Checking Buck-Regulator Minimum On-Time at Maximum Input Voltage

A buck regulator can support the required input voltage and output voltage individually while struggling with their combination at the selected switching frequency. At a high conversion ratio, the required high-side pulse becomes short. Minimum on-time is therefore a separate selection check.

Calculate the required pulse

Analog Devices’ minimum-on-time explanation relates the buck conversion ratio to duty cycle and explains why a regulator cannot always generate arbitrarily short pulses. The simple continuous-conduction approximation is useful for identifying combinations requiring closer review.

Use duty cycle approximately equal to output voltage divided by input voltage, then multiply by the switching period. For an illustrative 24 V to 1.2 V conversion at 1 MHz, the ideal duty cycle is 5% and the corresponding on-time is 50 ns. This is a screening calculation, not a complete converter design.

Use the relevant corners

Repeat the calculation at maximum input voltage and the selected frequency’s high tolerance. Include the lowest programmed output setting when the product supports several voltages. Compare against the selected regulator’s documented minimum on-time, including whether the value is typical or guaranteed.

Account for the operating mode and the conditions attached to that parameter. Do not apply a continuous-conduction approximation without checking how the device behaves at light load, during startup or when it changes its switching pattern.

Read what happens at the boundary

Find the manufacturer’s description of operation when the requested pulse is too short. The converter may alter switching behavior, and the resulting ripple or frequency content may matter even if average output voltage remains acceptable. Use the actual device explanation instead of assuming a universal response.

Evaluate possible changes such as reducing frequency, choosing a device with suitable timing capability or revisiting the conversion architecture. Carry each change through the inductor, capacitor, thermal and interference analysis rather than adjusting one setting in isolation.

Verify the selected operating envelope

Measure output regulation, switching behavior and ripple across the permitted input and load range. Use a probing arrangement appropriate for the fast switch node, and keep that measurement separate from the low-level output ripple connection. Record the actual switching frequency and mode.

Keep the calculation with the relevant data-sheet limits and bench traces. State the input-voltage range for which the design was verified. If a higher-voltage adapter becomes an approved accessory, this record reveals whether the new operating point crosses a timing boundary even though it remains below the regulator’s absolute input-voltage rating.