Resistor ladder-based multi-reference voltage generator with headroom increasing circuitry

1 . A multi-reference voltage generator, comprising:

a current source;

a set of resistors coupled in series with the current source between an upper voltage rail and a lower voltage rail;

a first set of switching devices coupled between a first output and a set of taps at respective terminals of the set of resistors, wherein the set of taps includes a first tap and a second tap;

a second set of switching devices coupled between a second output and the set of taps, respectively;

a switching device coupled between the first tap and the second tap; and

a control circuit including an output coupled to a control input of the switching device.

2 . The multi-reference voltage generator of claim 1 , wherein the first tap is situated between the current source and the set of resistors.

3 . The multi-reference voltage generator of claim 1 , wherein a subset of the set of resistors are situated between the current source and the second tap.

4 . The multi-reference voltage generator of claim 1 , wherein the control circuit includes an input coupled to a set of control inputs of the first set of switching devices, respectively.

5 . The multi-reference voltage generator of claim 1 , wherein:

the set of taps are arranged consecutively from the lower voltage rail to the current source;

the first set of switching devices are configured to receive a first control signal indicating a number of one of the set of taps, the first control signal closing the switching device of the first set of switching devices coupled between the one of the set of taps and the first output, and opening the remaining ones of the first set of switching devices, wherein a first reference voltage is generated at the first output; and

the first control signal is configured to:

close the switching device if the number of the one of the set of taps is at or below a tap threshold; and

open the switching device if the number of the one of the set of taps is above the tap threshold.

6 . The multi-reference voltage generator of claim 5 , wherein a number of the second tap is higher than the number of the one of the set of taps.

7 . The multi-reference voltage generator of claim 5 , wherein the second set of switching devices are configured to receive a second control signal indicating a number of the one or another one of the set of taps, the second control signal closing the switching device of the second set of switching devices coupled between the one or the another one of the set of taps and the second output, and opening the remaining ones of the second set of switching devices, wherein a second reference voltage is generated at the second output.

8 . The multi-reference voltage generator of claim 1 , wherein:

the first set of switching devices are configured to receive a first control signal identifying one of the first set of switching devices to close; and

the control circuit is configured to close the switching device based on the one of the first set of switching devices to close.

9 . The multi-reference voltage generator of claim 8 , wherein the control circuit is configured to close the switching device based on a number of the set of resistors between one of the set of taps to which the one of the first set of switching device is coupled and the lower voltage rail.

10 . The multi-reference voltage generator of claim 9 , wherein the control circuit is configured to close the switching device if the number of the set of resistors between the one of the set of taps and the lower voltage rail is at or below a tap threshold.

11 . A method of generating first and second reference voltages, comprising:

generating a current through a set of resistors to generate a set of voltages at a set of taps coupled to the set of resistors, respectively;

outputting one of the set of voltages at a first selected tap as the first reference voltage;

outputting the one or another of the set of voltages at a second selected tap as the second reference voltage; and

coupling a first tap to a second tap of the set of taps based on the first selected tap.

12 . The method of claim 11 , wherein the first tap is situated between a source of the current and the set of resistors.

13 . The method of claim 11 , wherein a subset of the set of resistors are situated between a source of the current and the second tap.

14 . The method of claim 11 , wherein:

the set of taps are numbered in consecutive order from the tap farthest away from to the tap closest to a source of the current; and

coupling the first tap to the second tap is based on the number of the first selected tap being at or below a tap threshold.

15 . The method of claim 14 , further comprising decoupling the first tap from the second tap based on the number of the first selected tap being above the tap threshold.

16 . The method of claim 14 , wherein the first tap is the tap closest to a source of the current.

17 . The method of claim 14 , wherein the number of the second tap is at or above the tap threshold.

18 . The method of claim 11 , wherein the coupling of the first tap to the second tap is based on a number of taps between the first selected tap and the tap farthest away from a source of the current.

19 . The method of claim 11 , wherein:

outputting the one of the set of voltages at the first selected tap as the first reference voltage comprises closing a first switching device coupled between the first selected tap and a first output; and

outputting the one or the another of the set of voltages at the second selected tap as the second reference voltage comprises closing a second switching device coupled between the second selected tap and a second output.

20 . The method of claim 11 , wherein coupling the first tap to the second tap comprises closing a switching device coupled between the first tap and the second tap.