EXPOSURE CONTROL FOR IMAGE SENSORS

• US 20140247378A1
• Filed: 03/01/2013
• Published: 09/04/2014
• Est. Priority Date: 03/01/2013
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Sharma, Anup K., Fan, Xiaofeng, Li, Xiangli, Wan, Chung Chun, Lee, Chiajen, Gilton, Terry L.
• Granted Patent: US 9,293,500 B2
• Time in Patent Office: Days
• US Class Current: 348/280
• CPC Class Codes: H01L 27/14641 Electronic components shared by two or more pixel-elements, e.g. one amplifier shared by two pixel elements
• CPC Class Codes: H04N 25/58 involving two or more exposures
• CPC Class Codes: H04N 25/621 for the control of blooming
• CPC Class Codes: H04N 25/771 comprising storage means other than floating diffusion

Abstract

A method of operating an image sensor. Charge accumulated in a photodiode during a first sub-exposure may be selectively stored in a storage node responsive to a first control signal. Charge accumulated in the photodiode during a first reset period may be selectively discarded responsive to a second control signal. Charge accumulated in the photodiode during a second sub-exposure may be selectively stored responsive to the first control signal. Charge stored in the storage node from the first and second sub-exposures may be transferred to a floating diffusion node responsive to a third control signal.

Claims

1. A method of operating an image sensor, comprising:

selectively storing, responsive to a first control signal, charge accumulated in a photodiode during a first sub-exposure in a storage node;

selectively discarding, responsive a second control signal, charge accumulated in the photodiode during a first reset period;

selectively storing, responsive to the first control signal, charge accumulated in the photodiode during a second sub-exposure in the storage node; and

transferring, responsive to a third control signal, charge stored in the storage node from the first and second sub-exposures to a floating diffusion node.

2. The method of claim 1, wherein the first control signal comprises a transfer signal provided to a storage gate coupled between the photodiode and the storage node.

3. The method of claim 2, wherein the second control signal comprises an anti-blooming signal provided to an anti-blooming gate coupled to the photodiode.

4. The method of claim 2, wherein the second control signal is the transfer signal provided to the storage gate.

5. The method of claim 2, wherein the third control signal comprises a transfer signal provided to a transfer gate coupled between the storage node and the floating diffusion node.

6. The method of claim 1, further comprising selectively storing, responsive to the first control signal, charge accumulated in the photodiode during third and fourth sub-exposures in the storage node, the first, second, third, and fourth sub-exposures corresponding to a single frame of the image sensor.

7. The method of claim 1, further comprising resetting the photodiode during the first reset period responsive to a reset signal.

8. The method of claim 1, wherein a respective duration of respective first and second sub-exposures is substantially the same.

9. The method of claim 1, wherein a first duration of the first sub-exposure is substantially the same as a second duration of the first reset period.

10. The method of claim 1, wherein a first duration of the first sub-exposure is substantially less than a second duration of the first reset period.

11. The method of claim 10, wherein the second duration is at least 5 times that of the first duration.

12. An image sensor pixel, comprising:

a photodiode;

a storage node;

a storage gate coupled between the photodiode and storage node, the storage gate selectively receiving an exposure control signal and being responsive thereto;

a floating diffusion node;

a transfer gate coupled between the storage node and floating diffusion node, the transfer gate selectively receiving a transfer control signal and being responsive thereto; and

control circuitry configured to provide the exposure control signal to the storage gate such that the charge accumulated in the photodiode during a plurality of sub-exposures is transferred to the storage node after or during each sub-exposure, the control circuitry further configured to provide the transfer signal to the transfer gate following the a plurality of sub-exposures such that charge stored in the storage node is transferred to the floating diffusion node.

13. The image sensor pixel of claim 12, further comprising an anti-blooming gate coupled to the photodiode and configured to drain excess charge from the photodiode responsive to an anti-blooming control signal.

14. The image sensor pixel of claim 12, wherein the storage node is electrically and optically shielded.

15. The image sensor pixel of claim 12, wherein the image sensor pixel is associated with a first color channel, further wherein another image sensor pixel associated with a different color channel receives the same first, second, or third control signals as the image sensor pixel.

16. A method of operating an image sensor, comprising:

selectively accumulating charge in a first photodiode associated with a first color channel during a first period of time responsive to a first exposure control signal; and

selectively accumulating charge in a second photodiode associated with a second color channel during a second period of time responsive to a second exposure control signal;

wherein the first color channel is different than the second color channel and the first period of time is shorter than the second period of time, the first and second periods of time being at least partially contemporaneous with each other.

17. The method of claim 16, further comprising draining charge away from the first photodiode before the first period of time begins but after the second period of time has started.

18. The method of claim 17, wherein the charge is drained away responsive to providing an anti-blooming signal to a transfer gate coupled to the first photodiode.

19. The method of claim 17, wherein the second photodiode continues to accumulate charge while first photodiode begins to accumulate charge.

20. The method of claim 17, further comprising transferring the accumulated charge from the first photodiode to a first floating diffusion or storage node at substantially the same time as transferring charge from the second diode to a second floating diffusion or storage node.

21. The method of claim 16, further comprising resetting the first and second photodiodes responsive to a reset signal before accumulating charge during the first and second periods of time.

22. An image sensor, comprising:

a first pixel associated with a first color channel and configured to accumulate charge for a first period of time responsive to a first control signal; and

a second pixel associated with a second color channel and configured to accumulate charge for a second period of time responsive to a second control signal;

wherein the first color channel is different than the second color channel and the first period of time is shorter than the second period of time.

23. The image sensor of claim 22, wherein the first color channel is clear and the second color channel is green.

24. The image sensor of claim 22, wherein the first color channel is green and the second color channel is red.

25. The image sensor of claim 24, further comprising an anti-blooming gate configured to drain excess charge away from the first pixel responsive to an anti-blooming signal.

26. The image sensor of claim 22, wherein the anti-blooming gate comprises a transfer gate coupled between the photodiode and a floating diffusion or a storage node.

27. The image sensor of claim 22, wherein the first pixel is a global shutter pixel and comprises a storage node.

28. The image sensor of claim 22, wherein the first pixel is a rolling shutter pixel.

29. The image sensor of claim 22, further comprising control circuitry for providing the first and second control signals, the control circuitry comprising one or more multiplexers configured to selectively provide one of a plurality of voltage levels as the first or second control signals.

30. A method of operating an image sensor, comprising:

selectively accumulating charge in a first photodiode associated with a first color channel during a first plurality of sub-exposures;

selectively transferring, responsive to a first control signal, charge accumulated in the first photodiode during each of the first plurality of sub-exposures to a first node;

selectively accumulating charge in a second photodiode associated with a second color channel during a second plurality of sub-exposures; and

selectively transferring, responsive to a second control signal, charge accumulated in the first photodiode during each of the second plurality of sub-exposures to a second node;

wherein the first color channel is different than the second color channel and at least one of the first plurality of sub-exposures is shorter than at least one of the second plurality of sub-exposures, the first and second plurality of exposures being at least partially contemporaneous.

31. The method of claim 30, wherein the image sensor is operated in an interleaved high dynamic range mode, and the first color channel is associated with a first set of pixels forming a first image, and the second color channel is associated with a second set of pixels forming a second image, the first and second images being acquired by the image sensor at substantially overlapping times.

32. The method of claim 30, wherein the image sensor has a rolling shutter architecture.

33. The method of claim 30, wherein the image sensor has a global shutter architecture.

34. The method of claim 30, wherein the image sensor is operated in a video mode.

35. The method of claim 30, wherein the first and second nodes respectively comprise first and second storage nodes.

36. A method of operating an image sensor, comprising:

partitioning a first exposure of a first pixel associated with a first color channel into a first plurality of sub-exposures, each of the first plurality of sub-exposures having a respective duration; and

partitioning a second exposure of a second pixel associated with a second color channel into a second plurality of sub-exposures, each of the second plurality of sub-exposures having a respective duration;

wherein the duration of at least one of the first plurality of sub-exposures is shorter than at least one of the second plurality of sub-exposures.

37. The method of claim 36, wherein all of the first plurality of sub-exposures have substantially a first duration and all of the second plurality of sub-exposures have substantially a second duration, the first duration being shorter than the second duration.

38. The method of claim 36, wherein the first color channel is clear or green, and the second channel is red or blue.

39. The method of claim 36, wherein a third exposure of a third pixel associated with a third color channel is not partitioned.

40. The method of claim 39, wherein the third color channel is red.

41. The method of claim 39, wherein the respective durations of the first and second plurality of sub-exposures is determined based on the relative sensitivity of the first and second color channels to the third color channel.

42. The method of claim 36, further comprising analog summing charge accumulated in each of the first plurality of sub-exposures in a first storage node and analog summing charge accumulated in each of the second plurality of sub-exposures in a second storage node.