LX1970IDU Datasheet LX1970IDU | P4-P6

Microsemi

Analog Mixed Signal Group

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 4

Rev. 1.5b, 2008-01-25

WWW.Microsemi .COM

LX1970

isible Li

ht Sensor

RODUCTION

ATA

HEET

INTEGRATED PRODUCTS

MEASUREMENT CIRCUIT CONFIGURATIONS

VDD

ISNK

ISRC

White

LED

Current

Source

LX1970

VDD

SRC

IR LED

910nm

LX1970

White

LED

Current

Sources

Figure 1 – Light Current Measurement Circuit Figure 2 – IR sensitivity Measurement Circuit

APPLICATION NOTES

IGHT UNITS

In converting from

µW/cm

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/cm

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/m

= 100µW/cm

. 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/m

. The units for luminance are Nits or

lumens/m

/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.

Microsemi

Analog Mixed Signal Group

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 5

Rev. 1.5b, 2008-01-25

WWW.Microsemi .COM

LX1970

isible Li

ht Sensor

RODUCTION

ATA

HEET

INTEGRATED PRODUCTS

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:

ALI

OUT

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

= 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

100

200

300

400

500

600

700

800

900

1000

0 200 400 600 800 1000 1200

Light (lux)

SRC Current (µA)

LX1970 SRC Actual SRC with 10K Resistor

100

150

200

250

300

350

400

450

0 200 400 600 800 1000 1200

Light (lux)

SRC Current (µA)

Microsemi

Analog Mixed Signal Group

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 6

Rev. 1.5b, 2008-01-25

WWW.Microsemi .COM

LX1970

isible Li

ht Sensor

RODUCTION

ATA

HEET

INTEGRATED PRODUCTS

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.

10µF

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.

40K

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

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.

P1-P3 P4-P6 P7-P9 P10-P10

LX1970IDU

Mfr. #:

Buy LX1970IDU

Manufacturer:

Microchip / Microsemi

Description:

Optical Sensors Ambient Light Sensors

Lifecycle:

New from this manufacturer.

Delivery:

DHL FedEx Ups TNT EMS

Payment:

T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet

LX1970IDU