Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
Copyright © 2007
Rev. 1.5b, 2008-01-25
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DESCRIPTION
The LX1970 is a new technology light
sensor with spectral response that
emulates the human eye.
This device is ideal for monitoring
ambient light for brightness control
systems in flat panel displays. It has a
unique photo diode arrangement (patents
pending) with a peak response at 520 nm
while sharply attenuating both ultra violet
and infrared wavelengths.
The photo sensor is a PIN diode array
with an accurate, linear, and very
repeatable current transfer function.
Photo current is multiplied by
integrated high gain amplifiers and is
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 10KΩ to 50KΩ range.
With accurate internal gain amplifiers,
design complexity and cost are greatly
reduced.
The LX1970 is available in the 8-pin
MSOP.
IMPORTANT: For the most current data, consult MICROSEMI’s website: http://www.microsemi.com
Protected By U.S. Patents: 6,787,757; Patents Pending
KEY FEATURES
Approximate Human Eye
Spectral Response
Low IR Sensitivity
Highly Accurate &
Repeatable Output Current
vs. Light
Voltage Scalable
Temperature Stable
Integrated High Gain Photo
Current Amplifiers
Complementary Current
Outputs
No Optical Filters Needed
APPLICATIONS
PDA
Notebook PC
LCD TV
Tablet PC
Cell phones
PRODUCT HIGHLIGHT
I SNK
I SRC
Ambient
Light
VOUT
VDD
GND
0.3V Typ
0.3V TYP
50K
4.7uF
LX1970
VDD
50K
4.7uF
SNK
SRC
PACKAGE ORDER INFO
DU Plastic MSOP
8-Pin
TA (°C)
RoHS Compliant / Pb-free
-40 to 85 LX1970IDU
Note: Available in Tape & Reel. Append the letters “TR” to
the part number. (i.e. LX1970IDU-TR)
L
LX
X1
19
97
70
0
Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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Copyright © 2007
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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.......................................................................... -40 to +100°C
Solder Reflow Peak Temperature (40 seconds maximum exposure) ............260°C (+0, -5)
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.
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
1970
xxxx
DU PACKAGE
(Bottom View)
xxxx = Denotes Date Code / Lot Information
RoHS / Pb-free NiPdAu Pin Finish
MSL 2 / 260°C / 1 Year
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
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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Copyright © 2007
Rev. 1.5b, 2008-01-25
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RECOMMENDED OPERATING CONDITIONS
LX1970
Parameter Min Typ Max
Units
Supply Voltage (VDD ) 2¹ 5.5 V
SNK Compliance Voltage Range VSS +0.5 VDD V
SRC Compliance Voltage Range VSS V
DD - 0.5 V
SNK/SRC Output Resistor Range 10 1000 KΩ
Note 1: SRC output will work down to VDD=1.8V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, the following specifications apply over the operating ambient temperature -40°C≤ TA≤ 85°C, VDD = 2V to
5.5V, RLOAD= 50Kohms, Direct Light Input² of 14.6µW/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.
LX1970
Parameter Symbol Test Conditions Min Typ Max
Units
Supply Voltage Range VDD 2 5.5 VDC
Input Supply Current IDD V
DD = 3.0V, ISRC = 38µA, ISNK= open 60 85 110 µA
ISRC V
DD = 3.0V 30 38 46 µA
Output Current 3
ISNK VDD = 3.0V -30 -38 -46 µA
Output SNK/SRC Current Matching IMATCH V
DD = 3.0V 0.5 2 %
SNK Minimum Compliance Voltage SNKVMIN VDD = 3.0V, @ 95% of nominal output
current V
SS +0.3 VSS +0.5 V
SRC Maximum Compliance Voltage SRCVMAX VDD = 3.0V, @ 95% of nominal output
current V
DD – 0.3 VDD – 0.5 V
SNK/SRC Output Dark Current IDARK V
DD = 5.5V, No Light 10 300 nA
Wavelength of Peak Sensitivity λPS 520 nm
Half Reception Angle θ½ ±60 deg
Supply Voltage Coefficient of Output
Current PSRR VDD = 2V to 5.5V 2.2 5 %/V
Sensitivity @ 540nm (peak) Irradiance current responsivity 2.6 A/(W/cm2)
Sensitivity Change @ 910nm 4
Current responsivity change with
additional direct light input of 14.6µW/cm2
at 910nm
-5 0 5 % of peak
Radiant Sensitive Area Photodiode area 0.369 mm2
Note 2: The input irradiance is supplied from a white light-emitting diode (LED) optical source adjusted to impose 14.6µW/cm2 at 555nm on the sensor’s
surface.
Note 3: See Figure 1
Note 4: See Figure 2
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Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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MEASUREMENT CIRCUIT CONFIGURATIONS
A
VDD
SRC
IR LED
910nm
LX1970
White
LED
Current
Sources
%
Figure 1 – Light Current Measurement Circuit Figure 2 – IR sensitivity Measurement Circuit
APPLICATION NOTES
LIGHT UNITS
In converting from µW/cm2 to Nits it is necessary to
define the light source. Nits are units for a measurement
of luminance, which is the apparent brightness of an
illuminated flat surface. µW/cm2 is a measurement of
irradiance or the measurement of electromagnetic
radiation flux both visible and invisible. The first step in
the conversion process is to convert irradiance to
illuminance, which essentially involves running the
irradiant flux through a photopic filter. In normal ambient
a photopic curve is used and in dark ambient a scotopic
curve (dark adapted eye) is used. If the light is composed
of only one wavelength, a conversion chart will tell the
conversion factor to convert µW/m2 to lux (lumens/m2).
If more than one wavelength is used, the light spectrum
of the irradiance must be applied to the photopic filter to
determine the resultant illuminance. The most sensitive
wavelength for the normal light adapted human eye is
555nm. At 555nm, the conversion factor is 683 Lux =
1W/m2 = 100µW/cm2. Therefore 14.6µW/cm² = 100 lux
at 555nm.
The next step in the conversion process is to convert
illuminance to luminance. The units for illuminance are lux or
lumens/m2. The units for luminance are Nits or
lumens/m2/steradian. Assuming the illuminance falls on a
Lambertian surface which has perfect dispersion and total
reflection, the conversion from lux to nits is 3.14 lux falling on
a Lambertian reflector produces 1 Nit. Therefore 100 Lux will
produce 31.4 Nits.
If the photo sensor had a truly photopic response, it would
produce the same output current for the same number of nits or
lux, regardless of the color of the light. However, because the
match is not perfect, there is still wavelength dependency
particularly at the ends of the visible spectrum.
In the case of the LX1970 the peak photo response is at
520nm, however depending on the light source, what the
human eye perceives as ‘white’ light may actually be composed
of peak wavelengths of light other than 520nm. For instance a
typical fluorescent lamp includes dominant light not only near
550nm but also at 404 and 435nm. Incandescent light sources
such as standard tungsten lights generate substantial IR
radiation out beyond 2000nm.
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Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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Rev. 1.5b, 2008-01-25
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APPLICATION INFORMATION
The LX1970 is a light sensor with a spectral response
that resembles the human visual system. It is packaged in
clear MSOP package. Microsemi also offers the LX1971
with the same pin out and similar supply voltage range as
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.
The LX1970 has a responsiveness that is directly
proportional to the intensity of light falling on the photo
receptors. Although the gain varies depending on the
wavelength of the light and the direction of light, in general
for a 555nm wavelength (yellow-green), the sensitivity is:
ALIOUT µ76.0×= for L < 1200 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 1200
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 first curve shows the calculated responsiveness of
the LX1970 without load resistors based on the formula
above. It 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-350mV @ about 100
lux. The second curve shows saturation of the output with
VIN = 5V and a 10K resistor SRC to ground, above about
300uA the output becomes non-linear as it begins to
saturate.
LX1970 Calculated SRC Full Range Response
0
100
200
300
400
500
600
700
800
900
1000
0 200 400 600 800 1000 1200
Light (lux)
S R C Curre nt (µ A)
LX1970 SRC Actual SRC w i th 10 K Resist or
0
50
100
150
200
250
300
350
400
450
0 200 400 600 800 1000 1200
Light (lu x )
S R C Curre nt (µ A)
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Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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Rev. 1.5b, 2008-01-25
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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 LX1970 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 and LX1993.
In each specific application it is important to recognize
the need to correlate the sink and source current of the
LX1970 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 LX1970 for a given
exposure of light.
R1
R2 C1
10µF
3V
To inverter brightness
input or LED driver
controller.
N/C
3.3V or 5V
VSS
VDD
SRC
SNK
Figure 2
The example in figure 2 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 and 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.
R2111
0.25V
3.0V
R2R1 ×=−= ⎥
⎦
⎤
⎢
⎣
⎡
Next the value of R2 can be calculated based on the
maximum output source current coming from the
LX1970 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:
275KΩR211R125KΩ
50µA
1.25V
R2 =×=∴== ⎥
⎦
⎤
⎢
⎣
⎡
The output node will actually reach 1.25V when the source
current from the LX1970 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 3 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
To inverter
brightness input or
LED driver
controller input.
N/C
3.3V or 5V
VSS
VDD
SRC
SNK
3.3V PWM
Figure 3
Figure 4 shows how a fully manual override can be quickly
added to the example in figure 3. In addition to the gate to
turn on and off the LX1970, a diode has been inserted to
isolate the LX1970 when it is shut down.
30K 30K
10µF
To inverter
brightness input or
LED driver
controller.
N/C
VSS
VDD
SRC
SNK
PWM
CMOS
Gate
60K
3.3V
Diable
control
Figure 4
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 reference voltage and develop a logic
shutdown signal. If the application is utilizing a transflective
or reflective LCD display such a signal could disable the
backlight or front light to the display when reaching sufficient
ambient light.
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Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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RESPONSE VS WAVELENGTH
ISNK STEP RESPONSE
0
2
4
6
8
10
12
14
16
00.51
T ime (Seconds)
Amplitude (µW/cm
2
)
Load = 10kΩ and 1µF
Photo Step = Direct Light Input of 14.6µW/cm2
SMALL SIGNAL FREQUENCY RESPONSE
DARK CURRENT VS TEMP
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
10 100 1000 10000 100000
Frequency (Hz)
Amplitude (dB)
0.73µW/cm2
2.63µW/cm2
13.14µW/cm2
VDD = 3.0V, SNK , Three Light Levels, No Filtering
0.01
0.1
1
10
100
1000
25 40 55 70 85
Temp er atu r e (ºC)
Dark Current (nA)
VDD=5.5V
VDD=1.8V
SNK OUTPUT PSRR
GAIN VS TEMP
-120.00
-100.00
-80.00
-60.00
-40.00
-20.00
0.00
20.00
1 10 100 1000 10000 100000
Freque ncy (Hz)
Amplitude (dB)
2.63µW/cm2
13.14µW/cm2
VDD = 3.0V
Load = 10kΩ and 1µF to Ground
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
-50 0 50 100
Temp er atu r e (ºC)
Gain/Gain(25ºC)
y = 1. 35E-03x + 9. 64E-0 1
VDD = 3.0V
Direct Light Input of 13.14µW/cm2
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Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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Rev. 1.5b, 2008-01-25
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VERTICAL DIRECTION RESPONSE
HORIZONTAL DIRECTION RESPONSE
1
1
SNK COMPLIANCE VS CURRENT
SRC COMPLIANCE VS CURRENT
0
20
40
60
80
100
120
140
0.1 0.2 0.3 0.4 0.5 0.6 0.7
SN K co mp l i an ce Vo l tag e (V)
Output Current (µA)
VDD = 3.0V
0
20
40
60
80
100
120
140
0.10.20.30.40.50.60.70.8
So u r ce C o mp l i an ce Vo l tag e (VD D -SR C )
Output Current (µA)
VDD = 3.0V
SRC CURRENT VS LIGHT (LUX)
TYPICAL LOW AMBIENT RESPONSE
0
100
200
300
0 100 200 300
Light Input (LUX)
SRC Current (µA)
cool white
Incandescent
VDD = 5.0V; SRC = 10K & 4.7µF to GND
0.000
0.200
0.400
0.600
0.800
1.000
1.200
0 0.2 0.4 0.6 0.8 1
Ambient Light (lux)
Output Current (µA)
85°C 75°C 55°C <25°C
5V Input
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Microsemi
Analog Mixed Signal Group
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
Top View
Side Views
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 footprint
Bonding / Wafer area
Examination of
Active Area
Active Area
D
C
CL
CL
A
B
P1
MILLIMETERS INCHES
Dim
A 0.225 0.0088
B 0.145 0.0057
C 0.600 0.0236
D 0.612 0.0241
P1 2.5 0.98
Note:
CL is the center of the package
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
Analog Mixed Signal Group
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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