Electronic – I²C level shifter pulls low side up

i2clevel-shifting

I am using the well-known I²C level shifter from that appnote from Philips.

schematics

The 3V side is supplied by an LP2950-3.0 regulator and is loaded only moderate (a few quite lazy 74's).

The problem I'm obserivng is that the 3V side is being pulled to 5V.

I loaded the 3V side with a resistor and calculated that the level shifter presents a resistance of only 2.7 kOhms. I consider this rather low and I wonder:

  1. Is that normal behaviour or did I make some mistake?

  2. Isn't this harmful for the devices on the 3V side? There are several ICs that according to the datasheet are explicitly not 5V tolerant, not to forget the regulator.

Best Answer

I've just checked quickly, but your diagram looks good to me. I suspect it might be some other problem, maybe assembly of the parts or something along that line. Could you please check the following for both the SCL and the SDA line, using an ordinary Multimeter:

  • Are you able to measure one diode drop (approx. 0.5-0.75V) across the parasitic diode in the MOSFETs from the 3V side to the 5V side?

  • Do you get a reading of nearly infinite resistance (or at least some kOhms) when connecting the Multimeter's test leads the other way round, i.e. from SCL's or SDA's 5V end to their 3V ends?

Please note that these single MOSFET transistors are very (!) sensitive to ESD discharge, so even if you assembled the right parts the right way, they may have some hidden type of damage.

Related Topic