LX1972
TM
廬
Ambient Light Detector
P
RODUCTION
S
PECIFICATION
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 LX1972 can be used to control the
brightness input of CCFL inverters (like Microsemi鈥檚
PanelMatch鈩?inverter family, or line of controller IC鈥檚).
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 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.
3.3V or 5V
VDD
VSS
3V
R1
R2
The output node will actually reach 1.25V when the source
current from the LX1972 is only about 44碌A(chǔ) since about
6碌A(chǔ) 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.
3.3V or 5V
VDD
VSS
3.3V PWM
WWW .
Microsemi
.C
OM
R1
40K
R2
25K
To inverter
brightness input or
LED driver
controller input.
10碌F
Figure 5
To inverter brightness
input or LED driver
controller.
C1
10碌F
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 LX1972, a diode has been inserted to
isolate the sensor when it is disabled.
Disable
control
CMOS
Gate
VDD
VSS
Figure 4
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鈥檚 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.
鈳?/div>
鈳?/div>
3.0V
鈭?/div>
1
鈳?/div>
=
11
脳
R2
鈳?/div>
0.25V
鈳?/div>
Next the value of R2 can be calculated based on the
maximum output source current coming from the
LX1972 under the application鈥檚 maximum light exposure,
lets say this has been determined to be about 50碌A(chǔ) .
Thus R2 can be calculated first order as follows:
鈳?/div>
1.25V
鈳?/div>
R2
=
鈳?/div>
=
25K鈩?/div>
鈭?/div>
R1
=
11
脳
R2
=
275K鈩?/div>
鈳?/div>
50碌A(chǔ)
鈳?/div>
鈳?/div>
R1
=
R2
鈳?/div>
3.3V
60K
PWM
30K
10碌F
30K
To inverter
brightness input or
LED driver
controller.
A
PPLICATIONS
A
PPLICATIONS
Figure 6
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 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.
Copyright
漏
2004
Rev. 1.1b, 2005-10-31
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 6
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