TSL2571

LIGHT-TO-DIGITAL CONVERTER

TAOS117A − FEBRUARY 2011

8

r

r

Copyright

E 2011, TAOS Inc.

The LUMENOLOGY

r Company

www.taosinc.com

ALS Operation

The ALS engine contains ALS gain control (AGAIN) and two integrating analog-to-digital converters (ADC) for

the Channel 0 and Channel 1 photodiodes. The ALS integration time (ATIME) impacts both the resolution and

the sensitivity of the ALS reading. Integration of both channels occurs simultaneously and upon completion of

the conversion cycle, the results are transferred to the data registers (C0DATA and C1DATA). This data is also

referred to as channel count. The transfers are double-buffered to ensure data integrity.

CH1

ADC

ALS Control

CH1

Data

CH0

ALS

CH0

Data

AGAIN(r 0x0F, b1:0)

1

y, 8y, 16y, 120y Gain

CH0

CH1

C0DATAH(r 0x15), C0DATA(r 0x14)

C1DATAH(r 0x17), C1DATA(r 0x16)

ATIME(r 1)

2.72 ms to 696 ms

Figure 6. ALS 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 / 2.72 ms

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

Integration Time = 2.72 ms

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 1

gain, in terms of amount of gain, will be represented by the value AGAINx, i.e. AGAINx = 1, 8, 16, or 120.

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 TAOS 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

Lux1 = (C0DATA − 2

Lux2 = (0.6

Lux = MAX(Lux1, Lux2, 0)