Microsemi

Analog Mixed Signal Group

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 5

Copyright

© 2007

Rev. 1.5b, 2008-01-25

LX1970

Visible Light Sensor

I N T E GRA T ED

PR ODUC T S

APPLICATION INFORMATION

The LX1970 is a light sensor with a spectral response

that resembles the human visual system. It is packaged in

clear MSOP package. Microsemi also offers the LX1971

with the same pin out and similar supply voltage range as

LX1970 light sensor.

The LX1971 responsiveness

however differs from that of the LX1970. In general the

LX1971 has lower sensitivity and a wider dynamic range.

The LX1970 has a responsiveness that is directly

proportional to the intensity of light falling on the photo

receptors. Although the gain varies depending on the

wavelength of the light and the direction of light, in general

for a 555nm wavelength (yellow-green), the sensitivity is:

A

L

I

OUT

µ

76

.

0

×

=

for L < 1200 lux

The LX1970 is best suited for applications where the

light sensor is an integral part of a continuous lighting

control system. For example, in an LCD backlighting

application, the level of brightness of the backlight should

be adjusted in proportion to the level of ambient lighting;

the LX1970 can provide closed loop brightness control for

this type of system. For most indoor applications, the 1200

lux saturation point of the LX1970 is usually not a limiting

affect.

The LX1971 is best suited for applications where the

system must respond to external events that affect the

user’s ability to see clearly. For example, a sensor to turn

on headlights or a sensor to adjust the reflectivity of a rear

view mirrors. These systems require the sensor to have

dynamic range similar to sight. Like the human eye, the

square root function of the LX1971 makes it extra sensitive

to small changes at lower light levels. The wide dynamic

range allows the LX1971 to sense the difference between

twilight and daylight or sunshine and heavy cloud cover.

The first curve shows the calculated responsiveness of

the LX1970 without load resistors based on the formula

above. It can be used as baseline guidelines to calculate

gain setting resistors. The compliance of the current source

output may result in premature saturation of the output

when load resistors are added.

The SRC compliance

voltage is specified typically at VDD-350mV @ about 100

lux. The second curve shows saturation of the output with

300uA the output becomes non-linear as it begins to

saturate.

LX1970 Calculated SRC Full Range Response

0

100

200

300

400

500

600

700

800

900

1000

0

200

400

600

800

1000

1200

Light (lux)

LX1970 SRC Actual SRC with 10K Resistor

0

50

100

150

200

250

300

350

400

450

0

200

400

600

800

1000

1200

Light (lux)