DATASHEET SEARCH SITE | WWW.ALLDATASHEET.COM
Document overview
- Manufacturer or author: Provided By ALLDATASHEET.COM(FREE DATASHEET DOWNLOAD SITE)
- PDF pages: 6
Technical content
Rev. 0.5 1/15 Copyright © 2015 by Silicon Laboratories AN523 AN523 OVERLAY C ONSIDERATIONS FOR THE Si114 X S ENSOR 1. Introduction The Si1141/42/43 infrared proximity detector with integr ated ambient light sensor (ALS) is a flexible, high- performance solution for proximity-detection applications and (visible) ambient-light sensing challenges. The sensor is most commonly used under a glass or plastic overlay whose optical properties modify both the ALS response and the proximity-detection distance. The Si114x can also function through sl otted optical ports or other apertures. This causes a certain amount of attenuation of transmitted LED power and received proximity and ALS light levels. An overlay (used with or without a slot opening) has the following effects: Reduced incoming light for both ALS and proximity detection due to the cover material. A slight reduction of the Si114x's effective view angle due to the cover material's thickness (unless the cover is spherical). Reduced transmitted LED power. Changes in the system's spectral response compared with an uncovered Si114x. This application note gives examples of common overla y materials with various s pectral characteristics and discusses their influence on ALS and proximity detect ion. For other system and mechanical considerations, including emitter-to-receiver optical isolation, refer to “AN498: Si114x Designer’s Guide”. 2. Typical Application Typically, the Si114x is configured to detect the proximity of an object to be detected, such as a user's hand, and may also need to report ambient light for the purpose of screen dimming. In addition to the photodiode used for proximity detection, the Si114x makes use of two photodiodes with different spectral responses for ALS. This scheme enables sensing of the visible light while rejectin g infrared light, which is present in large amounts in incandescent light sources and in sunlight. Infrared light affects the response of both ALS photod iodes to different extents. Rejection is accomplished by summing the two outputs with different coefficients so that information about visible light is obtained. The choice of coefficients varies depending on the cover material's spectral characteristics in the visible and infrared bands. Proximity sensing is affected by the cover material's transmission characteristics at the LED wavelength (typically 850 to 950 nm). Since ambient light affects the performa nce of proximity detection, proximity detection can be enhanced by rejecting as much of the non-LED spectrum as possible; this is because more ambient light forces the Si114x’s ADC to operate at a less sensitive gain range in order to avoid saturation. Thus, the function of an integrated ALS and proximity sensor depends on a compromise between favoring IR light at the LED wavelength for proximity and allowing enough visible light for proper ALS operation. This trade-off depends on the cover material selected.
for high-end products. It cannot be tinted; so, ink must be screen printed on it in order to obtain the desired tint.
- The Need for Opaque Covers
Figure 1. Spectral Response of Teikoku’s MRX IR Black Ink, Single Layer over Polycarbonate
Rev. 0.5 3 5. UV Stabilization In high-end products where aesthetics are of primary importance, UV stabilizat ion is desirable in order to prevent yellowing of a clear window. When a dark cover is selected, UV stabilization is of negligible value and can normally be dispensed with. 6. Evaluation of Various Materials and Colors Table 1 summarizes various material and color selections along with ALS coefficients and expected ALS and proximity performance. The ALS reading in lux is given by the following equation: This assumes a completely open field of view. In practice , an additional correction is needed to account for apertures and for the slight loss of Si114x viewing angl e, which depends on the cover material's thickness. This single “gain correction” factor is adjusted empirically once the product is assembled. Proximity performance assumes the LED and the Si114x are under the same overlay. Contact Silicon Labs for support on the use of other cover materials. Other assumptions for the above formula include the following: Small IR photodiode used for ALS (ALS_IR_ADCMUX = 0) Gain of 1 for both ALS channels (ALS_VIS_ADC_GAIN = ALS_IR_ADC_GAIN = 0) Normal sensitivity ranges for both ALS channels (VIS_RANGE = IR_RANGE = 0) If any of the above are modified, the ALS coefficient for th e respective channel must be scaled accordingly. Refer to the Si114x data sheet for more details on the effects of changing the above settings. In practice, the above settings must be changed in orde r to obtain the best resolution possible for the maximum expected ambient light. Careful attention must be given to the amount of infrared li ght present in various light sources, e.g. incandescent light can saturate the IR photodiode with a much lower lux level than fluorescent light. Refer to the Si114x data sheet for sensitivity data. If the overlay being considered is not described here, it is possible to determine ALS coefficients by prototyping the product and recording each ALS photodiode's response to two different light sources. Coefficients can then be adjusted so as to give the same lux value for both types of lamp. At Silicon Labs, best results were obtained using a rough-service, low-wattage incandescent light bulb with a color temperature of 2500 K (Westinghouse 03952 or equivalent) and a broad-spectrum metal-halide lamp with a color temperature of 6500 K and CRI (color-rendering index) of 96 (Iwasaki M150P36SD or e quivalent). The lamps must be calib rated using an illuminance photometer. Incandescent bu lbs must be allowed to stabilize for at least one minute after turni ng on. Metal-halide lamps typically require 15 mn of warm-up time. Narrow-spectrum lamps, such as tri-band fluorescents (including compact fluorescent bulbs), are not recommended for the computation of ALS coefficients. Coefficients can be scaled to account for different gain settings . In general, it is best to set the infrared and visible ALS gains to the highest ranges that will not be saturated under the expected light condit ions. However, if power consumption is critical, a lower gain setting may be desirable because the measuring time is proportional to the gain. On the other hand, in an application that also requires proximity detection, power consumption is usually dominated by the LED current. Lux computation using photodiode coefficients may be pr ototyped with various non-standard overlays using the Si114x Control Panel. Refer to “AN576: Si114x Control Panel Application User's Gu ide”, Section 7.1.4 “Making Accurate Lux Measurements” for more details. Lux level = [ [(ALS visible reading) - (ALS visible dark reading)] x (ALS visible coefficient) + [ (ALSIR reading) - (ALS IR dark reading)] x (ALS IR coefficient) ] x gain correction
Table 1. Material and Color Selections
10415 SIL IR Black4
- The reflected light power is usually inversely proportional to the square of the distance. Distance derating does not take
- Using an 850 nm infrared LED.
- Using a 950 nm infrared LED.
- Preferred solution for Si114x.
Rev. 0.5 5 DOCUMENT CHANGE LIST Revision 0.1 to Revision 0.2 Updated Figure 1 on page 2. Updated Equation on page 3. Updated Table 1 on page 4. Revision 0.2 to Revision 0.3 Details added for ALS formula assumptions. Custom Teikoku ink mix added. Coefficient table updated. Register setting discussion added. Revision 0.3 to Revision 0.4 Added reference to AN576 for lux computations.
Silicon Laboratories intends to provide customers with the latest, accurate, and in-depth documentation of all peripherals and modules available for system and software implementers using or intending to use the Silicon Laboratories products. Characterization data, available modules and peripherals, memory sizes and memory addresses refer to each specific device, and "Typical" parameters provided can and do vary in different applications. Application examples described herein are for illustrative purposes only. Silicon Laboratories reserves the right to make changes without further notice and limitation to product information, specifications, and descriptions herein, and does not give warranties as to the accuracy or completeness of the included information. Silicon Laboratories shall have no liability for the consequences of use of the information supplied herein. This document does not imply or express copyright licenses granted hereunder to design or fabricate any integrated circuits. The products must not be used within any Life Support System without the specific written consent of Silicon Laboratories. A "Life Support System" is any product or system intended to support or sustain life and/or health, which, if it fails, can be reasonably expected to result in significant personal injury or death. Silicon Laboratories products are generally not intended for military applications. Silicon Laboratories products shall under no circumstances be used in weapons of mass destruction including (but not limited to) nuclear, biological or chemical weapons, or missiles capable of delivering such weapons. Trademark Information Silicon Laboratories Inc., Silicon Laboratories, Silicon Labs, SiLabs and the Silicon Labs logo, CMEMS®, EFM, EFM32, EFR, Energy Micro, Energy Micro logo and combinations thereof, "the world’s most energy friendly microcontrollers", Ember®, EZLink®, EZMac®, EZRadio®, EZRadioPRO®, DSPLL®, ISOmodem ®, Precision32®, ProSLIC®, SiPHY®, USBXpress® and others are trademarks or registered trademarks of Silicon Laboratories Inc. ARM, CORTEX, Cortex-M3 and THUMB are trademarks or registered trademarks of ARM Holdings. Keil is a registered trademark of ARM Limited. All other products or brand names mentioned herein are trademarks of their respective holders. http://www.silabs.com Silicon Laboratories Inc.
400 West Cesar Chavez
Austin, TX 78701 USA Smart. Connected. Energy-Friendly Products www.silabs.com/products Quality www.silabs.com/quality Support and Community community.silabs.com