
KAI−0373
www.onsemi.com
4
Output Structure
Charge packets contained in the horizontal register are
dumped pixel by pixel, onto the floating diffusion output
node whose potential varies linearly with the quantity of
charge in each packet. The amount of potential change is
determined by the expression DVFD =DQ/C
FD. A three
stage source-follower amplifier is used to buffer this signal
voltage off chip with slightly less than unity gain.
The translation from the charge domain to the voltage
domain is quantified by the output sensitivity or charge to
voltage conversion in terms of mV/e−. After the signal has
been sampled off-chip, the reset clock (fR) removes the
charge from the floating diffusion and resets its potential to
the reset-drain voltage (VRD).
Figure 3. Output Structure
FD = Floating Diffusion
FD
VOUT
DD
fR
VRD
SUB
WELL
VLG
VSS
Electronic Shutter
The KAI−0373 provides a structure for the prevention of
blooming which may be used to realize a variable exposure
time as well as performing the anti-blooming function.
The anti-blooming function limits the char ge capacity of the
photodiode by draining excess electrons vertically into the
substrate (hence the name Vertical Overflow Drain or
VOD). This function is controlled by applying a large
potential to the device substrate (device terminal SUB). If
a sufficiently l a rge voltage pulse (VES ≈ 40 V) is applied to
the substrate, all photodiodes will be emptied of charge
through the substrate, beginning the integration period.
After returning the substrate voltage to the nominal value,
charge can accumulate in the diodes and the charge packet
is subsequently readout onto the VCCD at the next
occurrence o f the high level on fV2. The integration time is
then the time between the falling edges of the substrate
shutter pulse and fV2. This scheme allows electronic
variation of the exposure time by a variation in the clock
timing while maintaining a standard video frame rate.
Application of the large shutter pulse must be avoided
during the horizontal register readout or an image artifact
will appear due to feedthrough. The shutter pulse VES must
be “hidden” in the horizontal retrace interval.
The integration time is changed by skipping the shutter
pulse from one horizontal retrace interval to another.
The smear specification is not met under electronic shutter
operation. Under constant light intensity and spot size, if the
electronic exposure time is decreased, the smear signal will
remain the same while the image signal will decrease
linearly with exposure. Smear is quoted as a percentage of
the image signal and so the percent smear will increase by
the same factor that the integration time has decreased. This
effect is basic to interline devices.
Extremely bright light can potentially harm solid state
imagers such as Charge-Coupled Devices (CCDs). Refer to
Application Note Using Interline CCD Image Sensors in
High Intensity Visible Lighting Conditions.