LX1974

Microsemi

Integrated Products Division

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

Page 6

Ambient Light Detector

Copyright

© 2005

Rev. 1.0, 2006-05-09

TM

®

APPLICATION EXAMPLES

The following examples present both fully automatic

(no user input) and semi-automatic to fully manual

override implementations. These general guidelines are

applicable to a wide variety of potential light control

applications. The LX1974 can be used to control the

brightness input of CCFL inverters (like Microsemi’s

PanelMatch™ inverter family, or line of controller IC’s).

Likewise, it can interface well with LED drivers like the

LX1990 and LX1991 sink LED drivers, or boost drivers

like the LX1992, LX1993, LX1994, and LX1995.

In each specific application, it is important to recognize

the need to correlate the output current of the LX1974 for

the target environment and its ambient light conditions.

The mechanical mounting of the sensor, light aperture

hole size, use of a light pipe or bezel are critical in

determining the response of the LX1974 for a given

exposure of light.

R1

R2

C1

10µF

3V

To inverter brightness

input or LED driver

controller.

3.3V or 5V

VSS

VDD

Figure 4 –

Fully Automatic Dimming

The example in figure 4 shows a fully automatic

dimming solution with no user interaction. Choose R1

and R2 values for any desired minimum brightness and

slope. Choose C1 to adjust response time to filter 50/60

Hz room lighting. As an example, let’s say you wish to

generate an output voltage from 0.25V to 1.25V to drive

the input of an LED driver controller.

The 0.25V

represents the minimum LED brightness and 1.25V

represents the maximum. The first step would be to

determine the ratio of R1 and R2.

R2

11

1

0.25V

3.0V

R2

R1

×

=

−

=

⎥⎦

⎤

⎢⎣

⎡

Next the value of R2 can be calculated based on the

maximum output source current coming from the

LX1974 under the application’s maximum light exposure,

lets say this has been determined to be about 50µA .

Thus R2 can be calculated; first order as follows:

275KΩ

R2

11

R1

25KΩ

50µA

1.25V

R2

=

×

=

∴

=

=

⎥⎦

⎤

⎢⎣

⎡

The output node will actually reach 1.25V when the source

current from the LX1974 is only about 44µA since about

6µA of current will be contributed from R1. This assumes a

high impedance input to the LED driver. In Figure 5, user

adjustable bias control has been added to allow control over

the minimum and maximum output voltage. This allows the

user to adjust the output brightness to personal preference

over a limited range. In addition, an equivalent DC voltage

may replace the PWM input source.

R1

40K

R2

25K

10µF

To inverter

brightness input or

LED driver

controller input.

3.3V or 5V

VSS

VDD

3.3V PWM

Figure 5 –

Semi-Manual Controlled Dimming

Figure 6 shows how a fully manual override can be quickly

added to the example in figure 5. In addition to the gate to

turn on and off the LX1974, a diode has been inserted to

isolate the sensor when it is disabled.

30K

30K

10µF

To inverter

brightness input or

LED driver

controller.

VSS

VDD

PWM

CMOS

Gate

60K

3.3V

Disable

control

Figure 6 –

Fully Manual Controlled Dimming

The preceding examples represent just a few of the

potential sensor applications. Further details and additional

circuits can be found in the application note (AN-28) LX1970

Visible Light Sensor located in the application section of

application note is written around the LX1970 visible light

sensor the circuits can be easily adapted for use with the

LX1974.