Dual-voltage pixel circuitry for liquid crystal display
Systems and methods for a digital pixel circuit for liquid crystal displays are provided. The design includes a dual-voltage pixel design, a two-transistor level-shift circuit design, self-adjusting transistor bias circuitry; and an optional on-chip test-array to determine die-specific design-center values for critical transistor leakage and threshold parameters. Level shift design simplicity, small pixel pitch, and applicability for small display applications such as microdisplays, are among the various benefits and advantages obtained.
1. A pixel circuit for supplying an output voltage to a pixel electrode in a display, the pixel circuit comprising:
a plurality of static random-access memory (SRAM) units;
a level shift circuit connected to at least one of the plurality of SRAM units, wherein the level shift circuit converts a core voltage to the output voltage supplied to the pixel electrode, the core voltage being lower than the output voltage, the level shift circuit comprising a first transistor and a second transistor; and
an update circuit connected to the level shift circuit that toggles between a voltage V REFON and a voltage V REFOFF .
2. The pixel circuit of claim 1 , further comprising a V REFON generation circuit for generating and calibrating the voltage V REFON , and a V REFOFF generation circuit for generating and calibrating the voltage V REFOFF .
3. The pixel circuit of claim 2 , wherein each of the V REFON generation circuit and the V REFOFF generation circuit comprises a plurality of level-shift circuits.
4. The pixel circuit of claim 3 , wherein the plurality of level-shift circuits are located in a non-viewable portion of the display.
5. The pixel circuit of claim 2 , wherein the V REFON generation circuit and the V REFOFF generation circuit are both analog circuits.
6. The pixel circuit of claim 2 , wherein the V REFON generation circuit and the V REFOFF generation circuit employ A/D and D/A circuitry.
7. The pixel circuit of claim 1 , wherein the voltage V REFOFF IS selected to result in a higher subthreshold current of the first transistor relative to a leakage current of the second transistor.
8. The pixel circuit of claim 7 , wherein a value of the subthreshold current of the first transistor is approximately 1 nA.
9. The pixel circuit of claim 8 , wherein a value of the voltage V REFOFF is in a range of 0.3-0.4V below VPIX.
10. The pixel circuit of claim 1 , wherein a gate-voltage of the first transistor is controlled such that both an on-resistance and an off-resistance of the first transistor are lower than that of an off-resistance of the second transistor.
11. The pixel circuit of claim 1 , wherein the first transistor is a p-channel field-effect transistor (PFET) and the second transistor is an-channel field-effect transistor (NFET).
12. The pixel circuit of claim 1 , wherein a value of the core voltage is in a range of 0.9V-1.2V, and a value of the output voltage is 2-4V.
13. The pixel circuit of claim 1 , wherein the voltage V REFON and the voltage V REFOFF are analog voltages.
14. The pixel circuit of claim 1 , wherein a value of the voltage V REFOFF is below a turn-on threshold voltage of the first transistor.
15. The pixel circuit of claim 1 , wherein a dimension of the pixel circuit is at or less than 6 μm.
16. A method, comprising:
operating one portion of a pixel circuit at a core voltage, the one portion of the pixel circuit comprising a plurality of static random-access memory (SRAM) units;
operating another portion of the pixel circuit at an output voltage using a Level-Shift block, the core voltage being lower than the output voltage;
operating the pixel circuit using a V REFON and a V REFOFF and, using a test-array in a non-viewable portion of a display comprising copies of Level-Shift blocks identical to the Level-Shift block that are not associated with any pixels, averaging characteristics of the Level-Shift blocks in the test-array to provide a reference for the V REFON and the V REFOFF ; and
supplying the output voltage to a pixel electrode of the display.
17. The method of claim 16 , further comprising operating the pixel circuit using a gate-voltage that controls a first transistor of the Level-Shift block such that both an on-resistance and an off-resistance of the first transistor are lower than of an off-resistance of a second transistor of the Level-Shift block.
18. The method of claim 16 , wherein the voltage V REFON and the voltage V REFOFF are analog voltages.
19. The method of claim 16 , further comprising:
generating and calibrating the voltage V REFON ; and
generating and calibrating the voltage V REFOFF .
20. A display comprising a pixel circuit for supplying an output voltage to a pixel electrode in the display, the pixel circuit comprising:
a plurality of static random-access memory (SRAM) units;
a level shift circuit connected to at least one of the plurality of SRAM units, wherein the level shift circuit converts a core voltage to the output voltage supplied to the pixel electrode, the core voltage being lower than the output voltage, wherein the level shift circuit comprises a first transistor and a second transistor; and
an update circuit connected to the level shift circuit that toggles between a voltage V REFON and a voltage V REFOFF .