Low power active matrix display
Described herein are systems and methods for stress avoidance and stress compensation in low power Liquid Crystal Displays (LCDs). In an exemplary embodiment, two or more transistors in series are used to hold charge on an LCD pixel. To avoid negative stress on the transistors, the transistors are alternately driven to an “off” state so that no one transistor sees a long “off” time. In another embodiment, stress on transistors of a display circuit are measured and controlled negative stress is applied to the transistors to compensate for the measured stress.
1. A method of operating a display circuit, wherein the display circuit comprises a plurality of pixel circuits, each pixel circuit comprising at least two transistors in series connected to a pixel of a Liquid Crystal Display (LCD), the method comprising:
performing frame loading operations, wherein each frame loading operation updates a display image of the LCD; and
between frame loading operations, for each pixel circuit, holding a charge of the corresponding LCD pixel,
wherein to hold a charge of a LCD pixel of a pixel circuit between frame loading operations a negative gate bias voltage is alternately applied to the transistors of said transistors in series of the pixel circuit, and when the negative gate bias voltage is applied to a least one transistor of said transistors in series of the pixel circuit, a positive gate bias voltage is applied to the at least one other transistor of said transistors in series of the pixel circuit, thereby applying a positive stress to compensate for negative stress accumulated on said at least one other transistor of said transistors in series when the negative gate bias is applied,
wherein said negative gate bias voltage and positive gate bias voltage are alternately applied between frame loading operations to each one of said transistors in series.
2. The method of claim 1 , further comprising alternately applying the negative gate bias voltage and the positive gate bias voltage to one of the transistors of said transistors in series of the pixel circuit at a rate of 60 Hz or greater.
3. The method of claim 1 , wherein performing a frame loading operation comprises, for each pixel circuit:
applying a positive gate bias voltage to each transistor of said transistors in series of the pixel circuit to form a conduction path through the transistors to the corresponding LCD pixel; and
sending a charge through the conduction path to the LCD pixel to charge the corresponding LCD pixel.
4. The method of claim 1 , wherein the display circuit comprises a plurality of row and column select transistors connected to the pixel circuits, the method further comprising
performing frame loading operation, wherein each frame loading operation updates a display age of the LCD and comprises applying gate bias voltages to the row and column select transistors; and
between frame loading operations, applying a negative gate bias to the row and column select transistors, thereby applying negative stress to compensate for positive stress accumulated on said row and column select transistors during the frame loading operations.
5. The method of claim 4 , further comprising:
periodically measuring threshold voltage shifts in the row and column select transistors; and
adjusting the amount of negative stress applied to the row and column select transistors based on the measured threshold voltage shifts.
6. The method of claim 5 , wherein adjusting the amount of negative stress comprises adjusting an amount of time that a negative voltage is applied to the row and column select transistors.
7. The method of claim 5 , wherein adjusting the amount of negative stress comprises adjusting a waveform of a negative voltage applied to the row and column select transistors.
8. A display circuit for a pixel array, comprising:
a row and column driver; and
a plurality of pixel circuits coupled to the row and column driver, wherein each pixel circuit comprises at least two transistors in series connected to a pixel of a Liquid Crystal Display (LCD);
wherein the row and column driver is configured to load a frame onto the LCD by applying positive voltages to the transistors of said transistors in series of the pixel circuits to form conduction paths to the pixels of the LCD and sending charges to the pixels through the conduction paths, and
between frame load operations, for each pixel circuit, to alternately apply a negative gate bias voltage to one of the transistors of said transistors in series of the pixel circuit to hold the charge of the corresponding LCD pixel, and when the negative gate bias voltage is applied to a least one transistor of said transistors in series of the pixel circuit, a positive gate bias voltage is applied to the at least one other transistor of the pixel circuit, thereby applying a stress reducing voltage to the at least one other transistor of said transistors in series of the pixel circuit,
wherein said negative gate bias voltage and positive gate bias voltage are alternately applied between frame loading operations to each one of said transistors in series.
9. The display circuit of claim 8 , wherein the row and column driver is configured to apply the negative gate bias voltage to one of the transistors of said transistors in series of the pixel circuit at a rate of 60 Hz or greater.
10. The display circuit of claim 8 , wherein the row and column driver is configured to update the frame of the LCD at a rate of 200 milliseconds or slower.
11. The display circuit of claim 8 , wherein the row and column driver is configured to update the frame of the LCD at a rate of one second or slower.
12. The display circuit of claim 8 , wherein transistors comprise amorphous silicon hydrogenated thin film transistors (a-Si:H TFT).