ISL29044A_14 Datasheet, PDF (8/19 Pages) Intersil Corporation – Low Power Ambient Light and Proximity Sensor with Internal IR-LED and Digital Output

ISL29044A
Principles of Operation
Photodiodes and ADCs
The ISL29044A contains two photodiode arrays, which convert
photons (light) into current. The ALS photodiodes are constructed
to mimic the human eye’s wavelength response curve to visible
light (see Figure 5). The ALS photodiodes’ current output is
digitized by a 12-bit ADC in 100ms. These 12 bits can be
accessed by reading from I²C registers 0x9 and 0xA when the
ADC conversion is completed.
The ALS converter is a charge-balancing, integrating 12-bit ADC.
Charge-balancing is best for converting small current signals in
the presence of periodic AC noise. Integrating over 100ms highly
rejects both 50Hz and 60Hz light flicker by picking the lowest
integer number of cycles for both 50Hz/60Hz frequencies.
ALS CONVERSION TIME =
100ms (FIXED)
SEVERAL µs BETWEEN
CONVERSIONS
ALS
ACTIVE
100ms
PROX
SENSOR
ACTIVE
100ms
100ms
100ms
0.54ms FOR PROX
CONVERSION
100ms
TIME
TIME
IRDR
(CURRENT
DRIVER)
SERIES OF
CURRENT PULSES
TOTALING 0.1ms
TIME
SLEEP TIME
(PROX_SLP)
FIGURE 15. TIMING DIAGRAM FOR PROX/ALS EVENTS - NOT TO SCALE
The proximity sensor is an 8-bit ADC that operates in a similar
fashion. When proximity sensing is enabled, the IRDR pin will
drive the internal infrared LED, the emitted IR reflects off an object
(e.g., a human head) back into the ISL29044A, and a sensor
converts the reflected IR wave to a current signal in 0.54ms. The
ADC subtracts the IR reading before and after the LED is driven
(to remove ambient IR such as sunlight), and converts this value
to a digital count stored in Register 0x8.
The ISL29044A is designed to run two conversions concurrently:
a proximity conversion and an ALS (or IR) conversion. Please note
that because of the conversion times, the user must let the ADCs
perform one full conversion first before reading from I²C
Registers PROX_DATA (wait 0.54ms) or ALSIR_DT1/2 (wait
100ms). The timing between ALS and Prox conversions is
arbitrary, as shown in Figure 15. The ALS runs continuously with
new data available every 100ms. The proximity sensor runs
continuously with a time between conversions decided by
PROX_SLP (Register 1 Bits [6:4]).
Ambient Light and IR Sensing
The ISL29044A is set for ambient light sensing when Register bit
ALSIR_MODE = 0 and ALR_EN = 1. The light-wavelength
response of the ALS appears, as shown in Figure 5. ALS
measuring mode (as opposed to IR measuring mode) is set by
default.
When the part is programmed for infrared (IR) sensing
(ALSIR_MODE = 1; ALS_EN = 1), infrared light is converted into a
current and digitized by the same ALS ADC. The result of an IR
conversion is strongly related to the amount of IR energy incident
on our sensor, but is unitless and is referred to in digital counts.
Proximity Sensing
When proximity sensing is enabled (PROX_EN = 1), the internal
IR LED is driven for 0.1ms by the built-in IR LED driver through
the IRDR pin. The amplitude of the IR LED current depends on
Register 1 bit 3: PROX_DR. If this bit is low, the load will see a
fixed 110mA current pulse. If this bit is high, the load on IRDR
will see a fixed 220mA current pulse, as seen in Figure 16.
LED+
INTERNAL IR-LED
LED-
220mA
(PROX_DR = 1)
110mA
(PROX_DR = 0)
PCB TRACE
IRDR
(IRDR IS HI-Z WHEN
NOT DRIVING)
FIGURE 16. CURRENT DRIVE MODE OPTIONS
When the IR from the LED reaches an object and gets reflected
back into the ISL29044A, the reflected IR light is converted into
current as per the IR spectral response shown in Figure 5. One
entire proximity measurement takes 0.54ms for one conversion
(which includes 0.1ms spent driving the LED), and the period
between proximity measurements is decided by PROX_SLP
(sleep time) in Register 1 Bits 6:4.
Average LED driving current consumption is given by Equation 1.
I _{l R D R ;A V G}
=
I_{--l--R----D----R-----;-P----E----A----K-----}×_-----1_---0_---0_----μ_----s_-
540μs + t_SLEEP
(EQ. 1)
A typical IRDR scheme is 220mA amplitude pulses every 800ms,
which yields 28μA DC.
Total Current Consumption
Total current consumption is the sum of I_DD and I_IRDR. The IRDR
pin sinks current (see Figure 16) and the average IRDR current
can be calculated using Equation 1. I_DD depends on voltage and
the mode-of-operation, as seen in Figure 9.
Interrupt Function
The ISL29044A has an intelligent interrupt scheme designed to
shift some logic processing away from the 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 ISL29044A 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.
8
FN8419.1
April 19, 2013