ISL29021 Intersil Corporation, ISL29021 Datasheet - Page 8

• Manufacturer Part Number: ISL29021
• Description: Digital Proximity Sensor
• Manufacturer: Intersil Corporation

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background IR noise and from the IR LED driven by the
ISL29021 as shown in Equation 5.
Here, β and E
The constant γ depends on the spectrum of the used IR LED
and the ADC’s range and resolution selections. E
IR intensity which is emitted from the IR LED and reflected
by a specific objector to the ISL29021. E
current to the IR LED and the surface of the object. E
decreases with the square of the distance between the
object and the sensor.
If background IR noise is small, E
the ADC output directly decreases with the distance. If there
is significant background IR noise, ISL29021 is to do a
proximity sensing using Scheme 1 to do on-chip background
IR noise subtraction.
Figure 9 shows ISL29021 configured at 12-bit ADC
resolution and sensitivity range select 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 10 shows ISL29021 configured at 12-bit
ADC resolution and sensitivity range select 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.
ISL29021 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, ISL29021 is sensitive enough to detect a black ESD
foam, which reflects slightly less than 1% of IR, as shown in
Figure 9. For biological objects, blonde hair reflects more than
brunette hair, as expected and shown in Figure 11. 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
Depending on the mode of operation set by Bits 7, 6 and 5 of
command register 00 hex, the upper and lower interrupt
thresholds are for either infrared signal level or proximity
detection. After each change of mode of operation, it is
expected a new set of thresholds are loaded to interrupt
registers 04, 05, 06 and 07 hex for proper interrupt detection.
Also, the interrupt persist counter will be reset to 0 when the
mode of operation is changed.
DATA
PROX
=
IR
β
×
have the same meanings as in Equation 4.
E
IR
+
γ
×
E
LED
8
IR
can be neglected, and
LED
depends on the
LED
LED
(EQ. 5)
is the
ISL29021
LED Modulation for Proximity Detection
ISL29021 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 2x more current. The sensitivity
of LED 50mA, DC 50mA is identical to that of 100mA,
360kHz modulation. Please note that the ISL29021 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 3, the
device requires three different phases in serial during the
entire detection cycle to do 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 3 also shows the current consumption during each 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 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 6:
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 7:
Suggested PCB Footprint
It is important that the users check the “Surface Mount
Assembly Guidelines for Optical Dual FlatPack No Lead
(ODFN) Package” before starting ODFN product board
mounting.
http://www.intersil.com/data/tb/TB477.pdf
Layout Considerations
The ISL29021 is relatively insensitive to layout. Like other
I
even in significantly noisy environments. There are only a
few considerations that will ensure best performance.
Route the supply and I
sources of noise. Use two power-supply decoupling
capacitors 1µF and 0.1µF, placed close to the device.
[
[
2
(
(
0.05mA
0.05mA
C devices, it is intended to provide excellent performance
+
+
0.05mA
0.05mA
+
+
1mA
1mA
+
+
2
(50mA ∗ 50%)) ∗ 0.4ms
(50mA ∗ 50%)) ∗
C traces as far as possible from all
7
ms
)
]/100ms = 1.83mA
)
]/30ms = 0.35mA
March 3, 2009
FN6732.0
(EQ. 6)
(EQ. 7)