TSL1402R
256 × 1 LINEAR SENSOR ARRAY WITH HOLD
TAOS041F − APRIL 2007
APPLICATION INFORMATION
Integration Time
The integration time of the linear array is the period during which light is sampled and charge accumulates on
each pixel’s integrating capacitor. The flexibility to adjust the integration period is a powerful and useful feature
of the TAOS TSL14xx linear array family. By changing the integration time, a desired output voltage can be
obtained on the output pin while avoiding saturation for a wide range of light levels.
The integration time is the time between the SI (Start Integration) positive pulse and the HOLD positive pulse
minus the 18 setup clocks. The TSL14xx linear array is normally configured with the SI and HOLD pins tied
together. This configuration will be assumed unless otherwise noted. Sending a high pulse to SI (observing
timing rules for setup and hold to clock edge) starts a new cycle of pixel output and integration setup. However,
a minimum of (n+1) clocks, where n is the number of pixels, must occur before the next high pulse is applied
to SI. It is not necessary to send SI immediately on/after the (n+1) clocks. A wait time adding up to a maximum
total of 100 ms between SI pulses can be added to increase the integration time creating a higher output voltage
in low light applications.
Each pixel of the linear array consists of a light-sensitive photodiode. The photodiode converts light intensity
to a voltage. The voltage is sampled on the Sampling Capacitor by closing switch S2 (position 1) (see the
Functional Block Diagram on page 1). Logic controls the resetting of the Integrating Capacitor to zero by closing
switch S1 (position 2).
At SI input, all of the pixel voltages are simultaneously scanned and held by moving S2 to position 2 for all pixels.
During this event, S2 for pixel 1 is in position 3. This makes the voltage of pixel 1 available on the analog output.
On the next clock, S2 for pixel 1 is put into position 2 and S2 for pixel 2 is put into position 3 so that the voltage
of pixel 2 is available on the output.
Following the SI pulse and the next 17 clocks after the SI pulse is applied, the S1 switch for all pixels remains
in position 2 to reset (zero out) the integrating capacitor so that it is ready to begin the next integration cycle.
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On the rising edge of the 19 clock, the S1 switch for all the pixels is put into position 1 and all of the pixels begin
a new integration cycle.
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The first 18 pixel voltages are output during the time the integrating capacitor is being reset. On the 19 clock
following an SI pulse, pixels 1 through 18 have switch S2 in position 1 so that the sampling capacitor can begin
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storing charge. For the period from the 19 clock through the n clock, S2 is put into position 3 to read the output
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voltage during the n clock. On the next clock the previous pixel S2 switch is put into position 1 to start sampling
the integrating capacitor voltage. For example, S2 for pixel 19 moves to position 1 on the 20 clock. On the n+1
clock, the S2 switch for the last (n ) pixel is put into position 1 and the output goes to a high-impedance state.
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If a SI was initiated on the n+1 clock, there would be no time for the sampling capacitor of pixel n to charge to
the voltage level of the integrating capacitor. The minimum time needed to guarantee the sampling capacitor
for pixel n will charge to the voltage level of the integrating capacitor is the charge transfer time of 20 μs.
Therefore, after n+1 clocks, an extra 20 μs wait must occur before the next SI pulse to start a new integration
and output cycle.
The minimum integration time for any given array is determined by time required to clock out all the pixels
in the array and the time to discharge the pixels. The time required to discharge the pixels is a constant.
Therefore, the minimum integration period is simply a function of the clock frequency and the number of pixels
in the array. A slower clock speed increases the minimum integration time and reduces the maximum light level
for saturation on the output. The minimum integration time shown in this data sheet is based on the maximum
clock frequency of 8 MHz.
Copyright E 2007, TAOS Inc.
The LUMENOLOGY r Company
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