Resistive Level Shifter Ratio

Formula

Description

A resistive voltage divider can shift a higher-voltage logic signal down to a lower-voltage logic level (e.g., 5 V to 3.3 V). The required resistor ratio is determined by the input and output voltages. This is a simple and cost-effective unidirectional level shifting method suitable for low-speed signals. The output impedance of the divider limits the maximum signal frequency due to the capacitive loading of the receiver input. For bidirectional level shifting or high-speed signals, use dedicated level-shifting ICs or MOSFET-based circuits.

Variables

• R₁/R₂ — Ratio of upper to lower resistor
• V_in — Input logic high voltage (V)
• V_out — Desired output logic high voltage (V)

Practical Notes

For 5 V to 3.3 V: ratio = (5−3.3)/3.3 ≈ 0.515. Use R1 = 1 kΩ, R2 = 2 kΩ (ratio = 0.5, Vout = 3.33 V). Keep total resistance low enough (1-10 kΩ) for adequate speed, but high enough to avoid excessive current from the driver. The rise time through the divider is approximately 2.2 × (R1‖R2) × C_load. This method only works for unidirectional high-to-low translation. For low-to-high or bidirectional, use a BSS138 MOSFET level shifter or a dedicated IC like TXB0108.