Hi team,
My customer wants to use LSF0108 IBIS model to do some simulation. Here're some questions:
1. They're using LSF0101_IO_33_S model to do the behavior level simulation. Is that correct?
2. Use-case is 1.8V VDD1 and 3.3V VDD2. If customer use 50Ohm pull-up on B output, they found the A input will also be pulled up to ~2V. What do you think might cause this problem?
Thank you!
Hi Luke,
That model should be fine to use for signal integrity analysis.
Using a 50 ohm pull-up is extremely aggressive - that would require your driver circuit to sink at least 66mA. The below image shows how a driver on the 'B' side would force the 'A' side low. Note that the driver has to sink current from both pull-up resistors.
Based on the input voltage (3.3V), the output voltage (2V), and the pull-up resistor value of 50 ohms, I would guess that the driver is being simulated with a 75 ohm output impedance. If you change the pull-up resistors to 1kohm, you should see a significant improvement in signal integrity.
