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SN28837 Datasheet(PDF) 5 Page - Texas Instruments

Part No. SN28837
Description  1/2-INCH PAL TIMER
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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SN28837
1/2-INCH PAL TIMER
SOCS031B – JULY 1991
5
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
variable-integration-time mode
In addition to the normal TV mode of operation, the SN28837 timing generator offers an optional
variable-integration mode for use with the TC276 and TC277 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 302 pulses 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 13).
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 1/50 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 1/50-second readout interval. If the integration time is less than 1/50 of a second, normal output
operation occurs; if the integration time is greater than 1/50 of a second, a frame buffer may be required to
capture the image.
Integration times greater than 1/50 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
42
43
C1
≈ 40 pF
C2
≈ 40 pF
SN28837
NOTE: The SN28837 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 SN28837




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