ISL29028A Datasheet, PDF (9/16 Pages) Intersil Corporation – Low Power Ambient Light and Proximity Sensor with Intelligent Interrupt

ISL29028A
ALS
ACTIVE
ALS CONVERSION TIME =
100ms (FIXED)
SEVERAL µs BETWEEN
CONVERSIONS
100ms
100ms
100ms
100ms
100ms
PROX
SENSOR
ACTIVE
0.54ms FOR PROX
CONVERSION
TIME
TIME
IRDR
(CURRENT
DRIVER)
SERIES OF
CURRENT PULSES
TOTALING 0.1ms
TIME
SLEEP TIME
(PROX_SLP)
FIGURE 3. TIMING DIAGRAM FOR PROX/ALS EVENTS - NOT TO SCALE
The proximity sensor is an 8-bit ADC which operates in a similar
fashion. When proximity sensing is enabled, the IRDR pin will
drive a user-supplied infrared LED, the emitted IR reflects off an
object (i.e., a human head) back into the ISL29028A, 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 ISL29028A 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 3).
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 ISL29028A 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 6. 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 external
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 4.
220mA
(PROX_DR = 1)
110mA
(PROX_DR = 0)
PIN 8 - IRDR
(IRDR IS HI-Z WHEN
NOT DRIVING)
FIGURE 4. CURRENT DRIVE MODE OPTIONS
When the IR from the LED reaches an object and gets reflected
back into the ISL29028A, the reflected IR light is converted into
current as per the IR spectral response shown in Figure 7. 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 E}
=
I_{--l--R----D----R---;--P---E----A---K-----}×_-----1_---0_----0_----μ_----s_-
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 (as shown in Figure 4) 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 11.
Interrupt Function
The ISL29028A 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”.
9
FN7721.1
February 8, 2011