SLVSI31 Data sheet

SLVSI31 April 2026 TPS61371

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- MPQF832

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8.2.2.2 Selecting the Inductor

A boost converter normally requires two main passive components for storing the energy during power conversion: an inductor and an output capacitor. The inductor affects the steady state efficiency (including the ripple and efficiency) as well as the transient behavior and loop stability, which makes the inductor to be the most critical component in application.

When selecting the inductor, as well as the inductance, the other parameters of importance are:

The maximum current rating (consider RMS and peak current)
The series resistance
Operating temperature

Choosing the inductor ripple current with the low ripple percentage of the average inductor current results in a larger inductance value, maximizes the potential output current of the converter, and minimizes EMI. The larger ripple results in a smaller inductance value and a physically smaller inductor, which improves transient response, but results in potentially higher EMI.

The rule in choosing the inductor is to make sure the inductor ripple current (ΔI_L) is a certain percentage of the average current. Then use Equation 2, Equation 3, and Equation 4 to calculate the inductance:

Equation 2.

∆ I_{L} = \frac{V_{I N} \times D}{L \times f_{S W}}

Equation 3.

∆ I_{L_R} = R i p p l e % \times \frac{V_{O U T} \times I_{O U T}}{η \times V_{I N}}

Equation 4.

L = \frac{1}{R i p p l e %} \times \frac{η \times V_{I N}}{V_{O U T} \times I_{O U T}} \times \frac{V_{I N} \times D}{f_{S W}}

where

Δ_IL is the peak-peak inductor current ripple
V_IN is the input voltage
D is the duty cycle
L is the inductor
ƒ_SW is the switching frequency
Ripple% is the ripple ration versus the DC current
V_OUT is the output voltage
I_OUT is the output current
η is the efficiency

The current flowing through the inductor is the inductor ripple current plus the average input current. During power up, load faults, or transient load conditions, the inductor current can increase above the peak inductor current calculated.

Inductor values can have ±20% or even ±30% tolerance with no current bias. When the inductor current approaches the saturation level, the inductance can decrease 20% to 35% from the value at 0A bias current depending on how the inductor vendor defines saturation. When selecting an inductor, make sure the rated current, especially the saturation current, is larger than the peak current during the operation.

The inductor peak current varies as a function of the load, the switching frequency, and the input and output voltages and can be calculated by Equation 5 and Equation 6.

Equation 5.

I_{P E A K} = I_{I N} + \frac{1}{2} \times ∆ I_{L}

where

I_PEAK is the peak current of the inductor
I_IN is the input average current
ΔI_L is the ripple current of the inductor

The input DC current is determined by the output voltage, the output current, and efficiency can be calculated by:

Equation 6.

I_{I N} = \frac{V_{O U T} \times I_{O U T}}{V_{I N} \times η}

where

I_IN is the input current of the inductor
V_OUT is the output voltage
V_IN is the input voltage
η is the efficiency

While the inductor ripple current depends on the inductance, the frequency, the input voltage, and duty cycle calculated by Equation 2, replace Equation 2, Equation 6 into Equation 5 to calculate the inductor peak current:

Equation 7.

I_{P E A K} = \frac{I_{O U T}}{(1 - D) \times η} + \frac{1}{2} \times \frac{V_{I N} \times D}{L \times f_{S W}}

where

I_PEAK is the peak current of the inductor
I_OUT is the output current
D is the duty cycle
η is the efficiency
V_IN is the input voltage
L is the inductor
ƒ_SW is the switching frequency

The heat rating current (RMS) is calculated by Equation 8:

Equation 8.

I_{L_R M S} = \sqrt{{I_{I N}}^{2} + \frac{1}{12} {(∆ I_{L})}^{2}}

where

I_{L_RMS} is the RMS current of the inductor
I_IN is the input current of the inductor
ΔI_L is the ripple current of the inductor

Make sure that the peak current does not exceed the inductor saturation current and the RMS current is not over the temperature related rating current of the inductors.

For a given physical inductor size, increasing inductance typically results in an inductor with lower saturation current. The total losses of the coil consists of the DC resistance ( DCR ) loss and the following frequency dependent loss:

The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)
Additional losses in the conductor from the skin effect (current displacement at high frequencies)
Magnetic field losses of the neighboring windings (proximity effect)

For a certain inductor, the larger current ripple (smaller inductor) generates the higher DC and frequency-dependent loss. An inductor with lower DCR is basically recommended for higher efficiency. However, there is typically a tradeoff between the loss and footprint.

The following inductor series in Table 8-2 from the different suppliers are recommended.

Table 8-2 Recommended Inductors for TPS61371

PART NUMBER	L (μH)	DCR TYP (mΩ) TYP.	SATURATION CURRENT / TYP.	SIZE (L × W × H mm)	VENDOR⁽¹⁾
PIJT3225FE-1R0MSR	1	40	4.4	3.2 × 2.5 × 0.65	Cyntec
XAL4020-222ME	2.2	35	5.6	4 × 4 × 2	Coilcraft
DFE322512F-2R2M=P2	2.2	66	2.6	3.2 × 2.5 × 1.2	Murata
DFE322520FD-4R7M#	4.7	98	3.4	3.2 × 2.5 × 2.0	Murata

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