AP64352

Document number: DS41978 Rev. 5 - 2

16 of 21

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August 2019

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AP64352

Application Information (cont.)

13

Inductor

Calculating the inductor value is a critical factor in designing a buck converter. For most designs, the following equation can be used to calculate

the inductor value:

Eq. 9

Where:



∆IL is the inductor current ripple



fSW is the buck converter switching frequency

For AP64352

, choose ∆IL to be 30% to 50% of the maximum load current of 3.5A.

The inductor peak current is calculated by:

Eq. 10

Peak current determines the required saturation current rating, which influences the size of the inductor. Saturating the inductor decreases the

converter efficiency while increasing the temperatures of the inductor and the internal power MOSFETs. Therefore, choosing an inductor with the

appropriate saturation current rating is important. For most applications, it is recommended to select an inductor of approximately 2.2µH to 10µH

with a DC current rating of at least 35% higher than the maximum load current. For highest efficiency, the inductor’s DC resistance should be less

than 3

0mΩ. Use a larger inductance for improved efficiency under light load conditions.

14

Input Capacitor

The input capacitor reduces both the surge current drawn from the input supply as well as the switching noise from the device. The input capacitor

must sustain the ripple current produced during the on-time of Q1. It must have a low ESR to minimize power dissipation due to the RMS input

current.

The RMS current rating of the input capacitor is a critical parameter and must be higher than the RMS input current. As a rule of thumb, select an

input capacitor with an RMS current rating greater than half of the maximum load current.

Due to large dI/dt through the input capacitor, electrolytic or ceramic capacitors with low ESR should be used. If using a tantalum capacitor, it must

be surge protected or else capacitor failure could occur. Using a ceramic capacitor greater than 10µF is sufficient for most applications.