EVB90620 Datasheet, PDF (33/39 Pages) Melexis Microelectronic Systems – High precision non-contact temperature measurements

MLX90620
16x4 IR array
11 Applications Information
11.1 Use of the MLX90620 thermometer in SMBus configuration
MCU VDD = 2.5...5V ^VDD
MCU
2 - SDA
3 - VDD
SENSOR VDD = 2.6V
VDD1
1 - SCL
4 - VSS
Figure 26: MLX90620 SMBus connection
Figure 26 shows the connection of a MLX90620 to a SMBus with 3.3V power supply. The MLX90620
has diode clamps SDA/SCL to Vdd so it is necessary to provide MLX90620 with power in order not to load
the SMBus lines.
12 Application Comments
Significant contamination at the optical input side (sensor filter) might cause unknown additional
filtering/distortion of the optical signal and therefore result in unspecified errors.
IR sensors are inherently susceptible to errors caused by thermal gradients. There are physical
reasons for these phenomena and, in spite of the careful design of the MLX90620xxx, it is recommended not
to subject the MLX90620 to heat transfer and especially transient conditions.
The MLX90620 is designed and calibrated to operate as a non contact thermometer in settled
conditions. Using the thermometer in a very different way will result in unknown results.
Capacitive loading on a I²C can degrade the communication. Some improvement is possible with
use of current sources compared to resistors in pull-up circuitry. Further improvement is possible with
specialized commercially available bus accelerators. With the MLX90620 additional improvement is possible
by increasing the pull-up current (decreasing the pull-up resistor values). Input levels for I²C compatible mode
have higher overall tolerance than the I²C specification, but the output low level is rather low even with the
high-power I²C specification for pull-up currents. Another option might be to go for a slower communication
(clock speed), as the MLX90620 implements Schmidt triggers on its inputs in I²C compatible mode and is
therefore not really sensitive to rise time of the bus (it is more likely the rise time to be an issue than the fall
time, as far as the I²C systems are open drain with pull-up).
Power dissipation within the package may affect performance in two ways: by heating the “ambient”
sensitive element significantly beyond the actual ambient temperature, as well as by causing gradients over
the package that will inherently cause thermal gradient over the cap
3001090620
Rev 4.0
Page 33 of 40
Datasheet IR16x4
19 September 2012