TMD2771

DIGITAL ALS

and PROXIMITY MODULE

TAOS143B − SEPTEMBER 2012

9

The LUMENOLOGY

r Company

r

r

Copyright

E 2012, TAOS Inc.

www.taosinc.com

ALS Operation

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

for the two 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

ADC

CH0

Data

AGAIN(r 0x0F, b1:0)

1

y, 8y, 16y, 120y Gain

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

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

ATIME(r 1)

2.72 ms to 700 ms

Channel 1

IR Only

Channel 0

Visible and IR

Figure 7. 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, 700).

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

glass/plastic above the device, lux can be calculated using the following. 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 can be used

(referred to as GA in the equation below). For open air with no aperture, 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)