lx1972 Microsemi Corporation, lx1972 Datasheet - Page 6

• Manufacturer Part Number: lx1972
• Description: Ambient Light Detector
• Manufacturer: Microsemi Corporation

Copyright © 2004
Rev. 1.1b, 2005-10-31
(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 LX1972 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.
the need to correlate the output current of the LX1972 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 LX1972 for a given
exposure of light.
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.
represents the minimum LED brightness and 1.25V
represents the maximum.
determine the ratio of R1 and R2.
maximum output source current coming from the
LX1972 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:
The following examples present both fully automatic
In each specific application, it is important to recognize
The example in figure 4 shows a fully automatic
Next the value of R2 can be calculated based on the
TM
3V
R2
=
⎡
⎢ ⎣
1.25V
R1
50µA
R1
=
3.3V or 5V
⎤
⎥ ⎦
R2
R2
VDD
VSS
=
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
⎡
⎢
⎣
25KΩ
0.25V
Figure 4
3.0V
The first step would be to
∴
10µF
−
C1
1
R1
To inverter brightness
⎤
⎥
⎦
input or LED driver
=
=
11
11
controller.
×
×
R2
R2
A P P L I C A T I O N E X A M P L E S
=
275KΩ
The 0.25V
Integrated Products Division
®
Microsemi
current from the LX1972 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.
added to the example in figure 5. In addition to the gate to
turn on and off the LX1972, a diode has been inserted to
isolate the sensor when it is disabled.
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 our
website www.microsemi.com. Although this application note
is written around our LX1970 visible light sensor the circuits
can be easily adapted for use with the LX1972.
Figure 6 shows how a fully manual override can be quickly
The preceding examples represent just a few of the
The output node will actually reach 1.25V when the source
3.3V PWM
PWM
Disable
control
3.3V
Ambient Light Detector
P
RODUCTION
30K
CMOS
40K
R1
Gate
60K
3.3V or 5V
25K
R2
VDD
Figure 5
Figure 6
VSS
10µF
S
PECIFICATION
VDD
VSS
30K
10µF
brightness input or
brightness input or
controller input.
To inverter
LED driver
controller.
To inverter
LED driver
LX1972
Page 6