ISL29028A

9

FN7721.2

March 24, 2011

The proximity sensor is an 8-bit ADC which operates in a similar

fashion. When proximity sensing is enabled, the IRDR pin will

drive a user-supplied infrared LED, the emitted IR reflects off an

object (i.e., a human head) back into the ISL29028A, and a sensor

converts the reflected IR wave to a current signal in 0.54ms. The

ADC subtracts the IR reading before and after the LED is driven

(to remove ambient IR such as sunlight), and converts this value

to a digital count stored in Register 0x8.

The ISL29028A is designed to run two conversions concurrently: a

proximity conversion and an ALS (or IR) conversion. Please note that

because of the conversion times, the user must let the ADCs perform

PROX_DATA (wait 0.54ms) or ALSIR_DT1/2 (wait 100ms). The timing

between ALS and Prox conversions is arbitrary (as shown in Figure 3).

The ALS runs continuously with new data available every 100ms. The

proximity sensor runs continuously with a time between conversions

decided by PROX_SLP (Register 1 Bits [6:4]).

Ambient Light and IR Sensing

The ISL29028A is set for ambient light sensing when Register bit

ALSIR_MODE = 0 and ALR_EN = 1. The light-wavelength response of

the ALS appears as shown in Figure 6. ALS measuring mode (as

opposed to IR measuring mode) is set by default.

When the part is programmed for infrared (IR) sensing

(ALSIR_MODE = 1; ALS_EN = 1), infrared light is converted into a

current and digitized by the same ALS ADC. The result of an IR

conversion is strongly related to the amount of IR energy incident

on our sensor, but is unitless and is referred to in digital counts.

Proximity Sensing

When proximity sensing is enabled (PROX_EN = 1), the external

IR LED is driven for 0.1ms by the built-in IR LED driver through

the IRDR pin. The amplitude of the IR LED current depends on

Register 1 bit 3: PROX_DR. If this bit is low, the load will see a

fixed 110mA current pulse. If this bit is high, the load on IRDR

will see a fixed 220mA current pulse as seen in Figure 4.

When the IR from the LED reaches an object and gets reflected

back into the ISL29028A, the reflected IR light is converted into

current as per the IR spectral response shown in Figure 7. One

entire proximity measurement takes 0.54ms for one conversion

(which includes 0.1ms spent driving the LED), and the period

between proximity measurements is decided by PROX_SLP

(sleep time) in Register 1 Bits 6:4.

Average LED driving current consumption is given by Equation 1.

A typical IRDR scheme is 220mA amplitude pulses every 800ms,

which yields 28μA DC.

Total Current Consumption

pin sinks current (as shown in Figure 4) and the average IRDR

voltage and the mode-of-operation as seen in Figure 11.

Interrupt Function

The ISL29028A has an intelligent interrupt scheme designed to

shift some logic processing away from intensive microcontroller

independent light sensor which can instruct a system to “wake

up” or “go to sleep”.

ALS

ACTIVE

100ms

100ms

100ms

PROX

SENSOR

ACTIVE

IRDR

(CURRENT

DRIVER)

ALS CONVERSION TIME =

100ms (FIXED)

0.54ms FOR PROX

CONVERSION

SERIES OF

CURRENT PULSES

TOTALING 0.1ms

SLEEP TIME

(PROX_SLP)

100ms

100ms

SEVERAL µs BETWEEN

CONVERSIONS

TIME

TIME

TIME

FIGURE 3. TIMING DIAGRAM FOR PROX/ALS EVENTS - NOT TO SCALE

FIGURE 4. CURRENT DRIVE MODE OPTIONS

PIN 8 - IRDR

220mA

(PROX_DR = 1)

(IRDR IS HI-Z WHEN

NOT DRIVING)

110mA

(PROX_DR = 0)

;

;

100

μs

×

+

-----------------------------------------------------

=

(EQ. 1)