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ISL29011
incandescent light. The β also changes with the ADC’s range and
resolution selections.
ADC Output in Proximity Sensing
In the proximity sensing, the ADC output codes, DATA, are directly
proportional to the total IR intensity from the background IR
noise and from the IR LED driven by the ISL29011.
DATA PROX = β × E IR + γ × E LED (EQ. 8)
Here, β and E IR have the same meanings as in Equation 7. The
constant γ depends on the spectrum of the used IR LED and the
ADC’s range and resolution selections. E LED is the IR intensity
which is emitted from the IR LED and reflected by a specific
objector to the ISL29011. E LED depends on the current to the IR
LED and the surface of the object. E LED decreases with the
square of the distance between the object and the sensor.
If background IR noise is small, E IR can be neglected, and the
ADC output directly decreases with the distance. If there is
significant background IR noise, ISL29011 offers two schemes
to reduce the effect. The first way is to do a proximity sensing
using Scheme 0, immediately followed by an IR sensing. The
differential reading of ADC outputs from the proximity and IR
sensing will then reduce the effect of background IR noise and
directly decrease with the distance between the object and the
sensor. The second way is to do a proximity sensing using
Scheme 1 to do on-chip background IR noise subtraction. While
Scheme 0 has wider dynamic range, Scheme 1 proximity
detection is faster but with half the resolution. Please refer to
“Typical Performance Curves” on page 15 for ADC output versus
distance using Scheme 0 detection.
Figure 11 shows ISL29011 configured at 12-bit ADC resolution
and sensitivity range selected at 16000 (range 3) for the
proximity reading. A 12.5mA external LED current at 360kHz
modulation frequency detects three different sensing objects:
92% brightness paper, 18% gray card and ESD black foam.
Figure 12 shows ISL29011 configured at 12-bit ADC resolution
and sensitivity range selected at 1000 (range 1) for the proximity
reading, with a programmed external LED at 360kHz modulation
frequency, detecting the same sensing object: 18% gray card
under four different external LED current: 12.5mA, 25mA, 50mA
and 100mA to compare the proximity readout versus distance.
ISL29011 Proximity sensing relies on the amount of IR reflected
back from the objects to be detected. Clearly, it can not detect an
optically black object that reflects no light. However, ISL29011 is
sensitive enough to detect a black ESD foam, which reflects
slightly less than 1% of IR, as shown in Figure 11 on page 15. For
biological objects, blonde hair reflects more than brunette hair, as
expected and shown in Figure 13. Also 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. This characteristic is very different from that of a
plain paper reflector.
Interrupt Function
An interrupt event (FLAG) is governed by bit2 in COMMAND1. The
user must set bit2 in COMMAND1 to be logic low(0), which
means INT is cleared or not triggered yet. Then ISL29011 will
13
issue an ambient (ALS/IR) or proximity interrupt flag if the actual
count stored in Register 0x2 and 0x3 are outside the user's
programmed window. The user must read Register 0x0 to clear
interrupt.
Interrupt persistency at bit1 and bit0 of COMMAND1 is another
useful option available for both ambient/IR and proximity
measurement. Persistency requires x-in-a-row interrupt flags
before the INT pin is driven low. Then, user must read Register
0x0 to clear Interrupt.
V DD Power-up and Power Supply
Considerations
Upon power-up, please ensure a VDD 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 0x00 to two registers: 0x08, 0x00 (in
that order), wait ~1ms or more and then rewrite all registers as
desired.
LED Modulation for Proximity Detection
ISL29011 offers two ways to modulate the LED in the Proximity
Detection mode - DC or 360kHz (with 50% duty cycle) by bit 6 of
register 01h. At the IRDR pin, there are four different IRDR LED
currents; 12.5, 25, 50, and 100mA outputs selectable by bits 4
and 5 of register 01h. With the LED running in the DC mode, the
proximity detection is twice as sensitive but consumes 2 times
more current. The sensitivity of LED 50mA, DC 50mA is identical
to that of 100mA, 360kHz modulation. Please note that the
ISL29011 does not include a LED.
Current Consumption Estimation
The low power operation is achieved through sequential readout
in the serial fashion, as shown in Figure 5, the device requires
three different phases in serial during the entire detection cycle
to do ambient light sensing, infrared sensing and proximity
sensing. The external IR LED will only be turned on during the
proximity sensing phase under user program controlled current
at modulated frequency depends on user selections. Figure 5
also shows the current consumption during each ALS, IR sensing
and Proximity sensing phase. For example, at 8-bit ADC
resolution the integration time is 0.4ms. If user programed
50mA current to supply external IR LED at 360kHz modulated
frequency, during the entire operation cycle that includes ALS, IR
sensing and Proximity sensing three different serial phases, the
detection occurs once every 30ms, the average current
consumption including external IR LED drive current can be
calculated from Equation 9:
[ ( 0.07mA + 0.07mA + 1mA + (50mA ? 50%)) ? 0.4ms ) ] /30ms = 0.35mA
(EQ. 9)
If at a 12-bit ADC resolution where the integration time for each
serial phase becomes 7ms and the total detection time becomes
100ms, the average current can be calculated from Equation 10:
[ ( 0.07mA + 0.07mA + 1mA + (50mA ? 50%)) ? 7ms ) ] /100ms = 1.83mA
(EQ. 10)
FN6467.5
October 10, 2012
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