Pinned photodiode CMOS image sensor with a low supply voltage
A device for controlling an image sensor including at least one photosensitive cell including a photodiode capable of discharging into a sense node via a first MOS transistor, the sense node being connected to the gate of a second MOS transistor having its source connected to a processing system. The device includes a bias circuit capable of increasing the voltage of the source during the discharge of the photodiode into the sense node.
1. A circuit for a multi-pixel photosensor comprising a voltage bias circuit operatively adapted to alter a voltage on at least one read column coupled to at least one photosensitive cell, wherein the voltage bias circuit is configured to alter the voltage during charge transfer from a photodiode to a sensing node within the at least one photosensitive cell, and the voltage bias circuit is configured to change a bias of the sensing node during a reset period of the sensing node to a voltage level lower than a voltage level of the sensing node at a start of the reset period.
2. The circuit of claim 1 , wherein the bias circuit further comprises:
a switch operatively connected to the at least one read column;
a clamping voltage operatively connected to the switch; and
a current source operatively connected to the at least one read column.
3. The circuit of claim 1 , wherein the voltage bias circuit is configured to cause:
a first adjustment in the voltage from a first value to a second value prior to the charge transfer; and
a second adjustment in the voltage after charge transfer, the second adjustment made prior to reading a voltage value from the at least one read column representative of a number of charges transferred to the sensing node from the photodiode.
4. The circuit of claim 1 , wherein the multi-pixel photosensor comprises an imaging array of a camera, camcorder, cellular phone, or digital photographic device.
5. A method for operating at least one photosensitive cell of a multi-pixel photosensor, the method comprising:
altering a voltage on at least one read column coupled to the at least one photosensitive cell during charge transfer from a photodiode to a sensing node within the at least one photosensitive cell; and
changing a bias of the sensing node during a reset period of the sensing node to a voltage level lower than a voltage level of the sensing node at a start of the reset period.
6. The method of claim 5 , further comprising:
adjusting the voltage from a first value to a second value prior to the charge transfer;
changing the voltage after charge transfer; and
reading a column voltage value from the at least one read column after the act of changing the voltage, wherein the column voltage value is representative of a number of charges transferred to the sensing node from the photodiode.
7. The method of claim 5 , further comprising:
adjusting a reference voltage of a first MOS transistor from a first value to a second value prior to the charge transfer; and
changing the reference voltage after charge transfer,
wherein the first MOS transistor operatively couples the reference voltage and the sensing node.