TSL2571

LIGHT-TO-DIGITAL CONVERTER

TAOS117A − FEBRUARY 2011

7

The LUMENOLOGY

r Company

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Copyright

E 2011, TAOS Inc.

www.taosinc.com

PRINCIPLES OF OPERATION

System State Machine

The device provides control of ALS and power management functionality through an internal state machine

(Figure 5). After a power-on-reset, the device is in the sleep mode. As soon as the PON bit is set, the device

will move to the start state. It will then continue through the Wait and ALS states. If these states are enabled,

the device will execute each function. If the PON bit is set to 0, the state machine will continue until all

conversions are completed and then go into a low power sleep mode.

Sleep

Start

Wait

ALS

PON = 1 (r 0:b0)

PON = 0 (r 0:b0)

Figure 5. Simplified State Diagram

NOTE: In this document, the nomenclature uses the bit field name in italic followed by the register number and

bit number to allow the user to easily identify the register and bit that controls the function. For example, the

power on (PON) is in register 0, bit 0. This is represented as PON (r0:b0).

Photodiodes

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 through the use of two photodiodes. The channel 0 photodiode, referred to as the

CH0 channel, is sensitive to both visible and infrared light, while the channel 1 photodiode, referred to as CH1,

is sensitive primarily to infrared light. Two integrating ADCs convert the photodiode currents to digital outputs.

The ADC digital outputs from the two channels are used in a formula to obtain a value that approximates the

human eye response in units of lux.