User Manual

Application Note
SMBus communication with MLX90614
390119061402 Page 26 of 32 Jan-2008
Rev 004
8 Electrical considerations of SMBus applications with MLX90614
Vss Vss
PP/-OD ; SMBus
PWM/SDA
VddVdd
Vss
Dout
Din
Vss
+Vdd
Vss
SCLin
Vdd
SCL/Vz
Synthesized Zener diode
for building external HV voltage regulator
Fig.12: Input/Output pin schematics of MLX90614
For reliability reasons, the MLX90614 incorporates ESD clamp diodes to Vdd and from Vss.
Therefore a powered-down MLX90614 will load the SMBus. This differs from the SMBus
specification. In power-managed systems it is therefore needed to keep the MLX90614 powered
when the SMBus is needed. This is no issue with sleep mode.
Fig.13: SCL line undershoot with MLX90614
A synthesized Zener diode is integrated on the SCL/Vz pin of the device. It allows simple
implementation of higher voltage regulators. There are several things that influence the SMBus
applications with that feature:
Transient response of SCL pin of MLX90614 adds undershoot to the SMBus SCL line. Rising
edge on the bus results in partial opening of the zener diode, as shown on Fig.13. This
undershoot is typically well beyond the threshold level for high-to-low transition on SCL line.
When an external regulator is build the SCL can no longer be used. This would cause the
regulator to turn off and on with every clock cycle. Use of MLX90614 in >5V system is shown on
Fig. 14. However, if it is needed to have SMBus communication with an MLX90614 already
connected to a schematic like this, it will be necessary to override the power supply regulator as
shown on Fig. 14. Then the SCL pin can be toggled and the SMBus communication will run.
SCL input leakage is increased. In worst case (over temperature and voltage on the SCL pin)
this leakage may significantly exceed the sleep mode power drain of the MLX90614. Sleep