TMD27721 datasheet(21/60 Pages) OSRAM

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[v1-21] 2016-Feb-16

TMD2772/ TMD2772WA − Principles of Operation

Proximity Detection

Proximity detection is accomplished by measuring the amount

of IR energy, from the internal IR LED, reflected off an object to

determine its distance. The internal proximity IR LED is driven

by the integrated proximity LED current driver as shown in

Figure 24.The proximity detector will see light reflected from

the intended target as well as light reflected through any path.

Both surfaces of a transparent cover will reflect some of the IR

LEDs energy. An air gap of less the 0.5mm between the top of

the module and the cover is recommended. For a detailed

explanation of the of the effects of an air gap see ams

application note; Application Note DN58: Proximity Detection

Behind Glass for a detailed discussion of optical design

considerations.

Figure 24:

Proximity Detection

The LED current driver, output on the LDR terminal, provides a

regulated current sink that eliminates the need for an external

current limiting resistor. The combination of proximity LED

drive strength (PDRIVE) and proximity drive level (PDL)

determine the drive current. PDRIVE sets the drive current to

100%, 50%, 25%, or 12.5% when PDL is not asserted. However,

when PDL is asserted, the drive current is reduced by a factor

of 9.

Referring to the Detailed State Machine figure, the LED current

driver pulses the IR LED as shown in Figure 25 during the Prox

Accum state. Figure 25 also illustrates that the LED On pulse has

a fixed width of 7.3μs and period of 16.0μs. So, in addition to

setting the proximity drive current, 1 to 255 proximity pulses

(PPULSE) can be programmed. When deciding on the number

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