4

FN6732.0

March 3, 2009

Principles of Operation

Photodiodes and ADC

The ISL29021 contains a photodiode array which converts

infrared energy into current. The spectral response for IR

sensing is shown in Figure 6 in the performance curves section.

After IR radiation is converted to current during the infrared

signal processing, the current output is converted to digital by a

command reads the infrared light intensity in counts.

The converter is a charge-balancing integration type 16-bit

ADC. The chosen method for conversion is best for converting

small current signals in the presence of an AC periodic noise. A

100ms integration time, for instance, highly rejects 50Hz and

60Hz power line noise simultaneously. See “Integration and

Conversion Time” on page 7.

The built-in ADC offers user flexibility in integration time or

conversion time. Integration time is determined by an internal

the ADC. A good balancing act of integration time and

resolution depending on the application is required for optimal

results.

accommodate various IR conditions. For very dim

conditions, the ADC can be configured at its lowest range

(Range 1). For very bright conditions, the ADC can be

configured at its highest range (Range 4) in the proximity

sensing.

Low-Power Operation

The ISL29021 initial operation is at the power-down mode

after a supply voltage is provided. The data registers contain

it will start ADC conversion with proximity sensing. It will go to

the power-down mode automatically after one conversion is

finished and keep the conversion data available for the master

to fetch anytime afterwards. The ISL29021 will continuously

command of continuous measurement. It will continuously

update the data registers with the latest conversion data. It will

of power-down.

Infrared and Proximity Sensing

There are four operational modes in ISL29021: programmable

IR sensing once with auto power-down, programmable

proximity sensing once with auto power-down, programmable

continuous IR sensing and programmable continuous

proximity sensing. These four modes can be programmed in

series to fulfill the application needs. The detailed program

configuration is listed in “Register Set” on page 6.

When the part is programmed for infrared (IR) sensing, the

IR light with wavelength within the “IR or Proximity Sensing”

spectral response curve on Figure 6 is converted into

current. With ADC, the current is converted to an unsigned

n-bit (up to 16 bits) digital output.

When the part is programmed for proximity sensing, the

external IR LED is turned on by the built-in IR LED driver

through the IRDR pin. The amplitude of the IR LED current

and the IR LED modulation frequency can be programmed

through Command Register II. When the IR from the LED

reaches an object and gets reflected back, the reflected IR

light with wavelength within the “IR or Proximity Sensing”

spectral response curve in Figure 6 is converted into current.

With ADC, the current is converted to an unsigned n-bit (up

to 16 bits) digital output. The output reading is inversely

proportional to the square of the distance between the

sensor and the object.

Interrupt Function

The active low interrupt pin is an open drain pull-down

The interrupt serves as an alarm or monitoring function to

determine whether the infrared light level or the proximity

detection level exceeds the upper threshold or goes below the

lower threshold. The user can also configure the persistency

IS<1:0> = 0, Freq = 0 (Note 4)

101

mA

IS<1:0> = 0, Freq = 1 (Note 4)

51

mA

Duty Cycle

Duty Cycle of IR LED Modulation

50

%

PROX-IR

PROX

Differential ADC Output of IR and Proximity

Sensing With Object Far Away to Provide

No Reflection

IR and proximity sensing with Range 2; 15

Ω @ IRDR

pin, IS<1:0> = 0, Freq = 0; E = 210 lux, Sunlight.

1.0

%

NOTES:

3. 850nm infrared LED is used in production test. The 850nm LED irradiance is calibrated to produce the same DATA_IR count against an illuminance

level of 210 lux sunlight at sea level.

4. See “Register Set” on page 6.

Electrical Specifications

specified. (Continued)

PARAMETER

DESCRIPTION

CONDITION

MIN

TYP

MAX

UNIT

ISL29021