode, a second source electrode and a second drain electrode, and said third current-generating device comprises a third field effect transistor having a third gate electrode, a third source electrode and a third drain electrode, and wherein

said first source electrode is coupled to said first voltage supply terminal, said first drain electrode is coupled to said first output terminal and to said third source electrode, and said first gate electrode is coupled to said second gate electrode through said voltage-dropping circuit element, said second drain electrode is coupled to receive said reference current, and said second source electrode is coupled to said first supply terminal and to said third gate electrode, and wherein said third drain electrode is coupled through said second current mirror circuit to said voltage-dropping element.

8. A method according to claim 7, wherein said first, second and third field effect transistors comprise MOSFETs of a first channel polarity, and wherein said second current mirror circuit is comprised of MOSFETs of a second channel polarity.

9. A method according to claim 7, wherein said voltage-dropping circuit element comprises a resistor.

10. A method according to claim 9, wherein said second current mirror and said resistor have temperature dependent characteristics.

11. A method according to claim 7, wherein said step (b) comprises adjusting said second voltage as said first voltage is changed to a value which varies operation of said first field effect transistor from a saturated operating region to a linear operating region, and maintains the value of drain current of said first field effect transistor above a reference value for a range of further change in said first voltage.

12. A low voltage MOSFET-configured current sink/source comprising an output node coupled to the drain of an output MOSFET, said output MOSFET being current mirror-coupled with a reference current MOSFET, said output MOSFET having its gate resistor-coupled to the gate of said reference current MOSFET and its drain coupled to the source a feedback control MOSFET of a feedback circuit containing a further current mirror circuit, said feedback control MOSFET having its gate electrode coupled to said reference current MOSFET, and being operative, in response to a drop in drain-source voltage of said output MOSFET that would otherwise cause said output MOSFET to shift from its saturation region to its linear region of operation, to turn on and thereby cause said further current mirror circuit to mirror drain current in said feedback control MOSFET through said resistor, thereby increasing the value of gate-source voltage of said output MOSFET to a value that effectively shifts the saturation-linear transition region of the drain-to-source current versus drain-source voltage of said MOSFET to a lower drain-to-source voltage range.

13. A circuit for coupling an output current comprising:

a current mirror containing a first current-generating device having a first input terminal, a first output terminal and a first control terminal, and a second current-generating device having a second input terminal, a second output terminal and a second control terminal, said first input terminal being coupled to a first voltage supply terminal, said first control terminal being coupled to said second control terminal, said second input terminal being coupled to said first voltage supply terminal, said second output terminal being coupled to receive a reference current, and said first output terminal providing a prescribed output current in accordance with a first voltage applied across said first input terminal and said first output terminal, for a second voltage applied across said first control terminal and said first input terminal; and

a feedback circuit coupled in a feedback path with said first current-generating device and being operative to cause said first current-generating device to maintain said prescribed current in the presence of a change in said first voltage applied across said first input terminal and said first output terminal thereof.

14. A circuit according to claim 13, wherein said feedback circuit is operative, in the presence of said change in said first voltage across said first input terminal and said first output terminal, to controllably modify said second voltage across said first control terminal and said first output terminal to a value that causes said first current-generating device to maintain said prescribed output current.

15. A circuit according to claim 13, wherein a voltage-dropping circuit element is coupled between said first control terminal and said second control terminal, and wherein said feedback circuit is operative to supply a current through said voltage-dropping circuit element and thereby provide a voltage drop thereacross in the presence of said change in said first voltage across said first input terminal and said first output terminal, and thereby controllably modify said second voltage across said first control terminal and said first output terminal to a value that causes said first current-generating device to maintain said prescribed output current.

16. A circuit according to claim 15, wherein said feedback circuit includes a third current-generating device coupled in circuit with said first and second current-generating devices, and with said voltage-dropping circuit element, said third current-generating device being operative to provide a current that is coupled through said voltage-dropping circuit element, so as to modify said second voltage applied across said first control terminal and said first input terminal to a value that causes said first current-generating device to maintain said prescribed output current.

17. A circuit according to claim 16, wherein said feedback circuit further includes a second current mirror circuit coupled to said third current-generating device and to said voltage-dropping circuit element, and being operative to mirror a current generated by said third current-generating device through said voltage-dropping circuit element so as to modify said second voltage applied across said first control terminal and said first input terminal to a value that causes said first current-generating device to maintain said prescribed output current.

18. A circuit according to claim 17, wherein

said first current-generating device comprises a first field effect transistor having a first gate electrode, a first source electrode and a first drain electrode, said second current-generating device comprises a second field effect transistor having a second gate electrode, a second source electrode and a second drain electrode, and said third current-generating device comprises a third field effect transistor having a third gate electrode, a third source electrode and a third drain electrode, and wherein

said first source electrode is coupled to said first voltage supply terminal, said first drain electrode is coupled to said first output terminal and to said third source electrode, and said first gate electrode is coupled to said second gate electrode through said voltage-dropping circuit element, said second drain electrode is coupled to receive said reference current, and said second source electrode is coupled to said first supply terminal and to said third gate electrode, and wherein said third drain electrode is coupled through said second current mirror circuit to said voltage-dropping circuit element.

19. A circuit according to claim 18, wherein said first, second and third field effect transistors comprise MOSFETs of a first channel polarity, and wherein said second current mirror circuit is comprised of MOSFETs of a second channel polarity.

20. A circuit method according to claim 18, wherein said voltage-dropping circuit element and said second current mirror have temperature dependent characteristics.

21. A circuit according to claim 18, wherein said feedback circuit is operative to adjust said second voltage as said first voltage is changed to a value which varies operation of said first field effect transistor from a saturated operating region to a linear operating region, and maintains the value of drain current of said first field effect transistor above a reference value for a range of further change in said first voltage.