KAI-4021 datasheet(48/56 Pages) ONSEMI

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KAI-4021 Image Sensor

www.truesenseimaging.com

Revision 1.1 PS-0014 Pg 48

Electronic Shutter Description

The voltage on the substrate (SUB) determines the charge capacity of the photodiodes. When SUB is 8 volts the

photodiodes will be at their maximum charge capacity. Increasing VSUB above 8 volts decreases the charge capacity of

the photodiodes until 48 volts when the photodiodes have a charge capacity of zero electrons. Therefore, a short pulse

on SUB, with a peak amplitude greater than 48 volts, empties all photodiodes and provides the electronic shuttering

action.

It may appear the optimal substrate voltage setting is 8 volts to obtain the maximum charge capacity and dynamic

range. While setting VSUB to 8 volts will provide the maximum dynamic range, it will also provide the minimum

antiblooming protection.

The KAI-4021 VCCD has a charge capacity of 60,000 electrons (60 ke

-). If the SUB voltage is set such that the

photodiode holds more than 60 ke

-, then when the charge is transferred from a full photodiode to VCCD, the VCCD will

overflow. This overflow condition manifests itself in the image by making bright spots appear elongated in the vertical

direction. The size increase of a bright spot is called blooming when the spot doubles in size. The blooming can be

eliminated by increasing the voltage on SUB to lower the charge capacity of the photodiode. This ensures the VCCD

charge capacity is greater than the photodiode capacity. There are cases where an extremely bright spot will still cause

blooming in the VCCD. Normally, when the photodiode is full, any additional electrons generated by photons will spill

out of the photodiode. The excess electrons are drained harmlessly out to the substrate. There is a maximum rate at

which the electrons can be drained to the substrate. If that maximum rate is exceeded, (for example, by a very bright

light source) then it is possible for the total amount of charge in the photodiode to exceed the VCCD capacity. This

results in blooming. The amount of antiblooming protection also decreases when the integration time is decreased.

There is a compromise between photodiode dynamic range (controlled by VSUB) and the amount of antiblooming

protection. A low VSUB voltage provides the maximum dynamic range and minimum (or no) antiblooming protection. A

high VSUB voltage provides lower dynamic range and maximum antiblooming protection. The optimal setting of VSUB

is written on the container in which each KAI-4021 is shipped. The given VSUB voltage for each sensor is selected to

provide antiblooming protection for bright spots at least 100 times saturation, while maintaining at least 40ke- of

dynamic range.

The electronic shutter provides a method of precisely controlling the image exposure time without any mechanical

components. If an integration time of TINT is desired, then the substrate voltage of the sensor is pulsed to at least 40

volts TINT seconds before the photodiode to VCCD transfer pulse on V2. Use of the electronic shutter does not have to

wait until the previously acquired image has been completely read out of the VCCD.

KAI-4021 Datasheet(PDF) 48 Page - ON Semiconductor