tsl2550 ETC-unknow, tsl2550 Datasheet - Page 12

• Manufacturer Part Number: tsl2550
• Description: Ambient Light Sensor With Smbus Interface
• Manufacturer: ETC-unknow

TAOS029 – SEPTEMBER 2002
Power Supply Decoupling
12
Copyright E 2002, TAOS Inc.
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:
where:
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.
Table 5 contains a summary of the typical sensor outputs for several common light sources.
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.
The power supply lines must be decoupled with a 0.1 F 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.
Fluorescent
Daylight (shade)
Incandescent
LIGHT SOURCE
ILLUMINANCE
Light Level (lux) + (Ch0 Counts)
R = (Ch1 Counts) / (Ch0 Counts)
(LUX)
300
100
50
Table 5. Sensor Output Summary
APPLICATION INFORMATION
t
CHANNEL 0
(COUNTS)
www.taosinc.com
831
895
959
(0.46)
CHANNEL 1
(COUNTS)
343
671
68
(e
(*3.13R)
t
)
CH1/CH0
RATIO:
0.082
0.38
0.7
The LUMENOLOGY r Company
LUX per CH0
COUNT
0.052
0.36
0.11