TMD27721 datasheet(21/59 Pages) AMSCO

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ams Datasheet, Confidential

Page 21

[v1-20] 2014-Jul-21

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TMD2772/ TMD2772WA − Principles of Operation

of proximity pulses, keep in mind that the signal increases

proportionally to PPULSE, while noise increases by the square

root of PPULSE.

Figure 26:

Proximity LED Current Driver Waveform

Figure 25 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 26, 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×, 2×, 4×, or 8× gain. At power up, PGAIN

defaults to 1× gain, which is recommended for most

applications. For reference, PGAIN equal to 4× is comparable to

the TMD2771’s 1× gain setting. During LED On time integration,

the proximity saturation bit in the Status register (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”

on page 30.

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

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