TMD2772

DIGITAL ALS

and PROXIMITY MODULE

TAOS147

E − DECEMBER 2012

12

r

r

Copyright

E 2012, TAOS Inc.

The LUMENOLOGY

r Company

www.taosinc.com

Figure 8 illustrates light rays emitting from the internal IR LED, reflecting off an object, and being absorbed by

the CH0 and CH1 photodiodes. The proximity diode selector (PDIODE) determines which of the two

photodiodes is used for a given proximity measurement. Note that neither photodiode is selected when the

device first powers up, so PDIODE must be set for proximity detection to work.

Referring again to Figure 9, the reflected IR LED and the background energy is integrated during the LED On

time, then during the LED Off time, the integrated background energy is subtracted from the LED On time

energy, leaving the IR LED energy to accumulate from pulse to pulse. The proximity gain (PGAIN) determines

the integration rate, which can be programmed to 1

gain, which is recommended for most applications. For reference, PGAIN equal to 8

TMD2771’s 1

(0x13) will be set if the integrator saturates. This condition can occur if the proximity gain is set too high for the

lighting conditions, such as in the presence of bright sunlight. Once asserted, PSAT will remain set until a special

function proximity interrupt clear command is received from the host (see command register).

After the programmed number of proximity pulses have been generated, the proximity ADC converts and scales

the proximity measurement to a 16-bit value, then stores the result in two 8-bit proximity data (PDATAx)

registers. ADC scaling is controlled by the proximity ADC conversion time (PTIME) which is programmable from

1 to 256 2.73-ms time units. However, depending on the application, scaling the proximity data will equally scale

any accumulated noise. Therefore, in general, it is recommended to leave PTIME at the default value of one

2.73-ms ADC conversion time (0xFF).

In many practical proximity applications, a number of optical system and environmental conditions can produce

an offset in the proximity measurement result. To counter these effects, a proximity offset (POFFSET) is

provided which allows the proximity data to be shifted positive or negative. Additional information on the use

of the proximity offset feature is provided in available TAOS application notes.

Once the first proximity cycle has completed, the proximity valid (PVALID) bit in the Status register will be set

and remain set until the proximity detection function is disabled (PEN).

For additional information on using the proximity detection function behind glass and for optical system design

guidance, please see available TAOS application notes.