Image sensor with switchable in-pixel binning during readout
First and second readout circuits, each having a respective floating diffusion node, are coupled to a photodetection element within a pixel of an integrated-circuit image sensor. Following an exposure interval in which photocharge is accumulated within the photodetection element, a first portion of the accumulated photocharge is transferred from the photodetection element to the first floating diffusion node to enable generation of a first output signal within the first readout circuit, and a second portion of the accumulated photocharge is transferred from the photodetection element to the second floating diffusion node to enable generation of a second output signal within the second readout circuit. A digital pixel value is generated based on the first and second output signals.
1. An integrated-circuit image sensor, comprising:
a photodetection element to accumulate a photocharge;
first and second floating diffusion nodes switchably coupled to the photodetection element via first and second transfer gates, respectively;
an in-pixel binning transistor that switchably couples the first and second floating diffusion nodes; and
readout control circuitry to:
switch on the first transfer gate to enable transfer of a first portion of the photocharge from the photodetection element to the first floating diffusion node;
switch on the second transfer gate to enable transfer of a second portion of the photocharge from the photodetection element to the second floating diffusion node;
generate a first readout value based on the first portion of the photocharge in the first floating diffusion node;
switch on the in-pixel binning transistor to create a composite floating diffusion node that combines capacitances of the first and second floating diffusion nodes; and
generate a second readout value based on the second portion of the photocharge in the composite floating diffusion node.
2. The integrated-circuit image sensor of claim 1 , wherein the in-pixel binning transistor is controlled by a pixel control signal that is applied to all pixels of a pixel row.
3. The integrated-circuit image sensor of claim 1 , wherein:
the readout control circuitry includes a first readout channel and a second readout channel;
the first readout value is generated via the first readout channel; and
the second readout value is generated via the second readout channel.
4. The integrated-circuit image sensor of claim 3 , wherein:
the first readout channel generates the first readout value as a high-conversion-gain value; and
the first readout channel generates the second readout value as a low-conversion-gain value associated with a lower conversion gain than the high-conversion-gain value.
5. The integrated-circuit image sensor of claim 3 , wherein the readout control circuitry comprises:
a second binning transistor that switchably couples output lines of the first and second readout channels.
6. The integrated-circuit image sensor of claim 5 , wherein the second binning transistor is a column-binning transistor controlled by a column readout logic control signal.
7. The integrated-circuit image sensor of claim 6 , wherein the in-pixel binning transistor and the column-binning transistor are controlled by two different control signals.
8. The integrated-circuit image sensor of claim 6 , wherein the readout control circuitry implements three or more different readout modes using the two different control signals.
9. The integrated-circuit image sensor of claim 3 , wherein:
the in-pixel binning transistor is dedicated to the first readout channel;
the readout control circuitry includes a second in-pixel binning transistor dedicated to the second readout channel; and
the in-pixel binning transistor and the second in-pixel binning transistor are coupled to a reset transistor and controlled by separate control signals to enable selective reset of the first or second floating diffusion node.
10. The integrated-circuit image sensor of claim 9 , wherein:
the in-pixel binning transistor, the second in-pixel binning transistor, and the reset transistor are coupled via an interconnect; and
the interconnect is coupled to a capacitive node that increases a full-well capacity of the composite floating diffusion node.
11. A method of operation of an integrated-circuit image sensor, comprising:
accumulating a photocharge within a photodetection element;
operating readout control circuitry to generate a first readout value and a second readout value from the photodetection element, including:
switching on a first transfer gate to enable transfer of a first portion of the photocharge from the photodetection element to a first floating diffusion node;
switching on a second transfer gate to enable transfer of a second portion of the photocharge from the photodetection element to a second floating diffusion node;
generating the first readout value based on the first portion of the photocharge in the first floating diffusion node;
switching on an in-pixel binning transistor between the first and second floating diffusion nodes to create a composite floating diffusion node that combines capacitances of the first and second floating diffusion nodes; and
generating the second readout value based on the second portion of the photocharge in the composite floating diffusion node.
12. The method of claim 11 , wherein the in-pixel binning transistor is controlled by a pixel control signal that is applied to all pixels of a pixel row.
13. The method of claim 11 , wherein:
the readout control circuitry includes a first readout channel and a second readout channel;
the first readout value is generated via the first readout channel; and
the second readout value is generated via the second readout channel.
14. The method of claim 13 , further comprising:
generating the first readout value as a high-conversion-gain value; and
generating the second readout value as a low-conversion-gain value associated with a lower conversion gain than the high-conversion-gain value.
15. The method of claim 13 , further comprising:
operating a second binning transistor to switchably couple output lines of the first and second readout channels.
16. The method of claim 15 , wherein the second binning transistor is a column-binning transistor controlled by a column readout logic control signal.
17. The method of claim 16 , wherein the in-pixel binning transistor and the column-binning transistor are controlled by two different control signals.
18. The method of claim 16 , further comprising:
operating the readout control circuitry in three or more different readout modes using the two different control signals.
19. The method of claim 13 , wherein:
the in-pixel binning transistor is dedicated to the first readout channel;
the readout control circuitry includes a second in-pixel binning transistor dedicated to the second readout channel; and
the method further comprises selective resetting the first or second floating diffusion node based on controlling the in-pixel binning transistor and the second in-pixel binning transistor using separate control signals.
20. The method of claim 19 , wherein:
the in-pixel binning transistor and the second in-pixel binning transistor are coupled via an interconnect; and
the method further comprises controlling a capacitive node coupled to the interconnect to increases a full-well capacity of the composite floating diffusion node.