TSL27721 datasheet(19/56 Pages) AMSCO

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TSL2772 − Principles Of Operation

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 TSL2772 device, is driven by the integrated

proximity LED current driver as shown in Figure 23.

Figure 23:

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

120 mA, 60 mA, 30 mA, or 15 mA when PDL is not asserted.

However, when PDL is asserted, the drive current is reduced by

a factor of about 8 at VLDR = 1.6 V. To drive an external light

source with more than 120 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 24 during

the Prox Accum state. Figure 24 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.

CH1

Prox

Integration

Prox Control

Prox

ADC

Prox LED

Current Driver

CH0

PDATAH(r0x019)

PDRIVE(r0x0F, b7:6)

Prox

Data

External IR

LED

PTIME(r0x02)

PVALID(r0x13, b1)

VDD

PDL(r0x0D,b0)

PPULSE(r0x0E)

PGAIN(r0x0F, b3:2)

POFFSET(r0x1E)

PSAT(r0x13, b6)

PDIODE(r0x0F, b5:4)

Background Energy

PDATAL(r0x018)

LDR

Object

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