ISL29028A_14 Datasheet, PDF (10/16 Pages) Intersil Corporation – Low Power Ambient Light and Proximity Sensor with Intelligent Interrupt and Sleep Modes

ISL29028A
An ALS interrupt event (ALS_FLAG) is governed by Registers 5
through 7. The user writes a high and low threshold value to these
registers and the ISL29028A will issue an ALS interrupt flag if the
actual count stored in Registers 0x9 and 0xA are outside the user’s
programmed window. The user must write 0 to clear the ALS_FLAG.
A proximity interrupt event (PROX_FLAG) is governed by the high
and low thresholds in registers 3 and 4 (PROX_LT and PROX_HT).
PROX_FLAG is set when the measured proximity data is more
than the higher threshold X-times-in-a-row (X is set by user; see
next paragraph). The proximity interrupt flag is cleared when the
prox data is lower than the low proximity threshold
X-times-in-a-row, or when the user writes “0” to PROX_FLAG.
Interrupt persistency is another useful option available for both
ALS and proximity measurements. Persistency requires X-in-a-
row interrupt flags before the INT pin is driven low. Both ALS and
Prox have their own independent interrupt persistency options.
See ALS_PRST and PROX_PRST bits in Register 2.
The final interrupt option is the ability to AND or OR the two
interrupt flags using Register 2 Bit 0 (INT_CTRL). If the user
wants both ALS/Prox interrupts to happen at the same time
before changing the state of the interrupt pin, set this bit high. If
the user wants the interrupt pin to change state when either the
ALS or the Proximity interrupt flag goes high, leave this bit to its
default of 0.
ALS Range 1 Considerations
When measuring ALS counts higher than 1800 on range 1
(ALSIR_MODE = 0, ALS_RANGE = 0, ALS_DATA > 1800), switch
to range 2 (change the ALS_RANGE bit from “0” to “1”) and
remeasure ALS counts. This recommendation pertains only to
applications where the light incident upon the sensor is IR-heavy
and is distorted by tinted glass that increases the ratio of infrared
to visible light. For more information, contact the factory.
V_DD Power-up and Power Supply
Considerations
Upon power-up, please ensure a V_DD slew rate of 0.5V/ms or greater.
After power-up, or if the user’s power supply temporarily deviates
from our specification (2.25V to 3.63V), Intersil recommends the
user write the following: write 0x00 to register 0x01, write 0x29 to
register 0x0F, write 0x00 to register 0x0E, and write 0x00 to register
0x0F. The user should then wait ~1ms or more and then rewrite all
registers to the desired values. If the user prefers a hardware reset
method instead of writing to test registers: set V_DD = 0V for 1 second
or more, power back up at the required slew rate, and write registers
to the desired values.
Power-Down
To put the ISL29028A into a power-down state, the user can set both
PROX_EN and ALS_EN bits to 0 in Register 1. Or more simply, set all
of Register 1 to 0x00.
Calculating Lux
The ISL29028A’s ADC output codes are directly proportional to lux
when in ALS mode (see ALSIR_MODE bit).
E_calc = α_RANGE × OUT_ADC
(EQ. 2)
In Equation 2, E_calc is the calculated lux reading and OUT
represents the ADC code. The constant α to plug in is determined
by the range bit ALS_RANGE (register 0x1 bit 1) and is
independent of the light source type.
TABLE 15. ALS SENSITIVITY AT DIFFERENT RANGES
ALS_RANGE
α_RANGE
(Lux/Count)
0
0.0326
1
0.522
Table 15 shows two different scale factors: one for the low range
(ALS_RANGE = 0) and the other for the high range
(ALS_RANGE = 1).
Noise Rejection
Charge balancing ADC’s have excellent noise-rejection
characteristics for periodic noise sources whose frequency is an
integer multiple of the conversion rate. For instance, a 60Hz AC
unwanted signal’s sum from 0ms to k*16.66ms (k = 1,2...k_i) is zero.
Similarly, setting the device’s integration time to be an integer
multiple of the periodic noise signal greatly improves the light
sensor output signal in the presence of noise. Since wall sockets
may output at 60Hz or 50Hz, our integration time is 100ms: the
lowest common integer number of cycles for both frequencies.
Proximity Detection of Various Objects
Proximity sensing relies on the amount of IR reflected back from
objects. A perfectly black object would absorb all light and reflect
no photons. The ISL29028A is sensitive enough to detect black
ESD foam which reflects only 1% of IR. For biological objects,
blonde hair reflects more than brown hair and customers may
notice that skin tissue is much more reflective than hair. IR
penetrates into the skin and is reflected or scattered back from
within. As a result, the proximity count peaks at contact and
monotonically decreases as skin moves away. The reflective
characteristics of skin are very different from that of paper.
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FN7721.3
January 29, 2014