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LSF0102: I2C translation lipo battery powered RTC

Robert Smith
Prodigy 40 points
Part Number: LSF0102

Hi

I've got PCF8523 RTC that is powered directly from a LIPO battery with a voltage range of 3-4.28V. this must communicate with the microcontroller over I2C that is running at 3V IO. 

The RTC VIH is 0.7xVDD = 2.996V so conceivably i could just use the 3V supply for pullups to the RTC and it should work but it feels quite marginal having the pullups so close to the VIH limit hence (though maybe not?) I'd like to use a level translator to go from 3v to 3-4.28v.

I found this post : https://e2e.ti.com/support/logic-group/logic/f/logic-forum/576669/i2c-level-shifter-with-vary-supply-voltage-range-for-a-battery-application which uses the single supply app from the datasheet of the LSF0102 pg 17 (https://www.ti.com/lit/ds/symlink/lsf0102.pdf?ts=1690269956404&ref_url=https%253A%252F%252Fwww.google.com%252F) but this breaks the condition that "The single supply used must be at least 0.8 V larger than the lowest desired translation voltage." 

When i plug in the values to the eqn on the datasheet i don't get 276.9k found in the post. 

my questions are:

1) How was the resistor values for the single supply calculated in the linked post

2) Can you suggest a working configuration for my circuit requirements in the img below.

3) What pullup values are appropriate for both sides of the circuit if i'm using i2c fast mode. Trying to keep power consumption as low as possible for a battery powered device.

Thanks for the help!

  • +1
    Guru 316690 points

    1. The voltage divider should result in a voltage not above 1.8 V. But note that some current also flows into Vref_B and out of Vref_A, so you might have to experiment.

    2. There is a simpler configuration. Connect EN directly to 3 V, and leave Vref_A/B open. (Please note that the LSF0102 consists of three MOSFETs; this circuit makes it identical to NXP's AN10441's level shifter.)

    3. This depends on the total capacitance of the bus. Try with 4.7 kΩ or 10 kΩ on both sides, and check with an oscilloscope if the rising edges are too slow.