ISL29028IROZ Datasheet, PDF (10/16 Pages) Intersil Corporation – Low Power Ambient Light and Proximity Sensor with Intelligent Interrupt and Sleep Modes

ISL29028
Interrupt Function
The ISL29028 has an intelligent interrupt scheme
designed to shift some logic processing away from
intensive microcontroller I²C polling routines (which
consume power) and towards a more independent light
sensor which can instruct a system to “wake up” or “go
to sleep”.
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 ISL29028 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, see the separate ALS
Range 1 Considerations document.
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 ISL29028 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 ISL29028’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 ISL29028 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.
10
FN6780.2
November 4, 2011