TMD2672

DIGITAL PROXIMITY DETECTOR

TAOS149C − AUGUST 2012

9

The LUMENOLOGY

r Company

r

r

Copyright

E 2012, TAOS Inc.

www.taosinc.com

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 7.

CH1

Prox

Integration

Prox Control

Prox

ADC

Prox LED

Current Driver

CH0

PDATAH(r0x019)

PDRIVE(r0x0F, b7:6)

Prox

Data

IR

LED

PTIME(r0x02)

PVALID(r0x13, b1)

PPULSE(r0x0E)

POFFSET(r0x1E)

PSAT(r0x13, b6)

PDIODE(r0x0F, b5:4)

Background Energy

PDATAL(r0x018)

Object

LDR

LEDK

LEDA

Figure 7. 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. PDRIVE sets the drive current to one of four selectable levels.

Referring to the Detailed State Machine figure, the LED current driver pulses the IR LED as shown in Figure 8

during the Prox Accum state. Figure 8 also illustrates that the LED On pulse has a fixed width of 7.3

μs and

period of 16.0

μs. So, in addition to setting the proximity drive current, 1 to 255 proximity pulses (PPULSE) can

be programmed. When deciding on the number of proximity pulses, keep in mind that the signal increases

proportionally to PPULSE, while noise increases by the square root of PPULSE.

LED On

LED Off

IR LED Pulses

Background

Energy

Reflected IR LED +

Background Energy

16.0

ms

7.3

ms

Figure 8. Proximity LED Current Driver Waveform

Figure 7 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.