TC240 Datasheet, PDF (4/19 Pages) Texas Instruments – 780- × 488-PIXEL CCD IMAGE SENSOR

TC240
780- × 488-PIXEL CCD IMAGE SENSOR
SOCS005A – OCTOBER 1985 – REVISED DECEMBER 1991
detailed description
The TC240 consists of four basic functional blocks: (1) the image-sensing area, (2) the image-storage area,
(3) the multiplexer with serial registers and transfer gates, and (4) the buffer amplifier with charge-detection
nodes. Location of each of these blocks is shown in the functional block diagram.
image-sensing and storage areas
Cross sections with potential-well diagrams and top views of image-sensing and storage-area elements are
shown in Figure 1 and Figure 2. As light enters the silicon in the image-sensing area, free electrons are
generated and collected in the potential wells of the sensing elements. During this time, the antiblooming gate
is activated by the application of a burst of pulses every horizontal blanking interval. This prevents blooming
caused by the spilling of charge from overexposed elements into neighboring elements. After the completion
of integration, the signal charge is transferred into the storage area. To generate the dark reference that is
necessary in subsequent video processing circuits for restoration of the video black level, there are 23 full
columns and one half-column of elements at the left edge of the image-sensing area that are shielded from
incident light. Two full columns and one half-column of elements at the right of the image-sensing area are also
shielded from incident light. Thus, the total number of elements per row is 780 (753 active elements plus 25
shielded elements and 2 transitional elements).
multiplexer with transfer gates and serial registers
The color sensitivity of the TC240 is obtained by laminating a color stripe filter on top of the image-sensing area
and aligning it precisely with the vertical columns of sensing elements. This separates the columns into three
groups corresponding to the red, green, and blue colors used in the filter. The function of the multiplexer and
transfer gates is to transfer the charge line by line from each group of columns into the corresponding serial
register and to prepare it for readout. Multiplexing is activated during the horizontal blanking interval by applying
appropriate pulses to the transfer gates and serial registers. The required pulse timing is shown in Figure 3. A
drain is included in this area to provide the capability to quickly clear the image-sensing and storage areas of
unwanted charge. Such charge can accumulate in the imager during the start-up of operation or under special
circumstances when nonstandard TV operation is desired.
buffer amplifier with charge-detection nodes
The buffer amplifier converts charge into a video signal. Figure 4 shows the circuit diagram of a charge-detection
node and one of the three amplifiers. As charge is transferred into the detection node, the potential of this node
changes in proportion to the amount of signal received. This change is sensed by an MOS transistor and, after
proper buffering, the signal is supplied to the output terminal of the image sensor. After the potential change has
been sensed, the node is reset to a reference voltage supplied by an on-chip reference generator. The reset
is accomplished by a reset gate that is connected internally to the serial register. The detection nodes and
corresponding amplifiers are located some distance away from the edge of the storage area; six dummy
elements are used to span this distance. The location of the dummy elements is shown in the functional block
diagram.
2-104
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265