Method to provide a higher reference voltage at a lower power supply in flash memory devices

• US 20080136381A1
• Filed: 12/06/2006
• Published: 06/12/2008
• Est. Priority Date: 12/06/2006
• Status: Active Grant

• Current Assignee: Infineon Technologies Batam PT (Infineon Technologies AG)
• Original Assignee: Spansion LLC (Infineon Technologies AG)
• Inventors: Wu, Yonggang, Yang, Nian, Yang, Tien-Chun
• Granted Patent: US 7,724,075 B2
• Time in Patent Office: Days
• US Class Current: 323/220
• CPC Class Codes: G05F 3/08 wherein the variable is dc
• CPC Class Codes: G11C 16/30 Power supply circuits
• CPC Class Codes: G11C 5/147 Voltage reference generators, voltage or current regulators; Internally lowered supply levels; Compensation for voltage drops G11C5/141 takes precedence

Abstract

A fast reference circuit having active feedback includes a bias supply circuit and a variable divider circuit connected by an active feedback path to the bias supply circuit, and a comparator circuit connected to the variable divider circuit, the bias supply circuit, and a reference node of the variable divider circuit. In one embodiment, a start-up circuit initially discharges a potential at the bias supply and comparator circuits, then initializes a reference voltage at the reference node at about zero volts to improve repeatability. In one embodiment, the variable voltage divider comprises an impendence that is trimmed based on a sheet resistance of a process used to fabricate the fast reference circuit, and further comprises a variable reference current circuit coupled to the impedance and configured to generate a current having a value based on a desired reference voltage and to conduct the current through the impedance, thereby generating the reference voltage associated therewith. The comparator circuit is configured to compare the bias supply voltage to the reference voltage, and drive the bias supply and the variable divider circuit in response to the comparison, thereby quickly stabilizing the reference voltage.

Claims

1. A fast voltage reference circuit, comprising:

a bias supply configured to generate a bias supply voltage;

a variable divider circuit coupled to the bias supply by an active feedback path and configured to generate a variable reference voltage, the divider comprising;

an impedance;

a variable reference current circuit coupled to the impedance and configured to generate a current having a value based on a desired reference voltage and to conduct the current through the impedance, thereby generating the reference voltage associated therewith; and

a comparator circuit configured to compare the bias supply voltage to the reference voltage, and drive the bias supply and the variable divider circuit in response to the comparison, thereby quickly stabilizing the reference voltage.

2. The circuit of claim 1, wherein the impedance comprises a variable impedance configured to be selectively trimmed in response to a sheet resistance of a process employed to fabricate the fast voltage reference circuit.

3. The circuit of claim 1, wherein the impedance comprises a variable impedance circuit comprising a plurality of resistors and respective selection transistors configured to be selected and selectively trimmed in response to a sheet resistance of a process employed to fabricate the fast voltage reference circuit.

4. The circuit of claim 1, wherein the reference voltage is generated across the variable reference current circuit at a reference node arranged between the impedance and the reference current circuit.

5. The circuit of claim 1, further comprising a start-up circuit connected to the bias supply and the comparator circuit, the start-up circuit operable to initially discharge a residual potential in the bias supply and the comparator circuit, and operable to initialize the reference voltage output at zero volts, the start-up circuit comprising:

a first MOS transistor having a first terminal connected to the comparator circuit, a second terminal connected to circuit ground, and a control terminal connected to an enable input terminal;

a second MOS transistor having a first terminal connected to the output of the comparator circuit, a second terminal connected to circuit ground, and a control terminal connected to an enable bar input terminal;

a third MOS transistor having a first terminal connected to a power supply terminal, a second terminal connected to the comparator circuit, and a control terminal connected to the enable bar input terminal; and

a fourth MOS transistor having a first terminal connected to the bias supply end of the active feedback path, a second terminal connected to the bias supply, and a control terminal connected to the enable bar input terminal;

wherein the first, third, and fourth MOS transistors are operable to initially open and disable the bias supply and the comparator based on a signal at the control terminals, and the second MOS transistor is operable to conduct based on a signal at the control terminal to initially discharge a residual potential at the output of the comparator;

and wherein thereafter, the first, third, and fourth MOS transistors are operable to conduct enabling the bias supply and the comparator based on a signal at the control terminals, and the second MOS transistor is operable to open based on a signal at the control terminal to enable a signal at the output of the comparator, thereby enhancing repeatability and settling time of the output of the fast voltage reference circuit.

6. The circuit of claim 1, wherein the comparator circuit is further configured to amplify and drive the bias supply and the variable divider circuit in response to the comparison.

7. The circuit of claim 1, wherein the bias supply comprises a voltage divider coupled between the active feedback path and circuit ground to provide a bias voltage to the negative terminal input of the comparator circuit in response to active feedback from the variable divider circuit.

8. The circuit of claim 1, wherein the variable reference current circuit comprises a plurality of selectable reference current sources individually configured to provide a reference current, configured such that one or more of the plurality of reference currents may be selectively summed to generate a current thru the impedance having a value based on the desired reference voltage.

9. The circuit of claim 8, wherein the plurality of selectable reference current sources individually comprise a MOS selection transistor series connected to an enhancement type high voltage MOS transistor configured as a diode, wherein one or more of the plurality of reference current sources are selected by one or more respective selection transistors.

10. The circuit of claim 1, wherein the variable reference current circuit comprises a plurality of selectable reference current sources individually configured to provide a reference current, configured such that one or more of the plurality of reference currents may be selected by a digital to analog converter to generate a current thru the impedance having a value based on the desired reference voltage.

11. The circuit of claim 1, wherein the impedance comprises an unsilicided polysilicon material resistor.

12. A fast voltage reference circuit, comprising:

a bias supply configured to generate a bias supply voltage;

a divider circuit coupled to the bias supply by an active feedback path from a feedback node of the divider circuit and configured to provide a reference voltage as a function of a voltage divider voltage ratio, comprising;

a variable impedance configured to be selectively trimmed in response to a sheet resistance of a process employed to fabricate the fast voltage reference circuit;

a variable reference current circuit configured to generate a current having a value based on a desired reference voltage at a reference node arranged between the variable impedance and the variable reference current; and

a comparator circuit configured to compare the bias supply voltage to the reference voltage of the reference circuit, and operable to drive the bias supply and the variable divider circuit in response to the comparison, thereby quickly generating a stable reference voltage.

13. The circuit of claim 12, wherein the variable impedance comprises one or more unsilicided polysilicon resistors.

14. The circuit of claim 12, wherein the variable impedance comprises a plurality of resistors and respective MOS selection transistors configured to be selected and selectively trimmed in response to the sheet resistance of the process employed to fabricate the fast voltage reference circuit.

15. The circuit of claim 12, wherein the reference voltage is generated across the variable reference current circuit.

16. The circuit of claim 12, further comprising a start-up circuit connected to the bias supply and the comparator circuit, the start-up circuit operable to initially discharge a residual potential in the bias supply and the comparator circuit, and operable to initialize the reference voltage output at zero volts, the start-up circuit comprising:

a first MOS transistor having a first terminal connected to the comparator circuit, a second terminal connected to circuit ground, and a control terminal connected to an enable input terminal;

a second MOS transistor having a first terminal connected to the output of the comparator circuit, a second terminal connected to circuit ground, and a control terminal connected to an enable bar input terminal;

a third MOS transistor having a first terminal connected to a power supply terminal, a second terminal connected to the comparator circuit, and a control terminal connected to the enable bar input terminal; and

a fourth MOS transistor having a first terminal connected to the bias supply end of the active feedback path, a second terminal connected to the bias supply, and a control terminal connected to the enable bar input terminal;

wherein the first, third, and fourth MOS transistors are operable to initially open and disable the bias supply and the comparator based on a signal at the control terminals, and the second MOS transistor is operable to conduct based on a signal at the control terminal to initially discharge a residual potential at the output of the comparator;

and wherein thereafter, the first, third, and fourth MOS transistors are operable to conduct enabling the bias supply and the comparator based on a signal at the control terminals, and the second MOS transistor is operable to open based on a signal at the control terminal to enable a signal at the output of the comparator, thereby enhancing repeatability and settling time of the output of the fast voltage reference circuit.

17. The circuit of claim 12, wherein the bias supply comprises a voltage divider coupled between the active feedback path and circuit ground to provide a bias voltage to the negative terminal input of the comparator circuit in response to active feedback from the variable divider circuit.

18. The circuit of claim 12, wherein the variable reference current circuit comprises a plurality of selectable reference current sources individually configured to provide a reference current, and configured such that one or more of the plurality of reference currents may be selectively summed to generate a current thru the impedance having a value based on the desired reference voltage.

19. The circuit of claim 18, wherein the plurality of selectable reference current sources individually comprise a MOS selection transistor series connected to an enhancement type high voltage MOS transistor configured as a diode, wherein one or more of the plurality of reference current sources are selected by one or more respective selection transistors.

20. The circuit of claim 12, wherein the variable reference current circuit comprises a plurality of selectable reference current sources individually configured to provide a reference current, configured such that one or more of the plurality of reference currents may be selected by a digital to analog converter to generate a current thru the impedance having a value based on the desired reference voltage.

21. A method of providing a fast and stable reference voltage, comprising:

providing an impedance for a variable divider circuit;

selecting a variable current reference for the variable divider circuit based on a desired reference voltage;

translating the reference current through the impedance into the reference voltage;

comparing the reference voltage to a bias supply voltage which varies as a function of feedback from a node of the variable divider circuit having a negative function of a supply voltage and substantially no function of temperature; and

driving the differential voltage of the comparison into the variable divider circuit to rapidly stabilize the reference voltage substantially independent of variations in supply voltage and process variations.

22. The method of claim 21, further comprising initially discharging a residual potential at the variable divider circuit to initialize the reference voltage at about zero volts.

23. The method of claim 21, further comprising initially applying a start signal operable to initialize the reference voltage to a predetermined reference voltage.

24. The method of claim 21, further comprising initially trimming the impedance of the variable divider circuit based on a sheet resistance of a process employed to fabricate a circuit for the fast reference voltage.

25. The method of claim 21, wherein the selecting of the variable current reference for the variable divider circuit based on the desired reference voltage, comprises selecting one of a plurality of reference currents, and wherein one or more of the plurality of reference currents may be selectively summed together to generate a current thru the impedance having a value based on the desired reference voltage.

26. The method of claim 21, wherein translating the reference current through the impedance into the reference voltage comprises conducting the reference current through the impedance to generate the reference voltage across a reference current circuit of the reference current.

27. An electronic device, comprising:

a user input configured to allow a user to input data;

a user output configured to output data to a user;

a central processing unit (CPU) operatively coupled to the user input and the user output and configured to receive and process the user input and to output the user output; and

a memory operatively coupled to the CPU and configured to receive data from and send data to the CPU, the memory comprising a fast voltage reference circuit configured to supply a reference voltage (FVREF) to a wordline of the memory, the fast voltage reference circuit comprising a bias supply configured to generate a bias supply voltage, a variable divider circuit comprising an impedance and a variable reference current circuit, and a comparator circuit configured to compare the bias supply voltage to the reference voltage and drive the bias supply and the variable divider circuit in response to the comparison for outputting the reference voltage (FVREF) to the wordline as a function of variable reference current in the variable divider.

28. The device of claim 27, the fast voltage reference circuit comprising:

a bias supply configured to generate a bias supply voltage;

a variable divider circuit coupled to the bias supply by an active feedback path and configured to generate a variable reference voltage, the divider comprising;

an impedance;

a variable reference current circuit coupled to the impedance and configured to generate a current having a value based on a desired reference voltage and to conduct the current through the impedance, thereby generating the reference voltage associated therewith; and

a comparator circuit configured to compare the bias supply voltage to the reference voltage, and drive the bias supply and the variable divider circuit in response to the comparison, thereby quickly stabilizing the reference voltage.

29. The device of claim 27, the electronic device comprising at least one of a communication device, Personal Data Assistant (PDA), cell phone, memory stick, flash drive device, video camcorder, voice recorder, USB flash drive, fax machine, flash memory, laptop, computer, scanner, MP3 player, digital camera, home video game console, hard drive and memory card.