Series protection resistors on I2C lines
Rp and Rs form a voltage divider. Rs must be low enough to keep the voltage on the line below VIL-VnL. They must also be small enough to sink at least IOL.

We therefore have the following:

Is - Ip >= 3mA

Vs / VDD &lt;= 0.2

Vs + Vp = VDD

Solving for these simultaneously results in the output shown in the chart.