TCS3772

COLOR LIGHT-TO-DIGITAL CONVERTER

with PROXIMITY SENSING

TAOS145B − SEPTEMBER 2012

11

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 light energy, generally from an IR LED,

reflected off an object to determine its distance. The proximity light source, which is external to the TCS3772

device, is driven by the integrated proximity LED current driver as shown in Figure 6.

Prox

Integration

Prox Control

Prox

ADC

Prox LED

Current Driver

PDATAH(r0x019)

PDRIVE(r0x0F, b7:6)

Prox

Data

External IR

LED

PTIME(r0x02)

PVALID(r0x13, b1)

PPULSE(r0x0E)

Background Energy

PDATAL(r0x018)

LDR

Object

Figure 11. 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 100 mA, 50 mA, 25 mA. To drive an

external light source with more than 100 mA or to minimize on-chip ground bounce, LDR can be used to

drive an external p-type transistor, which, in turn, drives the light source.

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

Figure 12 during the Prox Accum state. Figure 12 also illustrates that the LED On pulse has a fixed width of

6.3

μs and period of 14.0 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

14.0

ms

6.3

ms

Figure 12. Proximity LED Current Driver Waveform

Figure 11 illustrates light rays emitting from an external IR LED, reflecting off an object, and being absorbed by

the proximity photodiode.