TIOL111: Pull up resistor size

Hello,

The typical concern with a higher resistor value is whether it is strong enough to overcome any leakage current through the various parasitic elements and whether it is strong enough to charge any parasitic capacitance on the net in a reasonable time.  Larger resistor values will take longer to pull the net back to High, and may not pull all the way up if there is a significant leakage current forming a voltage divider.  Typically I would recommend staying at 100kohm or below for the largest resistance possible, but this exact value depends on the system.  But 20kohm to 60kohm is generally fine.

Typically the more critical concern is how small can the resistor value be.  A smaller resistor value will require the device to sink more current in order to pull the voltage below the Vol threshold.  The TIOL111 specifies an output current abs max of -5mA to 5mA for the WAKE and NFAULT Pins.   And the datasheet specifies a maximum Vol for these pins of 0.5V with a 4mA output current.  Doing the math, this would make the pullup resistor 1.25kohm for a 5V signal, and 825ohm for a 3.3V signal.  If you use a resistors smaller than this, the Vol will be greater than the 0.5V level and potentially not register as a "low" to the MCU's digital inputs. 

However, if you use the 20kohm to 60kohm internal pullup resistors, you have plenty of margin to the 5mA abs max current spec and even the 4mA load current used to specify the Vol 0.5V threshold and should be fine with these values.

Regards,

Jonathan