MAX44004 Datasheet, PDF (6/17 Pages) Maxim Integrated Products – Digital Ambient Light Sensor

MAX44004
Digital Ambient Light Sensor
Detailed Description
The MAX44004 is a wide-dynamic-range ALS. The die is
placed inside an optically transparent (ODFN) package. A
photodiode array inside the device converts the light to a
current, which is then processed by low-power circuitry into
a digital value stream. The data is then stored in an output
register that is read by an I²C interface.
Two types of photodiodes are used in the device: a
green photodiode and an infrared photodiode. Ambient
light sensing is accomplished by subtracting the green
ALS photodiode signal and the infrared ALS photodiode
signals, after applying appropriate gains.
The photodiodes are connected to two ADCs. The user
can choose to view either just the green ALS signal, or
just the infrared ALS signal, or the difference of the green
and infrared ALS photodiodes.
Two key features of the device’s analog design are its
low-power design and interrupt pin operation.
The device can operate from a V_DD of 1.7V to 3.6V and
consumes just 5FA current. An on-chip programmable
interrupt function eliminates the need to continually poll
the device for data, resulting in a significant power saving.
Ambient-Light Sensing
Ambient-light sensors are designed to detect bright-
ness in the same way as human eyes do. To achieve
this, the light sensor needs to have a spectral sensitivity
that is identical to the photopic curve of the human eye
(Figure 1). Small deviations from the photopic curve
can affect perceived brightness by ambient light
sensors to be wildly different. However, there are practical
difficulties in trying to reproduce the ideal photopic curve
in a small cost-efficient package. The devices instead
use two types of photodiodes (green and infrared) that
have different spectral sensitivities—each of which is
amplified and subtracted on-chip with suitable gain
coefficients so that the most extreme light sources (fluo-
rescent and incandescent) are well matched to a com-
mercial illuminance lux meter.
The photopic curve represents a typical human eye’s
sensitivity to different wavelengths of light. As can be
seen in Figures 1 and 2, its peak sensitivity is at 555nm
(green). The human eye is insensitive to infrared (>
700nm) and ultraviolet (< 400nm) radiation.
Variation between light sources can extend beyond the
visible spectral range—fluorescent and incandescent
light sources, for example—with similar visible brightness
(lux) and can have substantially different IR radiation con-
tent (since the human eye is blind to it). Since this infrared
radiation can be picked up by silicon photodiodes, differ-
ences in light spectra can affect brightness measurement
of light sensors. For example, light sources with high IR
content such as an incandescent bulb or sunlight could
suggest a much brighter environment than our eyes would
perceive them to be. Other light sources, such as fluo-
rescent and LED-based systems, have very little infrared
content. The devices incorporate on-chip compensation
techniques to minimize these effects and still output an
accurate lux response in a variety of lighting conditions.
On-chip, user-programmable green channel and IR
channel gain trim registers allow the light-sensor response
to be tailored to the application, such as when the light
sensor is placed under a dark or colored glass.
120
100
STANDARD ALS
80
(GREEN-RED)
BLUE: IDEAL
60
PHOTOPIC CURVE
40
20
0
270 370 470 570 670 770 870 970 1070
WAVELENGTH (nm)
Figure 1. MAX44004 Spectral Response Compared to Ideal
Photopic Curve
120
100
GREEN CHANNEL
RED CHANNEL
80
IDEAL PHOTOPIC
CURVE
60
40
20
0
270 370 470 570 670 770 870 970 1070
WAVELENGTH (nm)
Figure 2. Green Channel and IR Channel Response at
Identical Gains on a Typical MAX44004
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