LM9823CCWM NSC [National Semiconductor], LM9823CCWM Datasheet - Page 19

• Manufacturer Part Number: LM9823CCWM
• Description: LM9823 3 Channel 48-Bit Color Scanner Analog Front End
• Manufacturer: NSC [National Semiconductor]

Applications Information
the OS input, the maximum allowable droop, the number of pixels
on the sensor, and the pixel conversion rate, f
the minimum clamp capacitor value:
For example, if the OS input leakage current is 25nA worst-case,
the sensor has 2700 active pixels, the conversion rate is 2MHz
(t
clamp capacitor value is:
6.1.2 CIS mode Minimum Clamp Capacitor Calculation:
If CDS is disabled, then the maximum LM9823 OS input leakage
current can be calculated from:
where V
and C
capacitor (2pF). Inserting this into Equation 6 results in:
with C
imum input signal), then Equation 9 reduces to:
In CIS mode (CDS disabled), the max droop limit must be much
more carefully chosen, since any change in the clamp capacitor’s
DC value will affect the LM9823 conversion results. If a droop of
one 10 bit LSB across a line is considered acceptable, then the
allowed droop voltage is calculated as: 2V/1024, or approximately
2mV. If there are 2700 active pixels on a line then:
6.1.3 Maximum Clamp Capacitor Calculation:
The maximum size of the clamp capacitor is determined by the
amount of time available to charge it to the desired value during
the optical black portion of the sensor output. The internal clamp
occurs when CLMP and VSMP are both high on a rising edge of
MCLK. If SMPCL=0, the clamps are on immediately before the
sample reference time, if SMPCL=1, the clamps are on immedi-
ately after the sample reference time. If the LM9823 is operated
in divide by 2 mode, then the clamp is on 50% of the time when
VSMP
Equation 8. CIS mode Input Leakage Current Calculation
Equation 11. CIS mode C
C CLAMP MIN
C CLAMP MIN
C CLAMP MIN
SAMP
SAMP
= 500ns), and the max droop desired is 0.1V, the minimum
Equation 9. CIS mode C
Equation 10. CIS mode C
Equation 6. CDS mode C
SAT
C CLAMP MIN
Equation 7. CDS mode C
=
equal to 2pF and V
is the capacitance of the LM9823 internal sampling
is the peak pixel signal swing of the CIS OS output
I
---------------------------C
t
leakage
SampCLK
V
SAT
=
C
=
CLAMP MIN
-------- - dt
=
=
dV
-------- - dt
dV
i
=
i
----------------------------------- - num pixels
max droop(V)
leakage current (A)
-------------------------------------------------- -
=
=
V
4p(F)(V)
max droop(V)
SAMP
5.4uF
4p(F)(V)
--------------------- - 2700
SAT
2mV
CLAMP MIN
SAT
f
SampCLK
----------------------------------- - num pixels
max droop(V)
=
CLAMP MIN
=
t
CLAMP MIN
CLAMP MIN
SampCLK
equal to 2V (the LM9823 max-
340pF
CLAMP MIN
25nA
------------- -
0.1V
Calculation Example
---------------
2MHz
C
2700
(Continued)
number of pixels
------------------------------------------- -
Calculation
SAMP
Calculation
Calculation
VSMP
Example
f
VSMP
, and provides
19
CLMP is high. In this case the available charge time per line can
be calculated using:
For example, if a sensor has 18 black reference pixels and f
is 2MHz with a 50% duty cycle, then t
“divide by” modes will have lower or higher clamp duty cycles
accordingly, depending on the SMPCL setting. See Diagram 8,
Clamp Timing With SMPCL = 0 and Diagram 9, Clamp Timing
With SMPCL = 1 .
The following equation takes the number of optical black pixels,
the amount of time (per pixel) that the clamp is closed, the sen-
sor’s output impedance, and the desired accuracy of the final
clamp voltage and provides the maximum clamp capacitor value
that allows the clamp capacitor to settle to the desired accuracy
within a single line:
Where t
on, R
LM9823 internal clamp switch, and accuracy is the ratio of the
worst-case initial capacitor voltage to the desired final capacitor
voltage. If t
1500 , the worst case voltage change required across the capac-
itor (before the first line) is 5V, and the desired accuracy after
clamping is to within 0.1V (accuracy = 5/0.1 = 50), then:
The final value for C
C
In some cases, depending primarily on the choice of sensor,
C
the capacitor can not be charged to its final voltage during the
black pixels at the beginning of a line and hold it’s voltage without
drooping for the duration of that line. This is usually not a problem
because in most applications the sensor is clocked continuously
as soon as power is applied. In this case, a larger capacitor can
be used (guaranteeing that the C
and the final clamp voltage is forced across the capacitor over
multiple lines. This equation calculates how many lines are
required before the capacitor settles to the desired accuracy:
Using the values shown before and a clamp capacitor value of
0.01µF, this works out to be:
In this example, a 0.01µF capacitor takes 14 lines after power-up
to charge to its final value. On subsequent lines, the only error will
be the droop across a single line which should be significantly
CLAMP MAX
CLAMP MAX
Equation 13. C
Equation 15. Number of Lines Required for Clamping
CLAMP
lines
Equation 16. Clamping Lines Required Example
Equation 12. Clamp Time Per Line Calculation
CLAMP
lines
C
t
CLAMP
=
CLAMP
is the output impedance of the CCD plus 50
, but no less than C
CLAMP MAX
may actually be less than C
Equation 14. C
=
R CLAMP
is the amount of time (per line) that the clamp is
C CLAMP MAX
1550
is 4.5µs, the output impedance of the sensor is
CLAMP MAX
=
Number of optical black pixels
------------------------------------------------------------------------------ -
CLAMP
0.01 F
-------------------
4.5 s
C
------------------------ -
t
CLAMP
CLAMP
=
=
----- -
--------------------------
CLAMP MAX
R
R CLAMP
should be less than or equal to
for a single line of charge time
t
t
CLAMP MIN
ln
------------------------------- -
ln(accuracy)
CLAMP
=
=
2f
CLAMP MIN
----------- -
0.1V
----------------- -
1550
728pF
4.5 s
ln
5V
VSMP
1
----------------------------------------------------
Initial Error Voltage
Final Error Voltage
------------------------------- -
ln(accuracy)
CLAMP MIN
Example
.
=
CLAMP
-------------- -
ln(50)
13.5 lines
1
requirement is met),
1
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is 4.5µs. Other
, meaning that
for the
VSMP