AN4501 datasheet(30/48 Pages) STMICROELECTRONICS

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Selection of the input capacitor

AN4501

30/48

DocID026394 Rev 1

Selection of the input capacitor

An input capacitor is required to provide the AC current to the inductor and to reduce the

input voltage ripple. Therefore, its choice has an impact on the electromagnetic emission at

the input.

Note: this section considers only the input voltage ripples caused by the operation of the

boost converter in steady state. An additional (and bigger) capacitor might be necessary to

buffer the input voltage, for example in case of line transients.

Figure 13. Current waveforms of the inductor and of the input capacitor for a boost

converter in CCM

7.1

Input voltage ripples

Like the output voltage ripples, the input voltage ripples are due to the charge and the

discharge of the (ideal) input bulk capacitance and of the current ripple across the ESR.

7.1.1

Contribution bulk capacitance to the input voltage ripple

The peak-to-peak voltage ripple in the input capacitor (noted VCIN,PP) corresponds to the

voltage increase during the charge of the capacitor (iCIN > 0).

We have:

is represented by the blue area (see Figure 14).

Δq

CIN

CIN t

---------

OFF

----------------------------



CIN PP

CIN t

---------

OFF

----------------------------



AN4501 Datasheet(PDF) 30 Page - STMicroelectronics