Datasheet NOA1212 (ON Semiconductor) - 7

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NOA1212 DESCRIPTION OF OPERATION Ambient Light Sensor Architecture
transmits photons in the visible spectrum which are The NOA1212 employs a sensitive photo diode fabricated primarily detected by the human eye and exhibits excellent in ON Semiconductor’s standard CMOS process IR rejection. The photo response of this sensor is as shown technology. The major components of this sensor are as in Figure 3. shown in Figure 2

. The photons which are to be detected The ambient light signal detected by the photo diode is pass through an ON Semiconductor proprietary color filter converted to an analog output current by an amplifier with limiting extraneous photons and thus performing as a band programmable gain. Table 4 shows the gain setting and the pass filter on the incident wave front. The filter only corresponding light sensitivity.
Table 4. PROGRAMMABLE GAIN SETTINGS Approximate Output Approximate Output GB2 GB1 Mode Current @ 100 lux Current @ 1000 lux Saturation
0 0 Power Down − − − 0 1 High Gain 51 mA 510 mA ~10,000 lux 1 0 Medium Gain 4.9 mA 49 mA ~100,000 lux 1 1 Low Gain 0.54 mA 5.4 mA > 100,000 lux
Power Down Mode
maximum desired EV as shown in Equation 3. Equation 4 This device can be placed in a power down mode by computes the value for RL (High−Gain mode). setting GB1 and GB2 to logic low level. V + ǒ51 mAń100 luxǓ * E * R (eq. 3) In order for proper operation of this mode GB1 and GB2 OMAX VMAX L should stay low 1.5 ms. R + ǒV * 0.4 VǓńE * ǒ100 luxń51 mAǓ (eq. 4) L DD VMAX
External Component Selection
For example, consider a 5 V supply with a desired EVMAX The NOA1212 outputs a current in direct response to the = 1000 lux, the value of RL would be 8.85 kW. The value for incident illumination. In many applications it is desirable to RL can easily be computed for different NOA1212 gain convert the output current into voltage. It may also be ranges by substituting the appropriate output current at desirable to filter the effects of 50/60 Hz flicker or other light 100 lux from Table 4. source transients. The optional capacitor CL can be used to form a low−pass Conversion from current to voltage may be accomplished filter to remove 50/60 Hz filter or other unwanted noise by adding load resistor RL to the output. The value of RL is sources as computed with Equation 5. bounded on the high side by the potential output saturation C + 1ń2p f (eq. 5) L c RL of the amplifier at high ambient light levels. RL is bounded For our example, to filter out 60Hz flicker the value of C on the low side by the output current limiting of the internal L would be 300 nF. amplifier and to minimize power consumption. Equation 1 describes the relationship of light input to
Power Supply Bypassing and Printed Circuit Board
current output for the High−Gain mode.
Design
I + ǒ51 mAń100 luxǓ * E (eq. 1) Power supply bypass and decoupling can typically be OUT V handled with a low cost 0.1 mF to 1.0 mF capacitor. By adding RL to the output, IOUT is converted into a The exposed pad on the bottom of the package is internally voltage according to Equation 2. connected to VSS pin 2 and should be soldered to the printed V + I * R + ǒ51 mAń100 luxǓ * E * R (eq. 2) OUT OUT L V L circuit board. The range of the output voltage is limited by the output stage to the VOMAX parameter value of VDD – 0.4 V at the
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