Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
Copyright © 2003
Rev. 1.2a,2007-01-09
WWW.Microsemi .COM
LX1971
Wide Ran
e Visible Li
ht Sensor
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 LX1971 can be used for the control of artificial
lighting, operation of shades, LED signage and display,
street lighting, automotive light sensors and control of
backlight dimming
.
In each specific application it is important to recognize
the need to correlate the sink and source current of the
LX1971 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 LX1971 for a given
exposure of light.
R1
R2
C1
10µF
3V
LED driver
controller or
dimming controller
input
N/C
3.3V or 5V
VSS
VDD
SRC
SNK
Figure 1
The example in figure one shows a fully automatic
lighting control solution with no user interaction. Choose
R1 and R2 values for any desired minimum brightness and
slope. Choose C1 to adjust response time. 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 represent the maximum. The first
step would be to determine the ratio of R1 and R2.
R1 = R2 [ 3.0V/0.25V –1] = 11 * R2
Next the value of R2 can be calculated based on the
maximum output source current coming from the LX1971
under the application’s maximum light exposure, lets say
this has been determined to be about 50uA . Thus R2 can
be calculated first order as follows:
R2 = [ 1.25V / 50uA ] = 25KΩ ∴ R1 = 11 * R2 = 275KΩ
The output node will actually reach 1.25V when the
source current from the LX1971 is only about 44uA since
about 6uA of current will be contributed from R1.
We are assuming a high impedance input to the LED driver.
In Figure 2 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. The PWM input source could
of course be replaced with an equivalent DC voltage.
R1
40K
R2
25K
10µF
LED driver
controller or
dimming controller
input
N/C
3.3V or 5V
VSS
VDD
SRC
SNK
3.3V PWM
Figure 2
Figure 3 shows how a fully manual override can be quickly
added to the example in figure 2. In addition to the gate to turn
on and off the LX1971, a diode has been inserted to isolate the
LX1971 when it is shut down.
30K
30K
10µF
LED driver
controller or
dimming controller
input
N/C
VSS
VDD
SRC
SNK
PWM
CMOS
Gate
60K
3.3V
Diable
control
Figure 3
The preceding examples represent just a few of the many
ways the sensor can be used. For example since there is also a
complimentary sink output a resistor from VDD to SNK could
develop a voltage that could be compared (with some
hysteresis) to a fixed (or adjustable) reference voltage and
develop a logic signal. In the case of street lighting or
operation of shades such a signal would engage the function
when reaching the correct ambient light level.
A
A
P
P
P
P
L
L
I
I
C
C
A
A
T
T
I
I
O
O
N
N
S
S
