TSL2550 Datasheet, PDF (11/20 Pages) List of Unclassifed Manufacturers – AMBIENT LIGHT SENSOR WITH SMBus INTERFACE

APPLICATION INFORMATION
TSL2550
AMBIENT LIGHT SENSOR
WITH SMBus INTERFACE
TAOS029L − OCTOBER 2007
The TSL2550 is intended for use in ambient light detection applications, such as display backlight control, where
adjustments are made to display brightness or contrast based on the brightness of the ambient light, as
perceived by the human eye. Conventional silicon detectors respond strongly to infrared light, which the human
eye does not see. This can lead to significant error when the infrared content of the ambient light is high, such
as with incandescent lighting, due to the difference between the silicon detector response and the brightness
perceived by the human eye.
This problem is overcome in the TSL2550 through the use of two photodiodes. One of the photodiodes
(channel 0) is sensitive to both visible and infrared light, while the second photodiode (channel 1) is sensitive
primarily to infrared light. An integrating ADC converts the photodiode currents to channel 0 and channel 1 digital
outputs. Channel 1 digital output is used to compensate for the effect of the infrared component of light on the
channel 0 digital output. The ADC digital outputs from the two channels are used in a formula to obtain a value
that approximates the human eye response in the commonly used Illuminance unit of Lux. For standard mode:
where:
Light Level (lux) + (Ch0 * Ch1) 0.39 e^{(*0.181R2 )}
R = Ch1 Counts / (Ch0 Counts − Ch1 Counts)
The formula above was obtained by optical testing with fluorescent and incandescent light sources. The light
level calculated from the formula will be slightly higher than the actual light level for sunlight and will be slightly
lower than the actual light level for composite fluorescent and incandescent light sources.
NOTE:
Please see TAOS application notes for additional information, including implementing a display
brightness control system with the TSL2550, and for a simple implementation of the equation shown
above suitable for use in embedded microcontrollers.
Table 4 contains a summary of the typical sensor outputs for several common light sources.
LIGHT SOURCE
Fluorescent
Daylight (shade)
Incandescent
Table 4. Sensor Output Summary (Standard Mode)
ILLUMINANCE
(LUX)
297
201
42
CHANNEL 0
(COUNTS)
831
895
959
CHANNEL 1
(COUNTS)
68
343
671
RATIO:
CH1/CH0
0.082
0.383
0.7
LUX per CH0
COUNT
0.36
0.22
0.04
Light from 50 or 60 Hz sources, and especially fluorescent lighting, has a high harmonic content. Since the
TSL2550 integrates the ambient light over an approximately 400 millisecond interval (per channel), this light
ripple is typically reduced to less than ¼ LSB.
Power Supply Decoupling
The power supply lines must be decoupled with a 0.1 μF capacitor placed as close to the device package as
possible. The bypass capacitor should have low effective series resistance (ESR) and effective series
inductance (ESI), such as the common ceramic types, which provide a low impedance path to ground at high
frequencies to handle transient currents caused by internal logic switching.
The LUMENOLOGY r Company
r
r
www.taosinc.com
Copyright E 2007, TAOS Inc.
11