How to determine the pull-up resistance, floating of voltage

Here explains floating of voltage due to pull-up resistance and how to select a pull-up resistance.

How to select a pull-up resistance

The general value of pull-up resistance of voltage detector is 10 kΩ to several hundred kΩ.

If the pull-up resistance value is small, the gap between "L" voltage and Vss increases due to ON resistance of driver FET. If the pull-up resistance value is large, the gap between "H" voltage and VIN (VDD) increases due to leak current of driver FET in the same way.

For this reason, select a pull-up resistance that satisfies logic of following IC under the conditions of actual specifications.

If you want to reduce the consumption of current that flows into a pull-up resistance when output is "L", set a pull-up resistance value a little higher.
If you want to prevent malfunction of application due to the influence of external noise, etc., set a pull-up resistance value a little lower.

【Appendix】How to calculate maximum value of output "L" / minimum value of "H"

XCalculate maximum value / minimum value of output "H" voltage and output "L" voltage referencing XC6138 series.

【Maximum value of output "L" voltage】

V_RESETB = Vpull / (1 + Rpull / R_{ON_MAX})
Vpull ：pull-up voltage
R_{ON_MAX} ：maximum ON resistance of Nch driver

Calculation example) when VDD=2.2V, Vpull=1.8V, Rpull=10kΩ
Step 1. Calculate R_{ON_MAX}
Calculate R_{ON_MAX} from V_RESETB/I_{RBOUT_MIN}
R_{ON_MAX} = 0.3V / 2.5mA ≒ 120Ω (MAX.)

Step 2. Calculate V_RESETB
Calculate the maximum value of output "L" voltage using R_{ON_MAX}
V_RESETB = Vpull / (1 + Rpull / R_{ON_MAX}) = 1.8V / (1 + 10kΩ / 120Ω) ≒ 21mV

【Minimum value of output "H" voltage】

V_RESETB = Vpull – Rpull × I_{LEAKN_MAX}
Vpull                ：pull-up voltage
I_{LEAKN_MAX}     ：maximum leak current of Nch driver

Calculation example) when Vpull=6.0V, Rpull=100kΩ
Step 1. I_{LEAKN_MAX}
Calculate I_{LEAKN_MAX} = 0.1μA (MAX.) depending on the electrical characteristics

Step 2. Calculate V_RESETB
Calculate the minimum value of output "H" voltage using I_{LEAKN_MAX}
V_RESETB= Vpull – Rpull × I_{LEAKN_MAX} = 6.0V - 100kΩ × 0.1μA = 5.99V