Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-05-23
2004-10-12
Smith, Zandra V. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100
Reexamination Certificate
active
06803553
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of integrated image sensor circuits.
2. Description of the Related Art
An integrated image sensor is used to convert light impinging on the sensor into electrical signals. An image sensor typically includes one or more (e.g., an array of) light-sensing photoelements such as photodiodes, phototransistors, or photodetectors, where electrical signals are generated via the well-known photoelectric effect. These signals may then be used, for example, to provide information about light intensity, color, or the optical image focused on the sensor. One common type of image sensor is a CMOS image sensor.
FIG. 1
shows a block diagram for a CMOS image sensor
100
implemented in a single integrated circuit or chip. Sensor
100
comprises a photoelement array
102
, a decoding/buffer area
104
, and control, processing, and input/output (I/O) circuitry
106
. Photoelement array
102
comprises an array of photoelements and associated circuitry such as switches and amplifiers. Each photoelement and its associated circuitry are collectively referred to as a pixel.
Sensor
100
can be switched into one of three operating modes: normal operation, standby, and power down. A typical operating cycle for sensor
100
in the normal operation mode has three steps: acquisition or integration, read-out, and reset During the integration step, incident photons generate electrical charge that is accumulated within each photoelement in photoelement array
102
where the voltage across the photoelement is changed due to this charge accumulation. During the read-out step, the voltage is read out passively, i.e., without destroying the accumulated charge. During the reset step, the accumulated charge is drained from the photoelement thereby preparing it to receive photons during the next operating cycle. During the standby mode, if light is impinging on photoelement array
102
, the photoelements are converting light to current but the charge is not accumulated. During the standby mode, the associated circuitry within photoelement array
102
and the circuitry within blocks
104
and
106
used to operate the photoelements are typically powered up with only certain biases and control signals turned off so that the sensor can be quickly switched to the normal operation mode. During the power down mode, power is completely removed from the sensor.
FIG. 2
shows a schematic circuit diagram of part of integrated CMOS image sensor
100
of FIG.
1
.
FIG. 2
shows a pixel
202
of photoelement array
102
of
FIG. 1
, a voltage source V
dd
204
, a bias transistor
206
, and a read-out circuit
208
. Bias transistor
206
and read-out circuit
208
are typically implemented in blocks
104
and/or
106
of FIG.
1
. Also shown are devices
210
that draw power from the same voltage source
204
. Devices
210
may include but are not limited to on-chip circuitry, such as in blocks
104
and
106
of FIG.
1
. Pixel
202
comprises a reset transistor
212
, a photoelement
214
, a source follower transistor
216
, and a row select transistor
218
.
During an integration step of the normal operation mode, reset transistor
212
and row select transistor
218
are turned off, and the voltage across photoelement
214
discharges in response to the incident light. After a predetermined integration time, transistor
218
is turned on to select the particular photoelement in the array and to initiate a read-out step of the normal operation mode. Read-out circuit
208
samples the gate voltage of transistor
216
. During a reset step of the normal operation mode, reset transistor
212
is turned on, which drains the charge accumulated by photoelement
214
into V
dd
204
. This charge is thus irreversibly lost. During the standby mode, transistors
212
and
218
are turned off, but power is still applied to circuitry within sensor
100
(e.g., read-out circuit
208
) and possibly devices
210
. During the power down mode, power supply
Vdd
204
is disconnected from sensor
100
.
Power consumption is important for portable and embedded imaging applications since the sensor is typically powered by a portable power source such as a battery. During the standby mode, the sensor consumes power due to off currents or other charge leakage through read-out circuit
208
, devices
210
, and/or other circuitry within sensor
100
. This problem may become particularly serious for CMOS technology as it scales down to deep submicron levels due to increasingly higher doping densities, thinner oxide layers, and shorter channel lengths. Often, standby power consumption will limit battery life and restrict use of image sensors.
SUMMARY OF THE INVENTION
Embodiments of the present invention are directed to a technique for reducing power consumption in integrated image sensors by which at least some of the charge generated by the sensor (e.g., during the sensor's normal operating mode) is used to power circuitry (e.g., during the sensor's standby mode), instead of drawing the entire current from a power source such as the external battery used to power the image sensor.
According to one embodiment, the present invention is a method for operating an integrated circuit having an image sensor with at least one photoelement comprising the steps of (a) generating charge by the photoelement and (b) applying at least a first portion of the charge generated by the photoelement to other circuitry to reduce consumption of power from a power supply.
According to another embodiment, the present invention is a circuit comprising (a) an image sensor having at least one photoelement implemented in an integrated circuit and (b) other circuitry, wherein the photoelement generates charge that is applied to the other circuitry to reduce consumption of power from a power supply.
REFERENCES:
patent: 3986176 (1976-10-01), Weimer
patent: 5900623 (1999-05-01), Tsang et al.
patent: 6008690 (1999-12-01), Takeshima et al.
patent: 6222390 (2001-04-01), Huang
No affiliations
Gruzdkov Yuri
Smith Zandra V.
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