Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I am designing an I2C level translator using MOSFET. I am using BSS138 DATA SHEET. My clock can go up to 1 MHz. The output is not coming properly for fast mode plus (CLK = 1 MHz, RISE TIME = FALL TIME = 120 ns). The 1.8 V output is peaking to 2.1 V and slowly settles to 1.8 V.Please find the attached circuit diagram and waveforms. Is it a pull-up resistor issue?
May I know what is wrong with my circuit?
I2C output is an open drain output. It means it can not provide 5V at output. It can only switch between 0 (low) and High-Z (high - in this case pullup resistor provides 5V at the line, and not i2c Output).
Your simulation should look like:
LT Spice uses some parameters when you don't give it right model. As you see it looks good.
But as you can see at Page 3 of AppNote (Chapter 2), this solution is not good for high speed mode.
The bidirectional level shifter can be used for both Standard-mode (up to100 kbit/s) or in Fast-mode (up to 400 kbit/s) I2C-bus systems. It is not intended for Hs-mode systems, which may have a bridge with a level shifting possibility.
If you try with BSS123 FET in your simulation you don't get perfect result. FET can't turn off fast, because of gate-drain and gate-source capacitance.
You could maybe try with other FETS and different values of pullup resistors.
