SN28835

1/2-INCH NTSC TIMER

SOCS023B – FEBRUARY 1991

5

POST OFFICE BOX 655303

variable-integration-time mode

In addition to the normal TV mode of operation, the SN28835 timing generator offers an optional

variable-integration mode for use with the TC244 and TC245 CCD area-array image sensors. The

variable-integration mode is selected by applying a low-logic level to GPS. This low-logic level disables the

vertical-drive (VD) signal from controlling, internal to the timer, the image-area and storage-area parallel transfer

signal (GP).

Prior to the start of a new integration period, the charge that has accumulated in the image area must be

transferred out. To flush this previous signal or dark-current charge from the image area, GP is pulsed low two

times. Each low pulse generates 256 image-area and storage-area gate and transfer signals that shift the

unwanted charge into the clearing drain. This clearing function should be performed during the high time of the

VD signal (see Figure 3 through Figure 12).

The new integration period continues as long as GP remains high. GPS must be held at a low-logic level to

prevent VD from controlling GP internally. The integration ceases and the readout occurs when VD and GP are

pulsed low simultaneously; this is accomplished by taking GPS to a high-logic level. The readout timing is

dependent on the vertical-drive pulse; this means that the total-integration time is a multiple of 1/60 of a second

plus the time interval between the last GP low pulse and the next VD low pulse. The image readout occurs within

the normal 1/60-second readout interval. If the integration time is less than 1/60 of a second, normal output

operation occurs; if the integration time is greater than 1/60 of a second, a frame buffer may be required to

capture the image.

Integration times greater than 1/60 of a second result in image degradation at temperatures greater than 25

°C

due to dark-current generation. The degradation is seen as a decrease in dynamic range (contrast) and an

increase in noise. It is recommended that the image sensor be cooled for long-exposure operation. The

dark-current generation is reduced by a factor of two for each 7

°C temperature decrease. The sensor operates

at – 30

°C. Cooling can be accomplished by using a thermoelectric or Peltier cooler attached to the image sensor.

Condensation on the header must be prevented by isolating the cooled sensor from moist air. Vacuum isolation

is preferred; however, the continual flushing of dry nitrogen across the header can also prevent condensation.

X1

X2

34

35

C1

≈ 40 pF

C2

≈ 40 pF

SN28835

NOTE: The SN28835 is designed for use with a

crystal oscillator. The X1 and X2

terminals should not connect directly to

external driver outputs.

Figure 2. Connection of an External Crystal Oscillator to the SN28835