NOA1212

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7

DESCRIPTION OF OPERATION

Ambient Light Sensor Architecture

The NOA1212 employs a sensitive photo diode fabricated

in ON Semiconductor’s

standard

CMOS

process

technology. The major components of this sensor are as

shown in Figure 2 . The photons which are to be detected

pass through an ON Semiconductor proprietary color filter

limiting extraneous photons and thus performing as a band

pass filter on the incident wave front. The filter only

transmits photons in the visible spectrum which are

primarily detected by the human eye and exhibits excellent

IR rejection. The photo response of this sensor is as shown

in Figure 3.

The ambient light signal detected by the photo diode is

converted to an analog output current by an amplifier with

programmable gain. Table 4 shows the gain setting and the

corresponding light sensitivity.

Table 4. PROGRAMMABLE GAIN SETTINGS

GB2

GB1

Mode

Approximate Output

Current @ 100 lux

Approximate Output

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

This device can be placed in a power down mode by

setting GB1 and GB2 to logic low level.

In order for proper operation of this mode GB1 and GB2

should stay low 1.5 ms.

External Component Selection

The NOA1212 outputs a current in direct response to the

incident illumination. In many applications it is desirable to

convert the output current into voltage. It may also be

desirable to filter the effects of 50/60 Hz flicker or other light

source transients.

Conversion from current to voltage may be accomplished

bounded on the high side by the potential output saturation

on the low side by the output current limiting of the internal

amplifier and to minimize power consumption.

Equation 1 describes the relationship of light input to

current output for the High−Gain mode.

I

(eq. 1)

voltage according to Equation 2.

V

(eq. 2)

The range of the output voltage is limited by the output

V

(eq. 3)

R

E

(eq. 4)

W. The value for

ranges by substituting the appropriate output current at

100 lux from Table 4.

filter to remove 50/60 Hz filter or other unwanted noise

sources as computed with Equation 5.

C

(eq. 5)

would be 300 nF.

Power Supply Bypassing and Printed Circuit Board

Design

Power supply bypass and decoupling can typically be

handled with a low cost 0.1

mF to 1.0 mF capacitor.

The exposed pad on the bottom of the package is internally

connected to VSS pin 2 and should be soldered to the printed

circuit board.