ISL29035
13
FN8371.3
December 12, 2016
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The transfer function used for n-bits ADC becomes Equation 3:
Where n = 4, 8, 12 or 16. This is the number of ADC bits
programmed in the command register. 2
n
represents the
maximum number of counts possible from the ADC output. Data is
the ADC output stored in the data registers (02 hex and 03 hex).
Enhancing EV Accuracy
The device has an on-chip passive optical filter designed to block
(reject) most of the incident Infrared. However, EV measurement
may vary under differing IR-content light sources. In order to
optimize the measurement variation between differing
IR-content light sources, ISL29035 provides IR channel, which is
programmed at COMMAND-I (Reg0x0) to measure the IR level of
differing IR-content light sources.
The ISL29035’s ADC output codes, DATA, are directly
proportional to the IR intensity received in the IR sensing.
Then EV
Accuracy
can be found in Equation 5:
Where DATA
EV
is the received ambient light intensity ADC output
codes. K is a resolution of visible portion. Its unit is Lux/count.
The typical value of K is 0.82. DATA
IR
is the received IR intensity.
The constant changes with the spectrum of background IR,
such as A, F2 and D65. The also changes with the ADC’s range
and resolution selections. A typical for Range1 and Range2 is
-11292.86 and for Range3 and Range4 it is 2137.14 without IR
tinted glass.
Noise Rejection
Electrical AC power worldwide is distributed at either 50Hz or
60Hz. Artificial light sources vary in intensity at the AC power
frequencies. The undesired interference frequencies are infused
on the electrical signals. This variation is one of the main sources
of noise for the light sensors. Integrating type ADC’s have
excellent noise-rejection characteristics for periodic noise
sources whose frequency is an integer multiple of the conversion
rate. By setting the sensor’s integration time to an integer
multiple of periodic noise signal, the performance of an ambient
light sensor can be improved greatly in the presence of noise. In
order to reject the AC noise, the integration time of the sensor
must be adjusted to match the AC noise cycle. 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.
Suggested PCB Footprint
It is important that users check the TB477, “Surface Mount
Assembly Guidelines for Optical Dual Flat Pack No Lead (ODFN)
Package” before starting ODFN product board mounting.
Board Mounting Considerations
For applications requiring the light measurement, the board
mounting location should be reviewed. The device uses an
Optical Dual Flat Pack No Lead (ODFN) package, which subjects
the die to mild stresses when the Printed Circuit (PC) board is
heated and cooled, which slightly changes the shape. Because of
these die stresses, placing the device in areas subject to slight
twisting can cause degradation of reference voltage accuracy. It
is normally best to place the device near the edge of a board, or
on the shortest side, because the axis of bending is most limited
in that location.
Layout Considerations
The ISL29035 is relatively insensitive to layout. Like other I
2
C
devices, it is intended to provide excellent performance even in
significantly noisy environments. There are only a few
considerations that will ensure best performance.
Route the supply and I
2
C traces as far as possible from all
sources of noise. Use two power-supply decoupling capacitors,
1µF and 0.1µF, placed close to the device.
Soldering Considerations
Convection heating is recommended for reflow soldering; direct
infrared heating is not recommended. The plastic ODFN package
does not require a custom reflow soldering profile, and is
qualified to +260°C. A standard reflow soldering profile with a
+260°C maximum is recommended.
Typical Circuit
A typical application for the ISL29035 is shown in Figure 16 on
page 14. The ISL29035’s I
2
C address is internally hard-wired as
1000100. The device can be tied onto a system’s I
2
C bus,
together with other I
2
C compliant devices.
(EQ. 3)
E
cal
Range
2
n
-------------------
DATA=
DATA
IR
E
IR
=
(EQ. 4)
EV
Accuracy
KxDATA
EV
DATA
IR
+=
(EQ. 5)
