ISL29038_15 Datasheet, PDF (10/15 Pages) Intersil Corporation – Low Power Ambient Light and Proximity Sensor with Enhanced Infrared Rejection

ISL29038
Registers 0x01, 0x02 are used to configure the primary proximity
and ALS parameters. Register 0x03 is used for optimizing IR
compensation in ALS measurements. A procedure to optimize IR
compensation is described in “ALS IR Compensation” on page 11.
Register 0x04 is the Interrupt Configuration and Status Register,
and is used primarily to indicate interrupt events from proximity
and ALS measurements. A PWR_FAIL bit to indicate a
‘Brown-Out’ event is available and is set in case of a power supply
interruption. A ‘Brown-Out’ event does not generate a hardware
interrupt. The host micro-controller must clear this bit by writing a
‘0’ to Reg 0x04[4].
Register 0x04 is also used to configure ALS and Proximity
interrupt persistency and the operation of the INT pin.
Registers 0x05 and 0x06 are used to set the proximity ‘LOW’ and
‘HIGH’ threshold for proximity interrupt event generation.
Registers 0x07, 0x08 and 0x09 are used to set the ALS ‘LOW’
and ‘HIGH’ threshold. Two 12-bit numbers span three address
locations as shown in Table 1.
Data registers 0x0A holds result of proximity conversion. The
proximity result should be validated by ‘Washout’ bit in Reg
0x0D[0]. Registers 0x0B and 0x0C hold results of an ALS
measurement.
The ALS data is 12 bits wide. Least Significant Byte of the ALS
data is available at address 0x0C and Most Significant Byte
(MSB) of ALS data is available at address 0x0B. The MSB is right
justified, i.e., the upper nibble is always zero and lower nibble
contains four data bits.
Register 0x0D[7:1] contains ambient IR measurement in
proximity measurement phase. This measurement is for
detecting ambient Washout condition, which is indicated by Reg
0x0D[0] being ‘HIGH’. Proximity ‘Washout’ is described in
“Proximity Ambient Washout Detection” on page 11.
A software reset can be initiated by writing 0x38 to Reg 0x0E.
ISL29038 Operation
Photodiodes and ADCs
The ISL29038 contains two photodiode arrays, which convert
photons (light) into current. The ALS photodiodes are designed to
mimic the human eye’s wavelength response curve to visible light.
The ALS photodiodes’ current output is digitized by a 12-bit ADC.
The ALS ADC output is accessed by reading from Reg 0x0B and
0x0C when the ADC conversion is completed.
The ALS ADC converter uses a charge-balancing architecture.
Charge-balancing is best suited for converting small current signals
in the presence of periodic AC noise. The ISL29038 targets an
integration time of 90ms, which can vary ±15% from nominal. The
ALS integration time is intended to minimize 60Hz flicker.
The proximity sensor uses an 8-bit ADC, which operates in a
similar fashion. The IRDR pin drives (pulses) an infrared LED, the
emitted IR reflects off an object back into the ISL29038, and the
photo diodes convert the reflected IR to a current signal in
0.5ms. The ADC subtracts the IR reading before and after the
LED is driven to remove ambient IR contribution.
The ALS runs continuously with new data available every 90ms.
The proximity sensor runs continuously with a time between
conversions controlled by PROX_SLP (Reg 0x01[6:4]).
Ambient Light Sensing
The ISL29038 is set for ambient light sensing when Register bit
ALS_EN = 1. Four measurement ranges from 125 Lux to 4000 Lux
are available. The ALS measurement range is configured via Reg
0x02[1:0].
Proximity Sensing
When proximity sensing is enabled (PROX_EN = 1), the external
IR LED is driven for 100µs by the built-in IR LED driver through
the IRDR pin.
ALS
CONVERSION TIME
(FIXED)
= 90ms
SEVERAL  s BETWEEN
CONVERSIONS
ALS
ACTIVE
90ms
PROX
SENSOR
ACTIVE
90ms 90ms 90ms
0.50ms FOR PROX
CONVERSION
90ms
TIME
TIME
IRDR
(CURRENT
DRIVER)
SERIES OF
CURRENT PULSES
TOTALING 0.09ms
TIME
SLEEP TIME
(PROX_SLP)
FIGURE 10. TIMING DIAGRAM FOR PROX/ALS EVENTS - NOT TO SCALE
The IR LED current depends on PROX_DRV (Reg 0x01[1:0]). Drive
current settings are as shown in Table 1. IR LED drive is in high
impedance state when not active.
When the IR from the LED reaches an object and gets reflected
back to the ISL29038, the reflected IR light is converted into a
current. This current is converted to digital data using an 8-bit
ADC. The proximity measurement takes 0.5ms for one
conversion including the 90μs LED drive time. The period
between proximity measurements is determined by PROX_SLP
(sleep time) in Reg 0x01[4:2].
Average LED driving current consumption is given by Equation 1.
I_lRDR;AVE = ^I--^l--^R----^D----^R----_T-^;-^P--_S--^E--_L--^A-_E---^K-_E----_P^-----⁹---⁰----^----^s-
(EQ. 1)
A typical IRDR scheme is 250mA pulses every 400ms, averaging
about 56μA DC.
Total Current Consumption
Total current consumption is the sum of I_DD and I_IRDR. The IRDR
pin sinks current and the average IRDR current is calculated using
Equation 1. The I_DD depends on voltage and the mode of
operation. For simplicity, Equation 1 ignores proximity ADC
conversion time since it is much smaller than the sleep time.
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FN7851.1
January 23, 2015