Trade-offs: 2-in-1 or 3-in-1 ambient light and proximity sensor
As sensors become more popular and systems become more complex, the idea of a plug-in module is gaining popularity. In terms of light sensors, these modules are referred to in terms of the number of functions incorporated in one package. The first devices are simple ambient, or visible, light sensors. (I guess these would be called 1-in-1.) When proximity sensors were invented, they existed as a solo product but also soon joined the ambient light sensor, or ALS. This is called a 2-in-1.
Please note that a proximity sensor needs an accompanying IR LED to generate the infrared signal it uses to bounce off of a nearby object and detect its presence. See Figure 1. Therefore the next generation, the 3-in-1, is a device where the IR LED is co packaged with the ALS and the proximity sensor. While this seems like a natural and easy progression, let me assure you that the optics and mechanics involved in this process are quite challenging. Of course, challenge is what makes engineering fun, usually… And remember that customers pay in proportion to the value the device has to them, not in proportion to the amount of effort that might have been exerted to create it. All of that said, let’s examine the trade-offs associated with using a 2-in-1 device versus a 3-in-1 device.
Figure 1. A proximity sensor needs an accompanying IR LED to generate the infrared signal it uses to bounce off of a nearby object and detect its presence.
For this discussion, the 2-in-1 device will be the ISL29038. Like the circuit on its evaluation board, it will be paired with an SFH4650 IR LED. (Figure 2) For the 3-in-1 device, I have chosen the ISL29044, shown in Figure 3.
Figure 2. Evaluation board of the ISL29038, a 2-in-1. The LED is larger than the sensor in this solution.
Figure 3. Evaluation board of the ISL29044, a 3-in-1.
Let’s start with mechanical considerations. Most obviously, since the 3-in-1 device is a single package, it has the smallest bill of materials (BOM) and smallest footprint as seen in Figure 3. This saves the manufacturer from both having to choose an IR LED and from deciding the best distance that should separate the IR LED and the sensor. On the other hand, the 2-in-1 sensor forces the user to choose an IR LED. The advantage of making this choice is that there are a wide range of IR LEDs available. The two most relevant issues are package size and half angle. Both will be dictated by the desired application.
While package size is somewhat self-explanatory, half angle may be a foreign concept. Some LEDs emit light with a narrow “beam” (like a good flashlight), while others have a wide “beam” (more like a ceiling light). The narrow “beam” is similar to a small half angle and the wide “beam” is like a large half angle. See Figure 4 for clarification. Choosing an LED with a narrow half-angle, like 15 degrees, will increase the proximity distance that the system can be measured. A wider-angle LED will spread the energy over a wider area, but not reach as far from the IR LED and sensor.
Figure 4. The half angle of the LED is a measurement of half of the arc of the spreading of the beam of light it emits.
Not only can the IR LED be chosen for optimum half angle, it can be placed at the optimum distance from the sensor. This is more than a subtle effect. If the IR LED and the sensor are too close together or placed too far below the bottom surface of the glass with improper isolation, internal reflections can saturate the system. It’ll become useless. Similarly, if the IR LED and the sensor are too far apart – or if they are placed too close to the bottom surface of the cover glass — there can be a “dead zone” near the surface of the glass. Systems cannot sense the presence of an object in dead zones. This is commonly referred to as its zero-distance behavior or a “blind zone”. It is critically important that a smart phone have the appropriate response when an object (like the user’s ear and cheek) are at zero distance or the screen may not be disabled and calls would be interrupted.
Different manufacturers use different apertures, that is, openings above the sensor. They also use different gaps, or the distance between the sensor and the back of the glass. The 2-in-1 solution allows the system designers the ability to optimize the mechanical design and optical design better than the 3-in-1. With the freedom to choose all distances, widths and isolations, the distance that the proximity sensor can reach will be maximized as well. The proximity distance of the ISL29038 is shown in Figure 5.
Of course, both solutions with share some attributes as well. The wavelength spectrum of the ink or film on the cover glass will challenge both the 2-in-1 and 3-in-1 solutions in the same manner. Also, both need to have a barrier, in the case of a 2-in-1, or a boot, in the case of a 3-in-1, for optimum performance. A barrier is a vertical wall usually made of some sort of rubber, foam, plastic or the smart phone housing that isolates the IR LED from the sensor. A boot is a cover that slips over the 3-in-1 package and extends upward, creating a tube filtering light down into the sensor and IR LED. At the same time, the boot has a built-in flap to separate the IR LED access from the sensor chamber. This is in addition to the typical isolation of the area as with non-reflective black foam, especially covering any nearby metal. One module commonly located near the ambient light and proximity sensor is the camera. Camera modules typically are housed in metal casings that could cause unwanted reflections in the proximity system. The side of the camera module that faces the sensor should be painted non-reflective black or covered in some kind of non-reflective material. Lastly, when comparing the entire bills of materials, both solutions cost about the same.
3-in-1s are gaining popularity in proximity and ambient light sensor solutions. While someday they might be good enough to completely replace 2-in-1, the ISL29038 offers features that are currently not available in a 3-in-1 device, even the ISL29044. These features, offset cancellation and IR cancellation, couple with the freedom of design choices to enable strong smart phone sensor solutions.
About the Author
Tamara Schmitz is a Senior Principal Applications Engineer and Global Technical Training Coordinator at Intersil Corporation, where she has been employed since mid 2007. Tamara holds a BSEE and MSEE in electrical engineering and Ph.D. in RF CMOS Circuit Design from Stanford University. From August 1997 until August 2002 she was a lecturer in electrical engineering at Stanford; from August 2002 until August 2007, she served as assistant professor of electrical engineering at San Jose State University.