Proximity Sensor SFH 7741
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Proximity Sensor SFH 7741 Application note 1. Introduction The SFH 7741 is a very small reflective optical sensor for short distances with digital output. With dimensions of only 3.7x3.7x1mm3, and surface-mount solder contacts, the device may be integrated in applications where reflective sensors have previously not been considered. A photograph of the part is shown in figure 1. This device was conceived to operate as a short- to medium-distance proximity sensor. The typical operating range is around 30 mm for a diffusely reflecting target with high reflection coefficient (for example, white nonglossy paper). Based on the low mean current consumption (45 µA) the target applications include especially mobile devices with touch-screens (e.g. cell phones, PDAs, portable entertainment devices) that must be turned off if placed near the user’s face. It could also be used in other proximity applications such as contactless switches (as in Figure 2 for example). This application note is structured as follows: • General description of the part’s design and operating principle • Application circuit recommendations • Customization to account for optical properties of the application • General application comments 10/27/2010 page 1 of 7 Figure 1: Photo of the SFH 7741. Contact pads are on the bottom side of the package Figure 2: The SFH 7741 may be used to implement a contactless or proximity switch. The presence of a hand or finger (the target) is detected by the reflection of light from the target.

2. Design and Operating Principle The part consists of an infrared light emitting diode (LED) with a wavelength of 850nm, a phototransistor, and an ASIC. A block schematic diagram is shown in Figure 3. For low current consumption (45µA average at Vdd=3V, Rprog=470 ) the ASIC performs a measurement every 90ms and the result is latched at the output and held until the next measurement result is present. When a measurement is performed, the emitter is driven with current If for 44µs. The phototransistor current IP is measured twice: immediately prior to the emitter pulse, and just before the...

The part is not qualified or warranted to run in test mode for any significant period of time. 3. Application Circuit The electrical set-up and the proposed external components are shown in Figure 3. An external pull-up resistor is required at the output. A value of 100k is recommended, but up to 1M can be used to keep the current consumption low. In this case the circuit will be more sensitive to noise. The power supply bypass capacitor C1 is necessary to reduce high frequency noise – 10-100nF is recommended. A further (larger) bypass capacitor C2 may be necessary to stabilize Vdd during...

A full discussion of these relationships is presented in the Application Note “Proximity Sensors - Part 1: Optics and Mechanics” (please contact your sales representative for a copy). As a point of reference, OSRAM OS is using a standard target with 90% reflectivity (KODAK neutral white paper– may be ordered from KODAK, Cat. No. E 152 7799, Publication R-270). effective target reflectivity scales with the target area). The typical switching distance of a human finger for example is 17 mm compared to 30mm with KODAK white paper (50x50mm², If = 50mA). Based on the necessary If (taken from...

Depending on the Vdd and Rprog the typical current draw Idd can be seen in Figure 6. Idd/µA 55 50 470 330 discussion of these issues, it is strongly recommended to consult the Application Note “Proximity sensors / Part 3: Crosstalk due to sensor packaging” (please contact your sales representative for a copy). 45 40 2k 35 30 inf 25 20 2.4 2.7 3.0 Vdd/V 3.3 3.6 Figure 6: SFH 7741; current consumption as a function of Vdd for different Rprog parameter. 4.2 Interference In any reflective optical sensor, the detector receives light not only from the desired target. Ambient light, as well as...

supplies should be avoided. Since the IR emitter pulse is much higher than the ASIC current draw (i.e. a high, low-duty cycle supply pulse), it can influence the stability of the supply voltage. This instability will influence the functioning of the SFH 7741. This effect is not observed to occur during normal operation of the sensor with batteries, storage batteries, or stabilized voltage supplies 5.3 Device handling Figure 8: SFH 7741 time sequence The measurement cycle tawake starts after 90ms sleep time, beginning with the reference measurement for the ambient light level (1), which is...

Figure 9: Package drawing and pin configuration of the SFH 7741 Pin 1 2 3 4 5 6 Table 2: Pin configuration of the SFH 7741 Description Anode LED (must not be connected) GND Out Test (must be connected to GND) Vcc Prog Authors: Dirk Sossenheimer, Jürgen Gärtner About Osram Opto Semiconductors Osram Opto Semiconductors GmbH, Regensburg, is a wholly owned subsidiary of Osram GmbH, one of the world’s three largest lamp manufacturers, and offers its customers a range of solutions based on semiconductor technology for lighting, sensor and visualisation applications. The company operates...