TSL27721 datasheet(18/56 Pages) AMSCO

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TSL2772 − Principles O f Operation

The registers for programming the integration and wait times

are a 2’s compliment values. The actual time can be calculated

as follows:

ATIME = 256 - Integration Time / 2.73 ms

Inversely, the time can be calculated from the register value as

follows:

Integration Time = 2.73 ms

× (256 - ATIME)

In order to reject 50/60-Hz ripple strongly present in fluorescent

lighting, the integration time needs to be programmed in

multiples of 10 / 8.3 ms or the half cycle time. Both frequencies

can be rejected with a programmed value of 50 ms (ATIME =

0xED) or multiples of 50 ms (i.e. 100, 150, 200, 400, 600).

The registers for programming the AGAIN hold a two-bit value

representing a gain of 1x, 8x, 16x, or 120x. The gain, in terms of

amount of gain, will be represented by the value AGAINx, i.e.

AGAINx = 1, 8, 16, or 120. With the AGL bit set, the 1x and 8x

gains are lowered to 1/6x and 8/6x, respectively, to allow for

operation up to 60k lux. Do not enable AGL when AGAIN is 16x

or 120x.

Lux Equation

The lux calculation is a function of CH0 channel count (C0DATA),

CH1 channel count (C1DATA), ALS gain (AGAINx), and ALS

integration time in milliseconds (ATIME_ms). If an aperture,

glass/plastic, or a light pipe attenuates the light equally across

the spectrum (300 nm to 1100 nm), then a scaling factor referred

to as glass attenuation (GA) can be used to compensate for

attenuation. For a device in open air with no aperture or

glass/plastic above the device, GA = 1. If it is not spectrally flat,

then a custom lux equation with new coefficients should be

generated. (See ams application note).

Counts per Lux (CPL) needs to be calculated only when ATIME

or AGAIN is changed, otherwise it remains a constant. The first

segment of the equation (Lux1) covers fluorescent and

incandescent light. The second segment (Lux2) covers dimmed

incandescent light. The final lux is the maximum of Lux1, Lux2,

or 0.

CPL = (ATIME_ms

× AGAINx) / (GA × 60)

Lux1 = (1

× C0DATA - 1.87 × C1DATA) / CPL

Lux2 = (0.63

× C0DATA - 1 × C1DATA) / CPL

Lux = MAX(Lux1, Lux2, 0)

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