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Design Articles

Use of Integrated Optical Proximity Sensors in Multi-Function Smart Phones

Integrated approach enables multiple features and reduces PCB real estate and design engineering steps

By Andreas Vogler, Product Marketing Manager for Infrared Products, OSRAM Opto Semiconductors

Today’s smart phones are the focal point of technological convergence, combining mobile phone, MP3 player, camera, video, wireless internet, e-mail, gaming, Bluetooth, and navigation systems into one small device with the slim profile and light weight that consumers have come to expect.

Short-range proximity sensors have become increasingly important components in managing a number of these features and making these feature-rich mobile devices more flexible and comfortable for the user, while simultaneously reducing power consumption and extending battery life.  Today’s proximity sensors are “smart” devices that are expected to provide real-time feedback that enables the mobile device to react automatically to the environment and to the user.

A new generation of reflective optical proximity sensors has been developed to meet the demands of multi-functional smart phones and other mobile devices.  They function by emitting invisible infrared light and detecting the amount of light reflected from a target (e.g. a finger, hand, or cheek).  These sensors integrate a high-efficiency infrared Thinfilm emitter, a photodetector, and an ASIC in a small surface-mountable package.  They enable both power-saving and user interface functions such as automatic turning on and off of the display and keypad backlight (e.g. if the device is placed in a pocket or face down on a table), deactivation of a touch screen interface to avoid unintended inputs (e.g. when engaged in a phone call), or automatic standby/ready state switching.  Optical proximity sensors may also enable automatic adjustment of the speaker volume when the phone is moved closer to the user’s ear.

These sensing devices take up very little PCB real estate due to the use of Thinfilm LED chip technology and innovative packaging techniques.  New short range optical proximity sensors with a detection range up to ~30 mm (approx. 1.12”) are as small as 3.7 x 3.7 x 1.0 mm (approx. 0.16” x 0.16” x 40 mil), with features such as digital output, low current consumption (50-75 µA average depending on operating range), integrated LED and driver, and ambient light suppression. Their digital signal output and integrated ambient light suppression eliminate the need for additional signal evaluation.

Curbing Power Consumption

figure 1
Figure 1: Thinfilm approach to chip construction directs emitted light
upwards and enables more of the generated light to be used

Thinfilm technology, developed by OSRAM in 1999, was designed to extract more light from the LED chip and direct the light emission almost entirely upwards.  Previously, though more than 90% of the electrical power of the LED chip could be converted into light, only a small proportion of that light could actually emerge from the chip in a “useful” way. Most of the generated light was reflected back into the chip and absorbed there or emitted out the sides of the chip. Thinfilm technology has significantly improved the light output from LED chips primarily by greatly decreasing the distance a photon must travel within the LED before escaping through the top surface.

The concept behind Thinfilm is shown in Figure 1.  A metal layer is integrated in the LED structure and functions as a mirror, reflecting the light generated in the chip to the top surface, from which it is emitted.  In the Thinfilm approach, a conventional LED structure is grown in reverse order on top of an appropriate substrate, with a contact metal deposited as the last layer.  This substrate with the metal layer on top is then flipped and bonded to a new metalized carrier substrate, a material with selected properties superior to the growth substrate (e.g. thermal conductivity).  Finally, the original growth substrate is removed and contacts are formed on the top and bottom surfaces. The resultant chip construction ensures that light is emitted exclusively upwards.

This concept has been applied to a variety of material systems and colors (red, yellow, blue) including, in 2004, infrared light emitting diodes (IREDs).  With LCDs consuming more than 50% of the power budget of a smart phone or other portable mobile device system, maximizing the efficiency of the LED backlight to generate more light without increasing power consumption is critical. The use of a surface emitter employing Thinfilm technology enables light to be emitted only from the top side, versus from all sides, eliminating the need for a reflector or lens to focus the light and enabling simpler and more efficient optical systems.  For the same reasons, this device structure also enables the efficient infrared emitters in advanced, compact packages that are used in state-of-the-art optical proximity sensors.

Addressing Optical Crosstalk

figure 2
Figure 2: Schematic representation of crosstalk in the form of a signal reflected
from the cover window. The reflector may be a finger, ear, or carrying case,
for example.

In designing new miniaturized optical proximity sensors into today’s multi-functional mobile devices, the issues of electrical interference, crosstalk, and signal-to-noise ratio need to be addressed.

There are two sources of interference in an optical proximity sensor: external (e.g. sunlight, indoor lighting, unintended targets) and internal (e.g. optical crosstalk between the subcomponents of the proximity sensor).  External interference can be suppressed in large part in the analog and/or signal processing domain.  For example, the LED signal may be modulated at a given carrier frequency, such that the DC or low-frequency light sources (e.g. sunlight, light bulbs) are ignored (similarly as in TV remote controls).  An often-used alternative is to take “background” measurements when the LED is off, and to subtract the background level from the signal measured when the LED is on.

Internal interference can prove challenging to manage in applications where the proximity sensor is mounted behind a transparent or semi-transparent panel (e.g. plexiglass), since the intensity of the light reflected from the panel/cover can be similar in magnitude to the signal of interest. The distinction between these two signals is shown in Figure 2.  It has been seen in the laboratory that, without appropriate counter-measures, the operating range of a proximity sensor may be reduced from over 25 mm to around 5 mm simply by placing a harmless piece of plexiglass about 1 mm above the sensor. 

figure 3
Figure 3: An appropriate choice of optical separator, extending from the sensor
to the panel/cover can reduce crosstalk between the emitter and detector.
Different separator widths are shown schematically in black, grey, and white, and
the correct dimensions depend on the relationship between the LED-
photodetector spacing, sensor-cover spacing, and cover thickness (s, d, and t
respectively).
Incorporating an optical barrier or separator between the emitter and detector components of the sensor, built from the base of the sensor to the panel/cover, can dramatically reduce internal crosstalk, as shown in Figure 3.  Ideally, the separator height should be as close to the sensor-cover separation as possible, not only to suppress the direct crosstalk from emitter to detector, but also the indirect crosstalk from reflections from both the inner and outer surfaces of the panel/cover. 

As a rule of thumb, the width of the separator should be comparable to the cover thickness to effectively suppress this indirect crosstalk.  Multiple reflections within the cover may generally be ignored as the reflectivity of a typical cover-air interface is in the neighborhood of only 4%.  As a wide separator limits the permissible light emission and detection angles, system designers typically (in particular when using an integrated design) use a panel/cover with minimal thickness and a small sensor-cover distance to maximize sensor performance.

figure 5
Figure 5: This size comparison dramatically
illustrates the tiny size of a short range optical
proximity sensor with integrated design.
figure 4
Figure 4: The integrated approach to optical proximity sensor chip design
dramatically reduces size, with increased functionality. The term “modulator”
refers in general to some driving of the LED, and “demodulator” refers to a
receiver or signal processor that interprets the light stream.


Discrete versus Integrated Design

Optical proximity sensors can be designed in two ways.  Traditionally, system designers built sensors from multiple discrete components, while more recently it is possible to purchase integrated sensors.  Figure 4 illustrates the significant space savings afforded by an integrated approach.  A proximity sensor design using a discrete driver, LED, detector, filter, and amplifier requires approximately 50 mm2 of real estate, while an integrated design requires only about 16 mm2.  Figure 5 graphically demonstrates the tiny size of a short range optical proximity sensor chip with integrated design. Moreover, some integrated proximity sensors incorporate additional functionality, increasing their advantage in reducing real estate, component count, and processor load.

The discrete design illustrated in Figure 4, for example, includes only the analog components required to drive an LED and receive a reflected signal.  It lacks the modulation function incorporated in the integrated design and, thus, requires an external logical drive signal to be generated elsewhere. The term “modulation is used very generally in this case, referring simply to turning the LED on and off in regular (not necessarily sine or square wave) intervals in order to support ambient light suppression. Moreover, the output of the discrete design is an analog signal, so an A/D converter or comparator must also be added to provide a digital output signal.  The ASIC in the integrated design has, in addition to the drive signal generator, an on-chip comparator, an integrated LED driver (and LED in the package), and an ambient light suppression function.  Its digital output signal, combined with these features, eliminates the need for additional signal evaluation.

The component count of a typical complete proximity sensor subsystem using the discrete design approach can reach up to 15 or more components, whereas the integrated approach requires only an external capacitor to stabilize the supply voltage, an optional resistor to adjust the detection distance, and a pull-up resistor for the output.  Integrated modules are also far easier to debug than discrete components within a device.

Conclusion

Mobile hand-held devices will continue to evolve into ever “smarter” consumer tools, with virtually every new technical innovation packed into one slim, compact, lightweight device.  The need for low voltage ICs, extended ranges, smaller and lower profile packages and more functionality will continue to drive the demand for optical proximity sensors that can be integrated into ever smaller multi-function chip packages. These LED sensors will continue to enable the increasing functionality of hand-held mobile devices such as smart phones, while helping designers manage power consumption and PCB real estate demands.

OSRAM Opto Semiconductors
Santa Clara, CA
(888) 446-7726
www.osram-os.com

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