ISL29038

11

FN7851.1

January 23, 2015

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ALS IR Compensation

The ISL29038 is designed for operation under dark glass cover.

Glass or plastic covers can significantly attenuate visible light

and pass the Infrared light without much attenuation.

Consequently, the ISL29038 under a glass cover experiences an

IR rich environment.

The on-chip ALS passive optical filter on the ISL29038 is designed

to block most of the IR incident on the ALS photo diodes. In

addition, the ISL29038 provides a programmable active IR

compensation that subtracts residual IR still reaching the sensor.

The ALS_IR_COMP register (Reg 0x03[4:0]) allows fine tuning of

the residual infrared component from the ALS output.

The recommended procedure for determining ALS IR

compensation is as follows:

• Illuminate the ISL29038 based product with a light source

without IR, such as a white LED. Record the ALS measurement

and the Lux level.

• Illuminate the device with an IR LED and the White LED. Take

an ALS measurement and Lux level measurement.

• Adjust the ALS_IR_COMP register (Reg 0x03, bits 4:0) to

compensate for the IR contribution.

• Repeat steps above until the IR light source contribution to the

ALS measurement is under 10 percent assuming no change in

Lux level due to IR light source.

Proximity Offset

Systems built with a protective glass cover over the ISL29038

can provide light ‘leakage’ or ‘crosstalk’ from the IR LED by

reflection from the glass saturating the proximity sensor

measurement system (Figure 11).

Saturation can occur when the reflection from the glass with no

object in the proximity detection space exceeds the full scale of

the measurement system. The ISL29038 proximity system

provides a user programmable proximity offset correction to

compensate for this reflection.

The PROX_IR_COMP register (Reg 0x02[6:3]) applies a corrective

offset to the received signal prior to ADC conversion, which

allows the signal to be brought within the usable range of the

proximity measurement system.

Proximity Ambient Washout Detection

Optical proximity sensor can saturate when illuminated with

excessive ambient light. The ISL29038 provides a warning flag

when the proximity measurement may be erroneous due to

excessive ambient light. The PROX_WASH register (Reg 0x0D[0])

reports this condition.

Interrupts Events

The ISL29038 interrupts are designed to minimize host

microcontroller overhead of continuous polling. The ISL29038

can generate interrupts on the results of an ALS measurement or

proximity measurement.

The ALS interrupt event ALS_FLAG (Reg 0x04[3]) is governed by

Reg 0x07 through 0x09. Two-12 bit high and low threshold

values are written to these registers. The ISL29038 will set the

ALS interrupt flag if the ADC conversion count in Registers 0x0B

and 0x0C are outside the programmed thresholds. The

ALS_FLAG is cleared by writing a ‘0’ to Reg 0x04[3].

A proximity interrupt event (PROX_FLAG) is governed by the high

and low thresholds in Reg 0x05 and 0x06 (PROX_LT and

PROX_HT) and is indicated by Reg 0x04[7]. PROX_FLAG is set

when the measured proximity data is more than the higher

threshold. The proximity interrupt flag is cleared when the

proximity data is lower than the low proximity threshold or by

writing a ‘0’ to Reg 0x04[7].

The Proximity interrupt generation can be selected between

‘out-of-window’ threshold and hysteresis schemes. When the

PROX_INT_ALG register (Reg 0x02, Bit 7) is set to 0, proximity

uses a window comparator scheme; when set to 1, proximity

uses a hysteresis scheme.

In hysteresis mode, the interrupt event is generated if the

proximity ADC count is higher than the PROX_HT threshold and

the interrupt event is cleared when the proximity ADC count is

less than the PROX_LT threshold. The interrupt event flag can

also be cleared by writing a ‘0’ to Reg 0x04[7].

INTERRUPT PERSISTENCE

To minimize interrupt events due to ‘transient’ conditions, an

interrupt persistency option is available for both ALS and proximity

measurements. Persistency requires ‘X-consecutive’ interrupt flags

before the INT pin is driven low. Both ALS and PROX have their own

independent interrupt persistency options. ALS_PRST and

PROX_PRST configuration are controlled from Reg 0x04.

Power-Up and ‘Brown-Out’ Reset

The ISL29038 has an enhanced power-on-reset system. A

‘Brown-Out’ detector flag in Reg 0x04[4] informs the system that

the device has powered-up properly. This flag should be reset as

part of the initialization sequence.

A ‘Brown-Out’ condition is defined as an operating condition

when the power supply voltage is not within the specified limits.

interface is re-initialized and the configuration registers are set to

power-up default values. After power-up and during device

initialization, host should examine that the PWR_FAIL flag

FIGURE 11. PROXIMITY SET-UP HIGHLIGHTING CROSSTALK

REFLECTED FROM COVER GLASS

LED

SENSOR

PCB

GLASS COVER