User Manual

Application Note
SMBus communication with MLX90614
390119061402 Page 27 of 32 Jan-2008
Rev 004
mode is available with the 3V version only, while the zener diode function can be used with the
5V version. However, the zener diode is present in all MLX90614 versions, so the leakage will
be seen on the 3V version too. It is recommended to disable the pull-up on the SCL line in order
to prevent the leakage from increasing the overall power-down power drain of the SMBus
system.
U2
C1R1
+V
+5V
SDA
Q1
+V
SCL
R1
SDA
+5V
Voltage regulator
Equivalent
schematics
C2
R1
+5V
U1
MLX90614Axx
1
2
3
4
Vss
SCL/Vz
SDA
Vdd
Q1
U1
MLX90614Axx
1
2
3
4
Vss
SCL/Vz
SDA
Vdd
R3
U1
5.7V
SDA
Vdd
SD A
Vss
SCL
MCU
+5V
C1
SCL
+V
R2
Q1
+5V
SCL
Fig.14: External voltage regulator with MLX90614
The input levels of the MLX90614 are not 100% compliant with the SMBus specification. The
SMBus specification states an input low voltage maximum value of 0.8V and a minimum high
voltage of 2.1V. For the MLX90614 (refer to the data sheet) the specifications differs. When 5V
(also applies for >5V applications) is used, the MLX90614 uses an on-chip voltage regulator (5V
– to – 3V±10%). With the 3V version, the power supply is used directly.
Then, at 5V (as well as at >5V) the internal circuitry of MLX90614 operates at 3V±10%, while at
3V the power supply specification covers 3V±20%. The higher tolerance of this power supply
results in a higher tolerance of the input levels. Worst-case values for MLX90614Axx are
Vin,L=0.5…1.5V and Vin,H=1.6…2.4V (over all temperatures and supply voltages), and for
MLX90614Bxx – 0.5…1,5V and 1.2…2.8V (idem). However, this does not mean, that
MLX90614Axx is likely to have Vin,L=1.5V and Vin,H=1.6V at the same time; also
MLX90614Bxx will not have Vin,L=1.5V and Vin,H=1.2V at the same time. Both thresholds
decrease as the power supply voltage decreases. The two thresholds are also affected by
temperature in the same direction. A hysteresis is provided on both SDA and SCL inputs for
noise immunity.
As a summary, keeping the logic levels on the bus Vlow<0.5V and Vhigh>2.8V will certainly
cover all operational cases with the MLX90614, but is not likely to be really necessary. Detailed
values (guaranteed by design, not test limits) are given below:
Table 8
Vdd (3V) 2.4 2.8 3 3.2 3.6
Vin,L,-40°C,min 0.57 0.75 0.84 0.94 1.13
Vin,L,-40°C,max 0.73 0.91 1.00 1.09 1.29
Vin,L,+27°C,min 0.63 0.82 0.91 1.01 1.20
Vin,L,+27°C,max 0.79 0.97 1.07 1.17 1.36
Vin,L,+125°C,min 0.72 0.91 1.01 1.11 1.30
Vin,L,+125°C,max 0.88 1.07 1.17 1.27 1.46
Vin,H,-40°C,min 1.23 1.61 1.81 2.01 2.40
Vin,H,-40°C,max 1.54 1.93 2.12 2.32 2.69
Vin,H,+27°C,min 1.41 1.80 1.99 2.19 2.57
Vin,H,+27°C,max 1.67 2.03 2.21 2.38 2.74
Vin,H,+125°C,min 1.57 1.93 2.11 2.29 2.64
Vin,H,+125°C,max 1.72 2.07 2.25 2.43 2.77