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schematic

simulate this circuit – Schematic created using CircuitLab

I have an ultra low power application where the minimum VDD of my temperature sensor (ADT7420) is 2.7V, while I need to run my MCU at 2.4V. The circuit is powered from either a 3V coin cell or USB 5V [debug mode]. I have space for a single 2.4V regulator on board.

The temperature sensor interface is I2C and it also has an interrupt output that is connected to the MCU. Sensor supply min and max is 2.7V and 5.5V.

My question is:

SDA/SCL and the interrupt pins on the sensor are configured as open drain. Therefore, would I be OK if I used pull up resistors to 2.4V and connected the signals directly to the MCU? i.e., because the signals to be interfaced are open drain, I do not really need level shifters OR even current limiting resistors b/w the sensor and the MCU.

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  • \$\begingroup\$ If the INT Out pin is not open drain, it could fry your MCU with 5 Volts. \$\endgroup\$ – Turbo J Jul 24 '16 at 14:54
  • \$\begingroup\$ And 2V4 may be too low for reliable I²C operation. Linking to the sensors datasheed sounds like a good idea. \$\endgroup\$ – Turbo J Jul 24 '16 at 14:55
  • \$\begingroup\$ I have added a link to the sensor datasheet. INT is specified to be open drain. \$\endgroup\$ – NK2020 Jul 24 '16 at 15:03
  • \$\begingroup\$ If you really want minimal power consumption, make sure you run the INT pin in interrupt active low mode and service the chip promptly after each interrupt, so that you don't leave that signal held low against the resistor and drawing current through it. \$\endgroup\$ – Chris Stratton Jul 24 '16 at 17:10
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This should work but there are a couple of things to watch:

First, the maximum voltage you can operate the temperature sensor at, for guaranteed I2C operation is 3.4V for interface operation at 2.4V

ADT4720 I/O levels

That is necessary to always meet the \$V_{ih}\$ specification of 0.7Vdd.

The maximum pull-up resistance depends on the speed at which you intend to operate the interface and you can go slower with larger values resistance. Smaller gulps of current generally tends to increase battery life if only by a very small amount; how much of an issue that is for you I do not know.

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