Enter your circuit's datasheet limits to choose a standard pull-down resistor that keeps an undriven digital input LOW and protects the HIGH driver.
Table of contents
How to use our Pull Down Resistor Calculator
- Enter the signal rail in "Supply voltage, Vcc (V)". Use the voltage that drives this node HIGH.
- Find the receiving device's guaranteed LOW-input limit in its electrical characteristics table, then enter it in "Maximum input LOW voltage, VIL max (V)".
- Add the worst-case leakage currents from every connected input, inactive output, or other device that can raise the released node. Enter that sum in "Worst-case total leakage current (uA)".
- Find a source-current condition in the driving output's electrical characteristics where its HIGH voltage is still guaranteed. Enter that current in "Safe HIGH source current (mA)".
- Choose the available resistor list in "Preferred resistor series", then select "Calculate".
- Use the suggested resistor only when "Pull-down range check" is valid. Check that the released-node voltage is no higher than VIL max and the HIGH-state current is no higher than your entered limit.
Definitions
Pull-down resistor: A resistor connected from a signal node to ground. It pulls an undriven input toward a LOW voltage.
Vcc: The positive supply voltage that drives the signal HIGH in this calculation.
VIL max: The highest input voltage that the receiving device guarantees it will read as a logic LOW.
Leakage current: Small unwanted current from connected devices that can raise a released signal node above ground.
IOH and VOH: Datasheet specifications for output source current and guaranteed HIGH output voltage. Together, they help identify a safe source-current limit.
E12 and E24: Preferred resistor-value series. E24 has more listed values in each tenfold resistance range than E12.
Common mistakes and quick fixes
Mistake: Using a typical switching threshold instead of guaranteed VIL max.
Fix: Copy the VIL maximum from the receiving device's electrical characteristics table. A typical threshold does not guarantee a valid LOW reading.
Mistake: Counting leakage from only one device on a shared signal node.
Fix: Add every worst-case leakage contribution that can raise the released node before entering "Worst-case total leakage current (uA)".
Mistake: Using an absolute-maximum pin-current rating as "Safe HIGH source current (mA)".
Fix: Use a current from the driver's IOH and VOH specifications where the required HIGH output voltage is guaranteed.
Mistake: Mixing uA and mA.
Fix: Enter leakage in uA and driver current in mA. For example, 10 uA equals 0.01 mA, not 10 mA.
Mistake: Assuming a familiar 10 kOhm resistor will work in every circuit.
Fix: Confirm that the chosen value falls between the calculated minimum and maximum resistance for this circuit.
Limitations & Key Assumptions / Boundary Conditions
- This is a static digital-input calculation. It does not calculate capacitance, RC settling time, noise coupling, transmission-line behavior, or MOSFET gate discharge.
- Total leakage must include every connected device that can raise the released node under the applicable worst-case datasheet conditions.
- The HIGH-state calculation treats the full entered supply voltage as appearing across the pull-down resistor. A real driver's output voltage can change under load.
- Resistor tolerance, temperature drift, supply variation, board contamination, and noise reduce margin. A value exactly on either calculated limit has no allowance for these effects.
- Reported resistor power is instantaneous while the node is HIGH. Average power depends on the signal's HIGH-time fraction.
- A valid static range does not prove that the finished circuit meets every connected device's requirements. Check the final schematic and all relevant datasheet limits.
Methodology
Static resistance limits
The calculator finds a minimum resistance from the active HIGH driver and a maximum resistance from leakage at the released input. The limits must overlap. In the formulas, R means resistance.
R minimum (kOhm) = Vcc (V) / safe HIGH source current (mA)
A smaller resistance would draw more current from the driver while the node is HIGH.
R maximum (kOhm) = 1000 x VIL max (V) / total leakage current (uA)
A larger resistance would let the entered leakage raise the released-node voltage above VIL max.
Standard resistor selection
The calculator finds the geometric middle of the valid range, scans E12 or E24 values across powers of ten, and keeps only values within the inclusive limits. It selects the listed value closest to that middle; an exact tie uses the lower value.
R middle (kOhm) = sqrt(R minimum x R maximum)
Checks after selection
For the selected resistance, the calculator estimates the released-node voltage, current while HIGH, and instantaneous resistor power.
released-node voltage (V) = leakage current (uA) x suggested resistance (kOhm) / 1000
HIGH-state current (mA) = Vcc (V) / suggested resistance (kOhm)
resistor power (mW) = Vcc (V) x Vcc (V) / suggested resistance (kOhm)
Worked example
For Vcc = 3.3 V, VIL max = 0.8 V, total leakage = 10 uA, and safe source current = 5 mA, the allowed range is 0.660 kOhm to 80.000 kOhm. The E24 suggestion is 7.5 kOhm. Its released-node voltage is 0.075 V, its HIGH-state current is 0.440 mA, and its instantaneous power is 1.452 mW.