MLX75031 Datasheet, PDF (19/67 Pages) Melexis Microelectronic Systems – Optical Gesture & Proximity Sensing IC

MLX75031
Optical Gesture & Proximity Sensing IC
with Integrated LED Drivers
Datasheet
During calibration at least 2 light levels (I_x1 and I_x2) have to be supplied to the target ambient channel (x)
with its photodiode at the same known temperature T. The closer these values are chosen to the range
used in application, the more accurate the final result will be. During the setting of these two light levels,
the output of ambient channel x: ambout_x1 and ambout_x2 are measured. This results in 2 equations and 2
unknowns: α_x and β_x. Both unknowns can be calculated from following formulas :
α=
T
ln_{}
I₁
I₂

ambout₁ − ambout₂
and
β
=
ln

I₁
1


−
α

ambout₁
T
−
2¹⁵

(2)
Note : these 2 values automatically correct any gain error of the connected photodiode and used lens
system.
After the calibration step, some error remains when the temperature deviates from the calibration
temperature. The error depends on the temperature drift of the reference current (TC_IrefΔT) and on the
offset Ox. In second order, it also depends on the temperature drift of gain G and of the offset Ox.
With TC_Iref = + 1230 ppm/K, the ambient current calculation error because of the temperature drift of I_ref is
from -10% at low temperature to +10% at high temperature.
The offset of the ambient signal conditioning chain is between 0 and -20mV (400 LSB). The corresponding
temperature coefficient would be between 0 and -670 ppm/°C. The corresponding error in the ambient light
calculation would be from 0 to -5.3%.
To reduce the MCU load a simple lookup table can be implemented instead of the complete formula.
8.2.4. Response Time
During operation, each ambient channel constantly shows a logarithmic output response towards the input
current that is applied. As a result, the time for the output to respond on a changing input current is defined
by a strongly non-linear function (similar but not equal to an RC-curve). Therefore a threshold crossing
criterion is used to define the time response.
The light threshold level is defined as a border between light and dark. This threshold can be defined as a
light level of a voltage level at the output of the ambient light channel. If the light suddenly crosses the
defined light threshold, the output of the ambient light channel will cross the corresponding voltage
threshold with a delay. This delay is the response time.
Figure 3 shows an example of such a threshold crossing. An input current step is used as this represents the
worst case condition. Note that starting from an input current level that is close to the threshold, the end
requirement of 20% below or 80% above can be heavily relaxed.
The ambient light response time is valid for any threshold level equal to or above the defined input
threshold level I_{ambx_detect} of any input channel.
REVISION 001 – JANUARY, 2013
39010xxxxx
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