Current ripple ratio is a practical starting point for choosing a buck converter’s inductor: it relates the inductor’s peak-to-peak ripple current to the converter’s average output current at a stated load. Use it to calculate a target inductance, then check the actual part’s current ratings, losses, bias behavior, size, and transient performance. A ratio near 30% is a common first calculation—not a universal requirement or proof that an inductor is safe.
What current ripple ratio means
Current ripple ratio describes the inductor’s alternating current relative to its average or DC current. In the common buck-converter design convention, the ratio is evaluated at maximum load in continuous-conduction mode (CCM):
r = ΔIL / IOUT
Here, ΔIL is the peak-to-peak inductor ripple current, and IOUT is the average output current at the specified operating point. Some documents call the ratio r, CR, or LIR. Check the source’s definition and load point before comparing values; notations are not guaranteed to use the same denominator or conditions. Analog Devices describes 0.3 as a typical LIR in its component-selection note. Application Note 1197 also defines the ratio at maximum rated load in CCM, but its available copy is hosted by a third party and does not establish the original publisher or date.
Choose a starting ratio, not a universal target
About 30% is a useful initial value when the regulator’s documentation does not prescribe another target. Published recommendations vary with the design context:
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| Source | Guidance | Context |
|---|---|---|
| Texas Instruments, Mathew Jacob, 2019 | 0.3–0.5; described as the “sweet spot” | Buck-inductor analysis in the Analog Design Journal article |
| ROHM, 2012 | 0.2–0.5 is usual | Application Note No. 12027ECY01 |
| Analog Devices | 0.3 typical; 30% presented as a common compromise | Selecting the Right Inductor Current Ripple; publication date not established in the retrieved page text |
| TDK | 20–30% of rated current | Power-inductor guidance for the step-down converter context it covers; page retrieved October 4, 2026 |
These are design recommendations, not a measured population statistic or a standard that every converter must meet. Start with the regulator IC’s datasheet or application note, then weigh footprint, efficiency, thermal limits, and load-transient requirements.
Calculate the inductance from your operating conditions
Ripple ratio alone cannot identify an inductance value. First establish the regulator and system’s operating envelope: input-voltage minimum and maximum, output voltage, maximum output current, switching-frequency range or tolerance, topology and control mode, temperature-rise limit, and available board area. The title does not supply these values, so it cannot support a responsible specific microhenry rating or part number.
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For an ideal buck converter, Analog Devices gives this relation using maximum input voltage:
L = VOUT × (VIN(MAX) − VOUT) / [VIN(MAX) × fSW × IOUT(MAX) × LIR]
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With the ratio defined at maximum load, first calculate ΔIL = r × IOUT(MAX), then use the regulator’s inductance equation to find a candidate value. The formula above is an explanatory idealized relation, not a substitute for the target IC’s design method. For example, ROHM’s fuller calculation accounts for switch and diode drops, with a simplified expression for higher output voltage. Use the controller-specific method where it addresses those drops, frequency limits, current limits, or topology-specific behavior.
- Set the operating point and limits. Record the input range, output voltage and maximum load, switching-frequency range, required operating mode, transient needs, thermal environment, and footprint constraint.
- Select a trial ratio. Use the regulator maker’s recommendation when available. Otherwise, about 0.3 is a reasonable first calculation point, not a pass/fail threshold.
- Calculate ripple and inductance. Find ΔIL from the chosen ratio and maximum load, then calculate L using the regulator’s specified equation and worst-case conditions.
- Choose a standard value and recalculate. Use the actual nominal value and tolerance of the candidate part to check ripple and current at the relevant operating extremes; do not assume the initial target is what the selected component will deliver.
Check peak current, operating mode, and part ratings
For a triangular ripple waveform in CCM, the steady-state inductor current swings around its average output current. Its peak and valley are:
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- IPEAK = IOUT + ΔIL/2
- IVALLEY = IOUT − ΔIL/2
Check that the valley stays above zero at loads where CCM is required. With a fixed inductor and input voltage, ripple may remain approximately fixed as load falls while average current declines; the ripple ratio therefore rises. Under Application Note 1197’s definition, it reaches 2 at the CCM-to-DCM boundary, when valley current reaches zero. That note’s original publisher and date are not established by the third-party-hosted copy. At lighter loads, confirm whether the regulator permits discontinuous-conduction mode (DCM), pulse skipping, or another mode, and whether its output ripple, noise, or control behavior is acceptable.
Compare candidate inductors using their datasheets and actual conditions, not just the nominal inductance or a single current number:
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- Inductance tolerance and inductance under DC bias.
- Saturation-current rating, its manufacturer-defined criterion, and margin to the controller’s switching-current limit.
- RMS or DC current rating and temperature rise in the intended board and thermal environment.
- DC resistance (DCR), conduction loss, and core loss at the switching frequency and ripple waveform.
- Package size, thermal conditions, and the effect of inductance on load-transient response.
Current-rating labels are not interchangeable across manufacturers. TI’s 2019 analysis, for instance, discusses saturation at a 30% inductance decline; another manufacturer may specify a different decline or test condition. Also account for faults and transients: ROHM cautions that coil current can exceed the steady-state calculated maximum and advises relating saturation rating to the IC’s switching-current limit. Do not infer a universal safety margin from a generic rule of thumb.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Balance ripple against efficiency, size, and response
Reducing ripple generally calls for more inductance, which can mean a larger or more expensive component and slower load-transient recovery. Increasing ripple can raise relevant RMS currents and increase inductor/core and surrounding-component losses. The right trade-off depends on the regulator, operating envelope, thermal budget, and required transient response; simply minimizing the ratio is not a complete selection strategy.
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