Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
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Rev. 1.0b,2004-05-19
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INTEGRATED PRODUCTS
DESCRIPTION
The LX1971 is a new technology light
sensor with spectral response that
emulates the human eye; it is pin to pin
compatible to Microsemi’s LX1970
visible light sensor however with a wider
dynamic range.
This device is ideal for monitoring
ambient light for the control of artificial
lighting, operation of shades, LED
signage and display, street lighting,
automotive light sensors and control of
backlight dimming. It has a unique photo
diode arrangement (patents pending) with
a peak response at 520nm while sharply
attenuating both ultra violet and infrared
wavelengths.
The photo sensor is a PIN diode array
with an accurate and very repeatable
current output.
Photo current is first multiplied by an
integrated high gain amplifier then the
square root is taken of the product of the
amplifier and an internal reference current.
Finally this is further multiplied converted
to current and made available at two
output pins; one a current source and the
other a current sink.
These currents can easily be converted
to voltage by adding a single resistor at
either or both outputs. Voltage gain is
determined by the resistor value typically
in the 5KΩ to 50KΩ range.
With accurate internal gain amplifiers
and square root transfer function, design
complexity and cost are greatly reduced.
The LX1971 is available in the Clear 8-
pin MSOP package.
IMPORTANT: For the most current data, consult MICROSEMI’s website: http://www.microsemi.com
KEY FEATURES
Approximate Human Eye
Spectral Response
Low IR Sensitivity
Highly Accurate &
Repeatable Output Current
vs. Light
Square root transfer function
Temperature Stable
Integrated High Gain Photo
Current Amplifiers
Complementary Current
Outputs
No Optical Filters Needed
BENIFITS
• Requires no wavelength
filters
• Insensitive to UV and IR
• More than 4 decades usable
light range
• Enables accurate lighting
control systems
PRODUCT HIGHLIGHT
SNK
SRC
Ambient
Light
VOUT
VDD
GND
0.45V Typ
0.60V Typ
25K
4.7uF
LX1971
VDD
25K
4.7uF
SNK
SRC
PACKAGE ORDER INFO
TA (°C) DU Plastic MSOP
8-Pin
-40 to 85 LX1971IDU
Note: Available in Tape & Reel.
Append the letter “T” to the part number. (i.e. LX1971IDUT)
L
LX
X1
19
97
71
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Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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ABSOLUTE MAXIMUM RATINGS
VDD ................................................................................................................. -0.3 to 6 VDC
SNK/SRC (Output Compliance Voltage)........................................... -0.3 to VDD + 0.3VDC
SNK/SRC (Maximum Output Current)................................................... Internally Limited
Operating Temperature Range ........................................................................ -40 to +85°C
Storage Temperature Range.............................................................................-55 to 125°C
Solder Reflow Peak Temperature (Soldering 10 seconds).........................................240°C
Notes: Exceeding these ratings could cause damage to the device. All voltages are with respect to
Ground. Currents are positive into, negative out of specified terminal.
Solder reflow to follow: IPC/JEDEC J-STD-020B 7/02 Pb-SN Small Body Profile
THERMAL DATA
DU Plastic MSOP 8-Pin
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA 206°C/W
THERMAL RESISTANCE-JUNCTION TO CASE, θJC 39°C/W
PACKAGE PIN OUT
VSS
NC
NC
SNK
VDD
NC
NC
SRC
1
2
3
54
6
7
8
DU PACKAGE
(Top View)
1
2
3
45
6
7
8
1971
xxxx
DU PACKAGE
(Bottom View)
xxxx = Denotes Date Code / Lot Information
FUNCTIONAL PIN DESCRIPTION
NAME DESCRIPTION
VDD Input Supply Voltage
VSS Ground Reference for Power and Signal Output
SNK Output Current Sink
SRC Output Current Source
SIMPLIFIED BLOCK DIAGRAM PACKAGE PHOTO
VDD
VSS
SNK
SRC
P
PA
AC
CK
KA
AG
GE
E
D
DA
AT
TA
A
Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 3
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RECOMMENDED OPERATING CONDITIONS
LX1971
Parameter Min Typ Max
Units
Supply Voltage (VDD ) 3¹ 5.5 V
SNK Compliance Voltage Range VSS +0.6 VDD V
SRC Compliance Voltage Range VSS V
DD - 0.75 V
SNK/SRC Output Resistor Range 5 100 KΩ
¹Outputs are functional down to VDD=2.7V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, the following specifications apply over the operating ambient temperature -40°C≤ TA≤ 85°C, VDD = 3V to
5.5V, RLOAD= 25Kohms, Direct Light Input² of 584µW/cm2 except where otherwise noted. Performance between -40°C and 0°C and
between 70°C and 85°C are assured by design and characterization.
LX1971
Parameter Symbol Test Conditions Min Typ Max
Units
Supply Voltage Range VDD 3 5.5 VDC
Input Supply Current IDD V
DD = 3.0V, ISRC = 62µA, ISNK = open 350 590 750 µA
ISRC VDD = 3.0V, Direct Light Input of
584µW/cm2 @ 25°C 55 61 67 µA
ISRC VDD = 3.0V, Direct Light Input of
584µW/cm2 52 61 70
ISNK
VDD = 3.0V, Direct Light Input of
584µW/cm2 -70 -61 -52 µA
ISRC VDD = 3.0V, Direct Light Input of
292µW/cm2 34 41 48 µA
Output Current
ISRC VDD = 3.0V, Direct Light Input of
9.34µW/cm2 5.5 7 8.5 µA
Output SNK/SRC Current Matching IMATCH V
DD = 3.0V 2 6 %
SNK Minimum Compliance Voltage SNKVMIN VDD = 3.0V, @ 95% of nominal output
current V
SS +0.45 VSS +0.6 V
SRC Maximum Compliance Voltage SRCVMAX VDD = 3.0V, @ 95% of nominal output
current V
DD – 0.6 VDD – 0.7 V
SNK/SRC Output Dark Current IDARK3.0V V
DD = 3V, No Light, 25°C 150 400 nA
SNK/SRC Output Dark Current IDARK3.0V V
DD = 3V, No Light, -40°C to 70°C 450 1000 nA
SNK/SRC Output Dark Current IDARK3.0V V
DD = 3V, No Light, 85°C 550 1150 nA
SNK/SRC Output Dark Current IDARK5.5V V
DD = 5.5V, No Light, 25°C 320 nA
Wavelength of Peak Sensitivity λPS 520 nm
Half Reception Angle θ½ ±60 deg
Supply Voltage Coefficient of Output
Current PSRR VDD = 3V to 5.5V 2.9 3.5 %/V
Resolution Limit Due To Random Noise IMIN Minimum usable resolution 146 nW/cm2
Sensitivity Change @ 910nm ISRC∆ IR
Current responsivity change with
additional direct light input of 584µW/cm2
at 910nm, TA = 25°C, VDD = 3V
-10 0 10 % of peak
Radiant Sensitive Area A Photodiode Area .369 mm2
²The input irradiance is supplied from a white light-emitting diode (LED) optical source adjusted to impose 584µW/cm2 at 555nm on the sensor’s surface.
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Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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APPLICATION INFORMATION
The LX1971 is a light sensor with a spectral response
that resembles the human visual system. It is packaged in
clear MSOP package with the same pin out and same
supply voltage range as Microsemi’s 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.
Whereas the LX1970 has a responsiveness that is
directly proportional to the intensity of light falling on the
photo receptors. There are two different response equations
for the LX1971 depending on the operating range (intensity
of the light). Below 2500 lux, the response resembles a
square root function. Above 2500 lux, the response fits a
straight line curve.
(
)
ALIOUT µ1.1808. +×= for L < 2500 lux
()
AALIOUT µ23µ0083. +×= for 2500 lux < L <
20000 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 2500
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 following curves show the calculated responsiveness o
f
the LX1971 without load resistors based on the formulas
above. They 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-600mV @ about 4000 lux.
LX1971 Calculated SRC Full Range Response
0
50
100
150
200
250
0 5000 10000 15000 20000 25000
Incandescent Light (lux)
S R C Current (µ A)
<2500 Lu x Formu la
>2500 Lu x Formu la
LX1971 Calculated SRC Low Level Response
0
10
20
30
40
50
0 500 1000 1500 2000 2500
Incandescent Ligh t (lux )
SRC Cur rent (µA)
<2500 Lu x Fo rmu la
LX1971 Calculated SRC High Level Resp onse
0
50
100
150
200
250
0 5000 10000 15000 20000 25000
Incandescent Light (lux)
SRC Cur rent (µA)
> 2500 L ux Fo rmul a
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Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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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.
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RESPONSE VS WAVELENGTH
ISRC STEP RESPONSE
0
2
4
6
8
10
12
14
16
0 0.5 1 1.5 2
Time (Seconds)
Amplitude (µW /cm
2
)
VDD = 3.0V; RSRC = 25K & 4.7uF
LINE REGULATION
DARK CURRENT VS TEMP
19
19.5
20
20.5
21
21.5
22
33.544.555.5
VDD (V)
SRC Current (µA)
VDD = 3.5V; RSRC = 25K & 4.7uF
1
10
100
1000
-40 -20 0 20 40 60 80 100
Temperature (ºC)
Dark Current (nA)
3V
5V
5.5V
RSRC = 25K & 4.7uF
SNK OUTPUT PSRR
GAIN VS TEMP
30
40
50
60
70
1 10 100 1000 10000 100000
Frequenc y (Hz)
P SRR (db)
VDD = 3.0V; RSNK = 25K & 4.7uF
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
-50 0 50 100
Temperature (ºC)
Gain/Gain(25ºC)
y = 1. 35E-03x + 9.64E-01
VDD = 3.0V Direct Light Input of 13.14µW/cm2
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Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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SNK COMPLIANCE VS CURRENT
SRC COMPLIANCE VS CURRENT
0
20
40
60
80
100
120
0.1 0.2 0.3 0.4 0.5
SNK compliance Voltage (V)
Output Curre n t (µA)
VDD = 3.0V; RSNK = 25K & 4.7µF
0
20
40
60
80
100
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Source Compliance Voltage (VDD-SRC)
Output Current (µA)
VDD = 3.0V; RSRC = 25K & 4.7µF
SRC OUTPUT VS LUMINANCE(LUX)
SRC OUTPUT VS LUMINANCE(LUX)
0
40
80
120
160
0 3000 6000 9000 12000 15000 18000
Lumina nc e (lux)
SRC (uA
)
Incandescent
VDD = 4.0V; RSNK = 10K & 4.7µF
0
10
20
30
40
50
0 500 1000 1500 2000 2500
Lumina nc e (lux)
SRC (uA
)
Incandescent
VDD = 4.0V; RSNK = 10K & 4.7µ
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Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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PACKAGE DIMENSIONS
DU 8-Pin Miniature Shrink Outline Package (MSOP)
A1 L
E
L1
E1
b
e
S
A
D
cΘ
MILLIMETERS INCHES
Dim MIN MAX MIN MAX
A – 1.10 – 0.043
A1 0.05 0.15 0.002 0.006
b 0.26 0.41 0.010 0.016
c 0.13 0.23 0.005 0.009
D 2.90 3.10 0.114 0.122
e 0.65 BSC 0.025 BSC
E 4.75 5.05 0.187 0.198
E1 2.90 3.10 0.114 0.122
L 0.41 0.71 0.016 0.028
L1 0.95 BSC 0.037 BSC
S 0.525 BSC 0.021 BSC
Θ 3° 3°
Active Area
Required Minimum Light footpr
i
Bonding / Wafer area
Examination of
Active Area
Recommended light footprint pattern
P1
Active Area
A
B
C
L
C
a
MILLIMETERS INCHES
Dim
A 1.22 0.048
a 0.85 0.033
B 0.60 0.024
C 0.60 0.024
P1 2.5 0.98
Note:
P1 represents a possible light footprint and its
dimensions are not subject to strict tolerances. Only
the active area of the device is required to be covered
with light. This larger footprint is designed to ensure
coverage of the device’s active area.
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Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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NOTES
PRODUCTION DATA – Information contained in this document is proprietary to
Microsemi and is current as of publication date. This document may not be modified in
any way without the express written consent of Microsemi. Product processing does not
necessarily include testing of all parameters. Microsemi reserves the right to change the
configuration and performance of the product and to discontinue product at any time.
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