Emissive pixel array and self-referencing system for driving same
The present invention is to improve on an emissive display by providing a backplane and modulation system that enables fabrication of multi-color or monochrome LED display systems that operate efficiently and without objectionable image artifacts. One aspect of the present invention is to implement the backplane of an emissive display that offers high precision across an array of pixels and extremely low variation. The present invention uses a large L FET to generate a reference current and then uses the same large L FET to act as a current source mirroring the reference current, thereby ensuring a substantially perfect match between reference current FET and current source FET.
1. A pixel drive circuit operative to function in a plurality of modes, wherein the modes comprise a discharge mode of operation and in a refresh mode of operation, the drive circuit comprising
a large L bias FET configured to provide a reference voltage based on a bias voltage on its gate; a large L FET configured as a current source; a capacitor operative to hold a stored voltage between the source and gate of the large L current source; a memory cell operative to hold an illumination state and assert that state on a modulation FET when the pixel drive circuit is in discharge mode; a modulation FET operative to modulate a current from the large L current source; a NAND gate operative to place the modulation FET in a non-conduct mode when the pixel drive circuit operates in a refresh mode or when the memory cell illumination state is off; and, a terminal to receive a refresh signal and switching FETs operative to configure the pixel drive circuit based on the refresh signal, and wherein
the pixel drive circuit operates in a refresh mode based on a first state of a refresh signal and in a discharge mode based on a second state of a refresh signal, and wherein
in a refresh mode of operation, the gate and drain of the large L current source FET connect to the drain of the bias FET, asserting the reference voltage on the gate and drain of the current source FET and on the capacitor operative to hold a stored voltage between the source and gate of the current source FET, thereby charging the capacitor to the reference voltage, and
wherein the voltage asserted on the source of the current source FET is one of V DDAR and V SS , and the voltage asserted on the source of the bias FET is the other of V SS and V DDAR not asserted on the source of the current source FET, and wherein
in a discharge mode of operation, the gate and drain of the current source FET are disconnected from one another and the drain of the bias FET is disconnected from the gate and drain of the current source FET, and the voltage on the gate of the current source FET is asserted by the capacitor, resulted in the discharge of current from the drain of the current source FET into the LED in those instances where the memory cell illumination state is on.
2. The pixel drive circuit of claim 1 , wherein the capacitor is a CMOS capacitor.
3. The pixel drive circuit of claim 2 , wherein the CMOS capacitor is operated in inversion mode.
4. The pixel drive circuit of claim 1 , wherein the memory cell is an SRAM circuit with complementary outputs, one of which is provides an input signal to a NAND gate.
5. The pixel drive circuit of claim 4 , wherein the output of the memory device that is used is S POS .
6. The pixel drive circuit of claim 1 , wherein the NAND gate comprises a pair of p-channel FETs in parallel and a pair of n-channel FETs in series wherein the n-channel FETs are in series with the two p-channel FETs, and wherein the refresh signal is asserted on the gate of one p-channel FET and on the gate of one n-channel FET and wherein the output of the memory cell is asserted on the gate of the remaining p-channel FET and on the gate of the remaining n-channel FET.
7. The pixel drive circuit of claim 1 , wherein the current source FET is a p-channel FET with its source connected to V DDAR and the bias FET is an n-channel FET with its source connected to V SS .
8. The pixel drive circuit of claim 7 , wherein, in discharge mode, the drain of the current source FET is connected through a modulation FET to the anode of a light emitting diode.
9. The pixel drive circuit of claim 8 , wherein the cathode of the LED is electrically connected to the cathodes of all LEDs in a common cathode arrangement.
10. The pixel drive circuit of claim 8 , wherein the cathode of the LED is connected to a universal voltage V_L.
11. The pixel drive circuit of claim 10 , wherein the universal voltage V_L is equal to V SS .
12. A current source circuit operative to function in a plurality of modes, wherein the modes comprise a discharge mode of operation and in a refresh mode of operation, the current source circuit comprising:
a large L bias FET configured to provide a reference voltage based on a bias voltage on its gate, a large L FET configured as a current source, a capacitor operative to hold a stored voltage between the source and gate of the large L current source, a switch FET operative to switch on or off a current from the large L current source, an inverter operative to place the switch FET in a non-conduct mode when the current source circuit operates in a refresh mode, a terminal to receive a refresh signal and switching FETs operative to configure the current source circuit based on the refresh signal, and wherein
the current source circuit operates in a refresh mode based on a first state of a refresh signal and in a discharge mode based on a second state of a refresh signal, and wherein
in a refresh mode of operation, the gate and drain of the large L current source FET connect to the drain of the bias FET, asserting the reference voltage on the gate and drain of the current source FET and on the capacitor operative to hold a stored voltage between the source and gate of the current source FET, thereby charging the capacitor to the reference voltage, and
wherein the voltage asserted on the source of the current source FET is one of V DDAR and V SS , and the voltage asserted on the source of the bias FET is the other of V SS and V DDAR not asserted on the source of the current source FET, and wherein
in a discharge mode of operation, the gate and drain of the current source FET are disconnected from one another and the drain of the bias FET is disconnected from the gate and drain of the current source FET, and the voltage on the gate of the current source FET is asserted by the capacitor, resulted in the discharge of current from the drain of the current source FET into a resistive load.
13. The current source circuit of claim 12 , wherein the capacitor is a CMOS capacitor.
14. The current source circuit of claim 13 , wherein the CMOS capacitor is operated in inversion mode.
15. The current source circuit of claim 12 , wherein the NAND gate comprises a pair of p-channel FETs in parallel and a pair of n-channel FETs in series wherein the n-channel FETs are in series with the two p-channel FETs, and wherein the refresh signal is asserted on the gate of one p-channel FET and on the gate of one n-channel FET and wherein the output of the memory cell is asserted on the gate of the remaining p-channel FET and on the gate of the remaining n-channel FET.
16. The current source circuit of claim 12 , wherein the current source FET is a p-channel FET with its source connected to V DDAR and the bias FET is an n-channel FET with its source connected to V SS .
17. The current source circuit of claim 16 , wherein, in discharge mode, the drain of the current source FET is connected through a switch FET to the anode of a light emitting diode.
18. The current source circuit of claim 17 , wherein the cathode of the LED is electrically connected to the cathodes of all LEDs in a common cathode arrangement.
19. The current source circuit of claim 17 , wherein the cathode of the LED is connected to a universal voltage V_L.
20. The current source circuit of claim 19 , wherein the universal voltage V_L is equal to V SS .