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.
During the brown-out period at power-up, the I
2
C interface and
the IR LED driver are inactive. Following brown-out, the I
2
C
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
LEDSENSOR
PCB
GLASS COVER
