Datasheet

PRODUCTPREVIEW
SR=
I
MAX
C
T=
C
16384 I´
MIN
Current
DAC
CPLOB
COB
DPGA
OBClamp
Level
Data
Out
16-BitADC
CDS
CCDIN
BYPP
Decoder
Not Recommended for New Designs
VSP2560
VSP2562
VSP2566
SBES008 –AUGUST 2008 www.ti.com
OPTICAL BLACK LEVEL (OB) LOOP AND OB CLAMP LEVEL
The VSP2560/62/66 have a built-in optical black (OB) offset self-calibration circuit (OB loop) that compensates
the OB level by using OB pixels output from the CCD image sensor. A block diagram of the OB loop and OB
clamp circuit is shown in Figure 5. CCD offset is compensated by converging this calibration circuit while
activating the CLPOB pin during a period when the OB pixels are output from the CCD.
Figure 5. OB Loop and OB Level Clamp
At the CDS circuit, the CCD offset is compensated as a difference between the reference level and the OB pixel
data level. These compensated signal levels are recognized as actual OB levels, and the outputs are clamped to
the OB levels set by the serial interface. These OB levels are the base of black for the effective pixel period
thereafter. Because DPGA, which is a gain stage, is outside the OB loop, the OB levels are not affected even
when the gain is changed.
The converging time of the OB loop is determined based on the capacitor value connected to the COB terminal
and output from the current output DAC (IDAC) of the loop. The time constant can be obtained from Equation 1:
Where:
• C is the capacitor value connected to COB
• I
MIN
is the minimum current (0.15 μA) of the IDAC, which is the current equivalent to 1 LSB of the IDAC
output. (1)
When C = 0.1 μF, T is 40.7 μs. Slew rate (SR) can be obtained from Equation 2:
Where:
• C is the capacitor value connected to COB
• I
MAX
is the maximum current (153 μA) of the IDAC, which is the current equivalent to 1023 LSB of the
IDAC output. (2)
IDAC output current multiplication is provided by the VSP2560/62/66. This function increases the IDAC output
current through the serial interface in multiples of 2, 4, and 8. Increased IDAC current shortens the time constant
of the OB loop. In this case, the OB level is drastically changed and must quickly settle the OB loop; this function
is effective.
Immediately after power-on, the COB capacitor voltage is not charged. For fast start up, a COB voltage boost-up
circuit is provided.
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Product Folder Links: VSP2560 VSP2562 VSP2566