ISL29043_14 Datasheet, PDF (11/16 Pages) Intersil Corporation – Low Power Ambient Light and Proximity Sensor with Internal IR-LED and Digital Output

ISL29043
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)
1
0.029
2
0.469
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 ISL29043 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.
Typical Opto-Mechanical Configuration
Typical applications for the ISL29043 involve use under a
cover-glass, or optical window. Typically, these glass components
are not coated to prevent unwanted reflections. Standard glass
and many plastic materials will reflect 4% of the incident light at
each surface. Reflected light emanating from the internal IR-LED
may be incident on the ALS/Proximity sensor and cause
significant DC-Offset in the detected signals. To prevent this
situation, the device should be used with a Light Baffle, as shown
in Figure 7. A Light Baffle prevents unwanted illumination from
the IR-LED from reaching the ALS/Proximity sensors while not
interfering with normal Ambient Light Sensing or Proximity
detection. The Baffle should be the limiting aperture for both the
IR-LED and the ALS/Prox sensor. Care should be taken to insure
there is no other obstruction in the light path.
A Light Baffle is made from a soft, compliant plastic, or rubber
material such as urethane, or silicone. The material should be
mechanically compliant since a designer desires it to fill the
separation between the PCB and the cover-glass and should not
produce undue stress on the thin cover-glass. A Light Baffle is
designed to fit completely over the ISL29043 package and may
be attached to the PCB with a dispensed adhesive. Typical
ISL29043 package height is 0.65 mm (see “Package Outline
Drawing” on page 16) and the inside lower cavity of the baffle is
0.4mm deep. With the cavity depth less than the package height,
the baffle does not reach fully to the PCB surface. This insures
that the internal barrier rests squarely on the top surface of the
package to prevent reflection of the IR-LED illumination toward
the sensor. The example Light Baffle in Figure 7 is shown with a
height of 1.1mm. However, the specific design-appropriate
height varies according to actual system design requirements. If
another material is chosen for a Light Baffle, the material should
be soft and compliant and also should be matte black in finish to
prevent reflection of the IR-LED illumination within a Light Baffle
and surrounding structures underneath the cover-glass.
Suggested Light Baffle PCB Footprint
The Light Baffle fits down over the entire ISL29043 package. The
lower wall thickness of the Light Baffle around the ISL29043
package is 0.3mm. Therefore, the PCB layout should allow for a
0.3mm clear-zone immediately around the ISL29043 with no
other surface components within this zone.
Operation Without a Light Baffle
For some product designs, it may be advantageous to use the
ISL29043 under the cover-glass without a Light Baffle. For these
applications, it is recommended that the opto-mechanical design
place the top surface of the ISL29043 package in direct contact
with the inside surface of the cover-glass. This configuration
significantly reduces the IR-LED illumination reflection from the
inside surface of the cover-glass and reduces the DC-Offset of the
proximity sensor. For typical operational performance
comparison, Figure 8 shows a graph of the proximity response
for a standard 18% Kodak Gray Card target over a range of 0 to
100 mm for the same ISL29043 device with:
a. No cover-glass,
b. Cover-glass (0.9 mm thick, ~75%T at 850nm) with Light Baffle,
c. Cover-glass (0.9 mm thick, ~75%T at 850nm) without Light
Baffle and in contact with cover-glass, and,
d. Cover-glass (0.9 mm thick, ~75%T at 850nm) without Light
Baffle and spaced 0.1 mm below cover-glass.
Also, it is highly recommended that only IRDR = 110mA be used
when operating the ISL29043 without a a Light Baffle as the
IRDR = 220mA setting may cause a large DC-Offset even with
the ISL29043 placed in direct contact with the inside surface of
the cover glass.
11
FN7935.0
February 9, 2012