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1 / 12 2018-06-04 | Document No.: AN093 www.osram-os.com Application Note High accuracy ambient light sensor SFH 5711 Abstract This application note describes the technical details as well as the operation of the ambient light sensor SFH 5711. Valid for: SFH 5711 Author: Lex Florian Further information: Ambient light sensor —general application note Product page SFH 5711 Application Note No. AN093

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Page 1: AN093 High accuracy ambient light sensor SFH 5711 · High accuracy ambient light sensor SFH 5711 ... Main characteristics of the SFH 5711 ... note about this topic available “Ambient

1 / 122018-06-04 | Document No.: AN093

www.osram-os.com

Application Note

High accuracy ambient light sensor SFH 5711

Abstract

This application note describes the technical details as well asthe operation of the ambient light sensor SFH 5711.

Valid for:SFH 5711

Author: Lex Florian

Further information:Ambient light sensor —general application noteProduct page SFH 5711

Application Note No. AN093

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Table of contents

A. Features of the SFH 5711 .......................................................................................2

B. Basic facts about SFH 5711 ...................................................................................3

Spectral sensitivity ..............................................................................................3

Logarithmic output ..............................................................................................4

Load resistance ..................................................................................................7

Temperature dependence ..................................................................................8

Sensitivity variation .............................................................................................9

Operation of the SFH 5711 .................................................................................9

Supply voltage ..................................................................................................10

Placement of the SFH 5711 ..............................................................................10

Main characteristics of the SFH 5711 ..............................................................11

A. Features of the SFH 5711

The ambient light sensor SFH 5711 is a photo detector with the followingfeatures:

• Perfect V (λ) characteristics

• Opto hybrid with logarithmic current output

• Low temperature coefficient

• High accuracy over wide illumination range

• 2.8 x 2.2 x 1.1 mm SMT package

• Automotive qualified

The SFH 5711 consists of a photodiode which is used for the light detection andan IC with the following functions: amplification of the photodiode output signal,logarithmic converter and temperature correction.

This application note describes the technical details of the sensor. For moregeneral information about ambient light sensing there is a special applicationnote about this topic available “Ambient light sensors — general applicationnote”.

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B. Basic facts about SFH 5711

Spectral sensitivity

Ambient light sensors are used wherever the settings of a system need to beadjusted to the ambient light conditions as perceived by humans. They aredesigned to detect light with a comparable spectral sensitivity as human eyesdo. Figure 1 shows the spectral sensitivity of a standard silicon (Si) photo sensor,the SFH 5711 and the human eye (V(λ)- curve).

Figure 1: Relative spectral sensitivity of a standard Si-detector and the SFH 5711 compared to the human eye

From Figure 1 one can see that a standard Si-detector has its maximumsensitivity in the IR range, which is invisible to human eyes. Incandescent lightsor the sun emit light in this “invisible range”, which then leads a standard Si-detector to “see” a higher brightness than a human would do. This match withthe human eye characteristics is the most important parameter for theperformance of an ambient light sensor.

The effect can be seen in Figure 2. It shows the detector signals for different lightspectra adjusted to the same brightness level for the human eye.

All signals in Figure 2 are normalized to standard light A (2856 K), which is ageneral point of reference for ambient light sensors. Figure 2 illustrates thebrightness measurement deviations of the different detectors compared to thehuman eye. An incandescent light bulb, for instance, emits a high portion of IRlight, which is fully detected by the standard Si-detector, but not seen by thehuman eye. Fluorescent lamps, on the other hand, do not emit much IR light.Hence the signals yielded by the standard Si-detectors are much higher for lightbulbs than they are for fluorescent lamps, even though both lamps appearequally bright to the human eye. The deviation of the brightness measurement

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for the different light sources can directly be derived from Figure 2. Compared tothe human eye, the standard Si-detector signal is 3 % too high in case of a lightbulb and over 90 % too low for a fluorescent lamp. The respective values for theSFH 5711 are ~1 % only. When designing an ambient light sensor application,all possible light sources have to be taken into account.

Figure 2: Detector readings for different light sources at the same brightness. The values are normalized to a standard light A (2856 K). The brightness measurement data of the standard Si-detector shows large deviations due to the high sensitivity to IR light.

Logarithmic output

In order to represent the wide dynamic range of ambient light illuminance EV(Lux) correctly, the SFH 5711 is equipped with an analog logarithmic currentoutput.

, with E0 =1 lx and sensitivity S = 10 μA/dec.

Figure 3 shows the output signal lout versus illuminance EV.

Advantages of a logarithmic output. For brightness measurements a goodrelative resolution over the entire brightness range is important. In other words:when measuring low brightness levels, small changes in those levels need to bedetected, whereas when high brightness levels are measured, only relativelylarge variations are of interest. A change from 100 lx to 200 lx is considerable,whereas the step from 10000 lx to 10200 lx is negligible. For linear outputdetectors such as phototransistors or photodiodes, brightness changes ΔEVresult in changes ΔIout of the output current, which are linear proportional to ΔEV.To resolve small variations in low illumination levels it is necessary to measure in

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small current steps. At high brightness levels, however, it makes little sense tocollect data with such fine absolute resolution.

Figure 3: Output current of the SFH 5711 versus illuminance

A logarithmic connection implements this selection already: Constant relativechanges of the input values are converted into constant absolute changes of theoutput value. Figure 4 explains this: Equal ratios of detected illuminance levelsare converted into output current levels of equal steps.

The following example illustrates this effect:

In many applications, the output signals are processed by an A/D converter. Inthis example an 8 bit converter is used. A linear output photo detector detects amaximum of 100 klx. Hence 256 different values are available to resolve the100 klx. Because of the linear relationship between output current andilluminance, the 256 values are equally distributed over the entire detectionrange, which yields a fixed value of 390 lx/bit. This setting cannot resolve lightlevels below 1000 lx with sufficient accuracy.

Therefore the operating range is changed to 0 lx to 1000 lx by switching to adifferent resistor value in order to achieve a resolution of 3.9 lx/bit for levelsbelow 1000 lx (for example see Figure 5). The relative accuracy of themeasurement is defined by the bit size and it varies depending on the operatingrange. For a typical value of 0.4 mA photocurrent at 1 klx, a 6.9 kΩ resistor yields2.8 V voltage drop at the A/D converter. To enhance the detection range to100 klx, the 2.8 V must resemble 100 klx and the resistor needs to be decreasedto 69 Ω.

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In the case of a logarithmic output, the 256 bits are not evenly distributed overthe detection range, but at a fixed ratio with the absolute detected value. Equalratios of detected illuminance levels are converted into output current levels ofequal steps (see Figure 4). The relative accuracy of the measurement in this caseis a constant value over the entire illuminance range.

Figure 4: A logarithmic output converts equal ratios of detected illuminance levels (EV1, EV2) into current levels of equal steps (I2, I1)

Figure 5 illustrates this procedure. A linear photodetector needs to be operatedwith different resistors to change between different detection ranges, which isnot necessary for the logarithmic output signal device. In the above example, thesensor can directly be connected to an ADC input via external load resistor.

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Figure 5: A linear output detector and a logarithmic output detector are used to detect ambient light. To achieve sufficient resolution over all brightness levels, the operating range of the linear detector is adjusted by using different resistors.

Load resistance

If the SFH 5711 is operated with an external load resistance RL, then the upperdetection limit of the sensor depends on the resistor value of RL. The loadresistance does not directly determine the maximum detection level, but it doesdetermine the output voltage of the sensor, which is limited by the supply voltageVcc. At high illuminance levels the output current Iout is high and the loadresistance must be reduced in order to stay below Vcc (Vout ≤ Vcc = Iout * RL)

Figure 6 shows the relationship between load resistance and maximumdetectable brightness levels. For 2.5 V supply voltage, the detection limit for a56 kΩ resistor is ~ 9 klx. To increase this level, a lower resistor value isnecessary. With 47 kΩ up to 60 klx can be detected. For higher Vcc, thereachable detection limit increases with the same resistor values.

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Figure 6: Typical maximum detectable light level vs. load resistance (resistance values are taken from E12 series)

Temperature dependence

The temperature coefficient of the SFH 5711 is not a constant value like it is thecase for phototransistors and photodiodes. It depends on the illuminance rangeunder which the device is operated.

Figure 7 shows the deviation of the brightness measurements relative to 25 °Cfor the SFH 5711 at different brightness levels. Corresponding data forphototransistors and photodiodes are also shown.

In comparison to a phototransistor, the SFH 5711 yields a better accuracy at allbrightness levels. Above 100 lx, the temperature behavior of the SFH 5711 iscomparable to that of a photodiode. At 10 lx and high temperatures, greaterdeviations than those of photodiodes have to be taken into account.

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Figure 7: Brightness measurement deviation relative to 25 °C for the SFH 5711 compared to photodiode and phototransistor

Sensitivity variation

Due to the manufacturing process there is a sensitivity variation within the SFH5711. To account this, OSRAM Opto Semiconductors offers a choice of binningoptions. Table 1 gives a summary of the binning options for the SFH 5711. Thewidth of each bin is 3 μA. This corresponds to a spread of 1:2 in detectedilluminance.

Operation of the SFH 5711

The SFH 5711 yields an analog output current Iout. This signal can either bedirectly transferred into a microcontroller or transformed into voltage. Thevoltage drop at the load resistor RL is then used as input signal into amicrocontroller. Many applications also use an A/D converter for signalprocessing. An additional capacitance C (see Figure 8 right) allows the

Table 1: Sensitivity binning for the SFH 5711. Each bin corresponds to a spread of 1:2 in detected illuminance

BinOutput current Iout [μA]λ = 560 nm , VDD = 2.5 V; Ev = 180 μW/m²

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adjustment of the reaction time of the sensor. The sensor has been tested up toC= 1 nF. Figure 8 show exemplary operating circuits for the SFH 5711.

Figure 8: Operating circuit for the SFH 5711. The analog output current is directly processed (A) or the voltage drop at the load resistance RL is used for further processing (B). An additional capacitance C can be used to adjust the reaction time

Supply voltage

The SFH 5711 is suitable for a supply voltage range of (2.3 ... 5) V.

Placement of the SFH 5711

The sensitive area of the SFH 5711 is 0.4 mm x 0.4 mm and much smaller thanthe device itself. When placing the part behind a light guide, only this sensitivearea has to be taken into account. Figure 9 shows the position and size of thesensitive area within the package. Table 2 provides the pin configuration.

Table 2: Pin configuration of the SFH 5711

Pin Description

1 VSS

2 VSS

3 VDD

4 Iout

EV

VDD

GND

Iout = S*log(EV/EO) EV

VDD

GND

log log

RL

C

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A B

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Figure 9: Position and size of the sensitive area within the SFH 5711

Main characteristics of the SFH 5711

Table 3 summarizes the main characteristics of the SFH 5711. For further details,please refer to the respective data sheet. The device is RoHS compliant.

Table 3: Main characteristics of SFH 5711

Parameter Value

Size L x W x H [mm] 2.8 x 2.2 x 1.1

Sensitive area [mm2] 0.4 x 0.4

typ. Signal Iout [μA] @1000 lx, VCC= 2.5V 30

Sensitivity variation 3 μA/ bin

Current consumption IDD [μA] 410 @ 0 lx; 460 @ 1000 lx

2.1

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7]

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Don't forget: LED Light for you is your place tobe whenever you are looking for information orworldwide partners for your LED Lightingproject.

www.ledlightforyou.com

ABOUT OSRAM OPTO SEMICONDUCTORS

OSRAM, Munich, Germany is one of the two leading light manufacturers in the world. Its subsidiary, OSRAMOpto Semiconductors GmbH in Regensburg (Germany), offers its customers solutions based on semiconduc-tor technology for lighting, sensor and visualization applications. OSRAM Opto Semiconductors has produc-tion sites in Regensburg (Germany), Penang (Malaysia) and Wuxi (China). Its headquarters for North Americais in Sunnyvale (USA), and for Asia in Hong Kong. OSRAM Opto Semiconductors also has sales offices th-roughout the world. For more information go to www.osram-os.com.

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PLEASE CAREFULLY READ THE BELOW TERMS AND CONDITIONS BEFORE USING THE INFORMA-TION SHOWN HEREIN. IF YOU DO NOT AGREE WITH ANY OF THESE TERMS AND CONDITIONS, DONOT USE THE INFORMATION.

The information provided in this general information document was formulated using the utmost care; howe-ver, it is provided by OSRAM Opto Semiconductors GmbH on an “as is” basis. Thus, OSRAM Opto Semicon-ductors GmbH does not expressly or implicitly assume any warranty or liability whatsoever in relation to thisinformation, including – but not limited to – warranties for correctness, completeness, marketability, fitnessfor any specific purpose, title, or non-infringement of rights. In no event shall OSRAM Opto SemiconductorsGmbH be liable – regardless of the legal theory – for any direct, indirect, special, incidental, exemplary, con-sequential, or punitive damages arising from the use of this information. This limitation shall apply even ifOSRAM Opto Semiconductors GmbH has been advised of possible damages. As some jurisdictions do notallow the exclusion of certain warranties or limitations of liabilities, the above limitations and exclusions mightnot apply. In such cases, the liability of OSRAM Opto Semiconductors GmbH is limited to the greatest extentpermitted in law.

OSRAM Opto Semiconductors GmbH may change the provided information at any time without giving noticeto users and is not obliged to provide any maintenance or support related to the provided information. Theprovided information is based on special conditions, which means that the possibility of changes cannot beprecluded.

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12 / 122018-06-04 | Document No.: AN093