SNVSCO1 Data sheet

SNVSCO1 November 2025 LM5126A-Q1

PRODUCTION DATA

7.2.2.4 Inductor Selection L_m

Three main parameters are considered when selecting the inductance value: Inductor current ripple ratio (RR), falling slope of the inductor current and the RHPZ frequency of the control loop.

The inductor current ripple ratio is selected to balance the winding loss and core loss of the inductor. As the ripple current increases the core loss increases and the copper loss decreases.
It is necessary for the falling slope of the inductor current to be small enough to prevent sub-harmonic oscillation. A larger inductance value results in a smaller falling slope of the inductor current.
Place the RHPZ at high frequency to allow a higher crossover frequency of the control loop. As the inductance value decreases the RHPZ frequency increases.

According to peak current mode control theory, it is necessary that the slope compensation ramp is greater than half of the sensed inductor current falling slope to prevent subharmonic oscillation at high duty cycle, that is:

Equation 32.

V_{s l o p e} \times f_{s w} > \frac{V_{o u t_m a x} - V_{i n_m i n}}{2 \times L_{m}} \times R_{c s}

where

V_slope is a 48mV peak (at 100% duty cycle) slope compensation ramp at the input of the current sense amplifier.

The lower limit of the inductance is found as,

Equation 33.

L_{m} > \frac{V_{o u t_m a x} - V_{i n_m i n}}{2 \times V_{s l o p e} \times f_{s w}} \times R_{c s}

R_cs is estimated to be 1.5mΩ, so the following is found,

Equation 34.

L_{m} > 1.4 µ H

The RHPZ frequency is found as,

Equation 35.

ω_{R H P Z} = \frac{R_{o u t} \times {D'}^{2}}{L_{m_e q}}

It is necessary that the crossover frequency is lower than 1/5 of RHPZ frequency,

Equation 36.

f_{c} < \frac{1}{5} \times \frac{ω_{R H P Z}}{2 π}

Assume a crossover frequency of 1kHz is desired, the upper limit of the inductance is found as,

Equation 37.

L_{m} < 5.2 µ H

The inductor ripple current is typically set between 30% and 70% of the full load current, known as a good compromise between core loss and winding loss of the inductor.

Per phase input current is calculated as,

Equation 38.

I_{i n_v i n m a x} = \frac{P_{o u t}}{{η \times V}_{i n_m a x}} = 29.2 A

In continuous conduction mode (CCM) operation, the maximum ripple ratio occurs at a duty cycle of 33%. The input voltage that result in a maximum ripple ratio is found as,

Equation 39.

V_{i n_R R m a x} = V_{o u t_m a x} \times (1 - 0.33) = 30 V

Thus, it is necessary to use the maximum input voltage V_{in_max} to calculate the maximum ripple ratio.

For this example, a ripple ratio of 0.3, 30% of the input current was chosen. Knowing the switching frequency and the typical output voltage, the inductor value is calculated as follows,

Equation 40.

L_{m} = \frac{V_{i n_m a x}}{I_{i n} \times R R} \times \frac{1}{f_{s w}} \times (1 - \frac{V_{i n_m a x}}{V_{o u t_m a x}}) = \frac{18 V}{29.2 A \times 0.3} \times \frac{1}{400 k H z} \times 0.6 = 3.1 µ H

The closest standard value of 3.3μH was chosen for L_m.

The inductor ripple current at typical input voltage is calculated as:

Equation 41.

I_{p p} = \frac{V_{i n_t y p}}{L_{m}} \times \frac{1}{f_{s w}} \times (1 - \frac{V_{i n_t y p}}{V_{o u t}}) = 7.4 A

If a ferrite core inductor is selected, make sure the inductor does not saturate at peak current limit. The inductance of a ferrite core inductor is almost constant until saturation. Ferrite core has low core loss with a big size.

For powder core inductor, the inductance decreases slowly with increased DC current. This action leads to higher ripple current at high inductor current. For this example, the inductance drops to 70% at peak current limit compared to 0A. The current ripple at peak current limit is found as,

Equation 42.

I_{p p_b i a s} = \frac{V_{i n_t y p}}{0.7 \times L_{m}} \times \frac{1}{f_{s w}} \times (1 - \frac{V_{i n_t y p}}{V_{o u t}}) = 10.6 A

7.2.2.4 Inductor Selection Lm

7.2.2.4 Inductor Selection L_m