Threshold-monitoring, conditional-reset image sensor

• US 10,136,090 B2
• Filed: 03/14/2014
• Issued: 11/20/2018
• Est. Priority Date: 03/15/2013
• Status: Active Grant

• Current Assignee: Rambus Inc.
• Original Assignee: Rambus Inc.
• Inventors: Vogelsang, Thomas, Guidash, Michael, Xue, Song, Smirnov, Maxim, Smith, Craig M., Endsley, Jay, Harris, James E.
• Primary Examiner(s): Haskins, Twyler L
• Assistant Examiner(s): Tejano, Dwight Alex C
• Application Number:
  US14/772,311
• Publication Number:
  US 20160028985A1
• Time in Patent Office: 1,712 Days
• CPC Class Codes: H01L 27/14634 Assemblies, i.e. Hybrid structures
• 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 25/46 by combining or binning pixels
• CPC Class Codes: H04N 25/57 Control of the dynamic range
• CPC Class Codes: H04N 25/74 Circuitry for scanning or addressing the pixel array
• CPC Class Codes: H04N 25/77 Pixel circuitry, e.g. memories, A/D converters, pixel amplifiers, shared circuits or shared components
• CPC Class Codes: H04N 25/779 Circuitry for scanning or addressing the pixel array

Abstract

An image sensor architecture with multi-bit sampling is implemented within an image sensor system. A pixel signal produced in response to light incident upon a photosensitive element is converted to a multiple-bit digital value representative of the pixel signal. If the pixel signal exceeds a sampling threshold, the photosensitive element is reset. During an image capture period, digital values associated with pixel signals that exceed a sampling threshold are accumulated into image data.

Claims

1. An integrated-circuit image sensor comprising:

a capacitive floating diffusion node;

first and second photosensitive elements switchably coupled to the capacitive floating diffusion node via first and second transfer gates, respectively;

readout circuitry coupled to the floating diffusion node and the first and second transfer gates to;

determine during a first interval whether a collective level of accumulated photocharge within the first and second photosensitive elements exceeds a read-out threshold, including applying a partial-read voltage to the first and second transfer gates during the first interval; and

apply a full-read voltage to the first and second transfer gates during a second interval in response to determining during the first interval that the collective level of accumulated photocharge exceeds the read-out threshold, the full-read voltage exceeding the partial-read voltage;

wherein the readout circuitry to apply the partial-read voltage to the first and second transfer gates during the first interval comprises circuitry to apply a voltage level to the first transfer gate that enables photocharge transfer from the first photosensitive element to the first floating diffusion node only if accumulated photocharge within the first photosensitive element exceeds a first charge level.

2. The integrated-circuit image sensor of claim 1 wherein if the accumulated photocharge exceeds the first charge level, application of the voltage level to the first transfer gate enables a limited portion of the accumulated photocharge within the first photosensitive element to be transferred to the first floating diffusion node such that a residual portion of the accumulated photocharge remains within the first photosensitive element at the conclusion of the first interval.

3. The integrated-circuit image sensor of claim 2 wherein the readout circuitry to apply the full-read voltage to the first and second transfer gates during the second interval comprises circuitry to apply a voltage level to the first transfer gate sufficient to enable the residual portion of the accumulated photocharge within the first photosensitive element to be transferred to the first floating diffusion node.

4. The integrated-circuit image sensor of claim 1 wherein the readout circuitry to apply the partial-read voltage to the first and second transfer gates during the first interval further comprises circuitry to apply a voltage level to the second transfer gates that enables photocharge transfer from the second photosensitive element to the first floating diffusion node only if accumulated photocharge within the second photosensitive element exceeds the first charge level.

5. An integrated-circuit image sensor comprising:

a capacitive floating diffusion node;

first and second photosensitive elements switchably coupled to the capacitive floating diffusion node via first and second transfer gates , respectively;

readout circuitry coupled to the floating diffusion node and the first and secod transfer gates to;

determine during a first interval whether a collective level of accumulated photocharge within the first and second photosensitive elements exceeds a readout threshold, including applying a partial-read voltage to the first and second transfer gates during the first interval; and

apply a full-read voltage to the first and second transfer gates during a second interval in response to determining during the first interval that the collective level of accumulated photocharge exceeds the read-out threshold, the full-read voltage exceeding the partial-read voltage;

wherein the readout circuitry comprises control circuitry to enable an output signal representative of a charge level of the first floating diffusion node to be driven continuously onto a shared output line during a readout interval that spans the first and second intervals, and wherein the readout circuitry to determine whether the collective level of photocharge exceeds the readout threshold comprises comparator circuitry to compare the output signal to the readout threshold.

6. The integrated-circuit image sensor of claim 5 further comprising a pixel array having a plurality of photosensitive elements in addition to the first and second photosensitive elements, and wherein the control circuitry to enable the output signal to be driven continuously onto the shared output line during the readout interval comprises read-select circuitry to enable the output signal to be driven onto a column output line that extends column-wise across the pixel array, and wherein the readout circuitry to apply the full-read voltage to the first and second transfer gates during the second interval in response to determining that the collective level of accumulated photocharge exceeds the readout threshold comprises:

first row control circuitry to assert a first transfer-gate control pulse that propagates row-wise across the pixel array during the second interval; and

first column control circuitry to assert or deassert a second transfer-gate control pulse that, if asserted, propagates column-wise across the pixel array to effect, in combination with the first transfer-gate control pulse, application of the full-read voltage to the first transfer gate.

7. The integrated-circuit image sensor of claim 6 wherein the readout circuitry to apply the full-read voltage to the first and second transfer gates during the second interval in response to determining that the collective level of accumulated photocharge exceeds the readout threshold further comprises second row control circuitry to assert a third transfer-gate control pulse that propagates row-wise across the pixel array during the second interval and that effects, in combination with the second transfer-gate control pulse, application of the full-read voltage to the second transfer gate.

8. The integrated-circuit image sensor of claim 6 wherein the readout circuitry to apply the full-read voltage to the first and second transfer gates during the second interval in response to determining that the collective level of accumulated photocharge exceeds the readout threshold further comprises second column control circuitry to assert or deassert a third transfer-gate control pulse that, if asserted, propagates column-wise across the pixel array to effect, in combination with the first transfer-gate control pulse, application of the full-read voltage to the second transfer gate.

9. A method of operation within an integrated-circuit image sensor having a first floating diffusion node switchably coupled to first and second photosensitive elements via respective first and second transfer gates, the method comprising:

determining during a first interval whether a collective level of accumulated photocharge within the first and second photosensitive elements exceeds a readout threshold, including applying a partial-read voltage to the first and second transfer gates during the first interval;

applying a full-read voltage to the first and second transfer gates during a second interval in response to determining during the first interval that the collective level of accumulated photocharge exceeds the readout threshold, the full-read voltage exceeding the partial-read voltage;

wherein applying the partial-read voltage to the first and second transfer gates during the first interval comprises applying a voltage level to the first transfer gate that enables photocharge transfer from the first photosensitive element to the first floating diffusion node only if accumulated photocharge within the first photosensitive element exceeds a first charge level.

10. The method of claim 9 further comprising:

resetting the first photosensitive element during a reset interval that precedes the first interval;

enabling the first photosensitive element to accumulate photocharge between the reset interval and the first interval; and

wherein applying the voltage level to the first transfer gate that enables photocharge transfer from the first photosensitive element to the first floating diffusion node only if accumulated photocharge within the first photosensitive element exceeds the first charge level comprises applying a voltage level to the first transfer gate that is insufficient to enable an entirety of the photocharge accumulated within the first photosensitive element between the reset interval and the first interval to be transferred to the first floating diffusion node such that a residual portion of the photocharge remains within the first photosensitive element.

11. The method of claim 10 wherein applying the full-read voltage to the first and second transfer gates during the second interval comprises applying a voltage level to the first transfer gate sufficient to enable the residual portion of the photocharge to be transferred to the first floating diffusion node.

12. The method of claim 9 wherein, if the accumulated photocharge exceeds the first charge level, applying the voltage level to the first transfer gate enables a limited portion of the accumulated photocharge within the first photosensitive element to be transferred to the first floating diffusion node such that a residual portion of the accumulated photocharge remains within the first photosensitive element at the conclusion of the first interval.

13. The method of claim 12 wherein applying the full-read voltage to the first and second transfer gates during the second interval comprises applying a voltage level to the first transfer gate sufficient to enable the residual portion of the accumulated photocharge within the first photosensitive element to be transferred to the first floating diffusion node.

14. The method of claim 9 wherein applying the partial-read voltage to the first and second transfer gates during the first interval further comprises applying a voltage level to the second transfer gate that enables photocharge transfer from the second photosensitive element to the first floating diffusion node only if accumulated photocharge within the second photosensitive element exceeds the first charge level.

15. The method of claim 9 further comprising enabling an output signal representative of a charge level of the first floating diffusion node to be driven continuously onto a shared output line during a readout interval that spans the first and second intervals, and wherein determining whether the collective level of photocharge exceeds the readout threshold comprises comparing the output signal to the readout threshold.

16. The method of claim 15 wherein the first and second photosensitive elements are disposed together with other photosensitive elements within a pixel array, and wherein enabling the output signal to be driven onto the shared output line comprises enabling the output signal to be driven onto a column output line that extends across column-wise across the pixel array, and wherein applying the full-read voltage to the first and second transfer gates during the second interval in response to determining that the collective level of accumulated photocharge exceeds the readout threshold comprises:

asserting a first transfer-gate pulse that propagates row-wise across the pixel array during the second interval; and

asserting or deasserting a second transfer-gate pulse during the second interval according to whether the output signal exceeds the threshold, the second transfer-gate control pulse, if asserted, propagating column-wise across the pixel array and effecting, in combination with the first transfer-gate control pulse, application of the full-read voltage to the first transfer gate.

17. The method of claim 16 wherein applying the full-read voltage to the first and second transfer gates during the second interval in response to determining that the collective level of accumulated photocharge exceeds the readout threshold further comprises asserting a third transfer-gate pulse that propagates row-wise across the pixel array during the second interval and that effects, in combination with the second transfer-gate control pulse, application of the full-read voltage to the second transfer gate.