8.2.2.2 Inductor Calculation

For peak current mode control, sub-harmonic oscillation occurs with a duty cycle greater than 50% and is characterized by alternating wide and narrow pulses at the SW pin. By adding a slope compensating ramp equal to at least one-half the inductor current down-slope, any tendency toward sub-harmonic oscillation is damped within one switching cycle. For design simplification, the LM25141-Q1 has an internal slope compensation ramp added to the current sense signal.

For the slope compensation ramp to dampen sub-harmonic oscillation, the inductor value should be calculated based on the following guidelines (Equation 15 assumes an inductor ripple current 30%):

Equation 15.

• Lower inductor values increase the peak-to-peak inductor current, which minimizes size and cost and improves transient response at the expense of reduced efficiency due to higher peak currents.
• Higher inductance values decrease the peak-to-peak inductor current, which typically increases efficiency by reducing the RMS current but requires larger output capacitors to meet load-transient specifications.

Equation 16.

A standard inductor value of 1.5 µH was selected

Equation 17.

Equation 18.

The peak-to-peak inductor current is:

Equation 19.

Equation 20.

Equation 21.

Equation 22.