Photodiode limiter

1. An image sensor comprising:

at least one pixel including:

a photodiode that generates electrons in response to light detected during an integration time,

a storage circuit that, in response to the electrons, generates an output voltage that is applied to an output line, and

a transfer circuit that transfers the electrons from the photodiode to the storage circuit upon activation of the transfer circuit;

a shutter circuit configured to control the integration time of the photodiode; and

a shutter control circuit coupled to the shutter circuit and configured to maintain a node voltage between the photodiode and the shutter circuit at or above a threshold voltage to prevent overload of the output voltage and erroneous activation of the transfer circuit,

wherein the shutter control circuit controls the shutter circuit to apply a pulsed voltage during the integration time of the photodiode to maintain the node voltage at or above the threshold voltage and control a limiting level of a voltage differential between a supply voltage and the node voltage during the integration time.

2. The image sensor according to claim 1 , wherein the shutter circuit is a transistor connecting the photodiode of the at least one pixel to the supply voltage and that limits the voltage differential between the supply voltage and the node voltage during the integration time.

3. The image sensor according to claim 2 , wherein the shutter control circuit is coupled to a gate of the transistor and configured to apply a first voltage to the gate to reset the node voltage and a subsequent second voltage to the gate to set the threshold voltage as the second voltage, wherein the second voltage is lower than the first voltage.

4. The image sensor according to claim 3 , wherein a transfer gate is activated after the shutter control circuit applies the second voltage to transfer the electrons from the photodiode to the storage circuit.

5. The image sensor according to claim 3 , wherein the shutter control circuit comprises:

a first multiplexor having a pair of inputs receiving the first and second voltages, respectively;

a second multiplexor having a pair of inputs respectively coupled to an output of the first multiplexor and a third voltage lower than the second voltage that turns off the transistor; and

a control circuit configured to control the first and second multiplexors to select one of the first, second and third voltages to be applied to the gate of the transistor.

6. An image sensor comprising:

a charge generating circuit that generates a charge in response to light detected during an integration time and a transfer circuit that transfers the charge to a storage circuit that generates an output voltage;

a shutter circuit configured to control the integration time of the charge generating circuit; and

a shutter control circuit coupled to the shutter circuit and configured to set a voltage threshold of the charge generating circuit during the integration time of the charge generating circuit to prevent overload of the output voltage,

wherein the shutter control circuit is configured to control the shutter circuit to apply a pulsed voltage during the integration time of the charge generating circuit to set the voltage threshold and control a limiting level of a voltage differential between a supply voltage and a voltage of the charge generating circuit during the integration time.

7. The image sensor according to claim 6 , wherein the voltage threshold is set by the shutter control circuit to prevent erroneous activation of the transfer circuit.

8. The image sensor according to claim 6 , wherein the shutter circuit is a transistor connecting the charge generating circuit to a supply voltage and that limits a voltage differential between the supply voltage and a voltage of the charge generating circuit during the integration time.

9. The image sensor according to claim 8 , wherein the shutter control circuit applies a first voltage to a gate of the transistor to reset a voltage of the charge generating circuit and subsequently a second voltage to the gate to set the voltage threshold as the second voltage lower than the first voltage.

10. The image sensor according to claim 9 , wherein a transfer gate is activated after the shutter control circuit applies the second voltage to transfer the charge from the charge generating circuit to the storage circuit.

11. An image sensor comprising:

at least one pixel including a photodiode that generates electrons in response to light detected during an integration time; and

a photodiode limiter configured to control the integration time of the photodiode and maintain a node voltage of the photodiode at or above a threshold voltage during the integration time to prevent overload of an output voltage of the image sensor,

wherein the photodiode limiter is configured to apply a pulsed voltage during the integration time to set the threshold voltage and control a limiting level of a voltage differential between a supply voltage and the node voltage during the integration time.

12. The image sensor according to claim 11 , wherein the photodiode limiter includes a transistor connecting the photodiode of the at least one pixel to a supply voltage and that limits a voltage differential between the supply voltage and the node voltage during the integration time.

13. The image sensor according to claim 12 , wherein the photodiode limiter includes a shutter control circuit that is coupled to a gate of the transistor and configured to apply a first voltage to the gate to reset the node voltage and a subsequent second voltage to the gate to set the threshold voltage as the second voltage, wherein the second voltage is lower than the first voltage.

14. The image sensor according to claim 13 , wherein a transfer gate is activated after the shutter control circuit applies the second voltage to transfer the electrons from the photodiode to a storage circuit.

15. The image sensor according to claim 13 , wherein the shutter control circuit comprises:

a first multiplexor having a pair of inputs receiving to the first and second voltages, respectively;

a second multiplexor having a pair of inputs coupled to an output of the first multiplexor and a third voltage lower than the second voltage that turns off the transistor; and

a control circuit configured to control the first and second multiplexors to select one of the first, second and third voltages to be applied to the gate of the transistor.

16. An image sensor comprising:

at least one pixel including a photodiode that generates a charge in response to light and a transfer circuit that transfers the charge to a storage circuit to generate an output voltage for an image; and

a photodiode limiter configured to maintain a photodiode voltage while the photodiode is generating the charge at or above a threshold voltage set to prevent at least one of overload of the output voltage and erroneous activation of the transfer circuit,

wherein the photodiode limiter is configured to apply a pulsed voltage during an integration time to set the voltage threshold and control a limiting level of a voltage differential between a supply voltage and the photodiode voltage during the integration time.

17. The image sensor according to claim 16 , further comprising a transistor connecting the photodiode of the at least one pixel to a supply voltage and that limits a voltage differential between the supply voltage and the photodiode voltage during integration of the photodiode.

18. The image sensor according to claim 17 , wherein the photodiode limiter is coupled to a gate of the transistor and configured to apply a first voltage to the gate to reset the photodiode voltage and a subsequent second voltage to the gate to set the threshold voltage as the second voltage, wherein the second voltage is lower than the first voltage.

19. The image sensor according to claim 18 , wherein the photodiode limiter comprises:

a first multiplexor having a pair of inputs receiving the first and second voltages, respectively;

a second multiplexor having a pair of inputs coupled to an output of the first multiplexor and a third voltage lower than the second voltage that turns off the transistor; and

a control circuit configured to control the first and second multiplexors to select one of the first, second and third voltages to be applied to the gate of the transistor.

20. A method for operating an image sensor including a pixel having a photodiode that generates a charge in response to light and a transfer circuit that transfers the charge to a storage circuit to generate an output voltage for the image, the method comprising:

applying a first voltage level to a shutter circuit coupled to the pixel to reset a photodiode voltage of the photodiode;

applying a second voltage level to the shutter circuit while the photodiode is generating the charge to maintain the photodiode voltage at a threshold voltage to prevent at least one of overload of the output voltage and erroneous activation of the transfer circuit; and

applying the second voltage level to the shutter circuit as a pulsed voltage while the photodiode is generating the charge to set the voltage threshold and control a limiting level of a voltage differential between a supply voltage and the output voltage during an integration time.

21. The method according to claim 20 , further comprising limiting, by the shutter circuit, a voltage differential between a supply voltage and the photodiode voltage during integration of the photodiode.

22. The method according to claim 20 , further comprising:

applying a first voltage to the shutter circuit to reset the photodiode voltage; and

applying a subsequent second voltage to the shutter circuit to set the threshold voltage as the second voltage, wherein the second voltage is lower than the first voltage.