Application information

PM6644

22/35

Doc ID 023203 Rev 1

.

3.1.2

Input capacitor selection

In a buck topology converter the current that flows into the input capacitor is a pulsed current

with zero average value. The input RMS current of the switching regulator can be roughly

estimated as follows:

Equation 11

regulator. The input capacitor should be chosen with an RMS rated current higher than the

maximum RMS current given by the formula.

Tantalum capacitors are good in terms of low ESR and small size, but they can occasionally

burn out if subjected to very high current during the charge.

Ceramic capacitors usually have a higher RMS current rating with smaller size and they

remain the best choice. In battery-powered applications, a 1-2.2

µF input ceramic capacitor

can be enough.

Table 11 shows an example of ceramic capacitor part numbers.

3.1.3

Output capacitor selection

The controller can work with ceramic or tantalum output capacitors.

The selection of the output capacitor impacts on the stability of the controller:

Equation 12

Equation 13

Table 10.

Inductor part number

Manufacturer

Part number

Inductance

(

µH)

DCR (

Ω)

RMS current

1.

40 °C temperature rise.

Saturation

2.

20% inductance drop.

Coilcraft

EPL3015-333ML

33

0.989

0.59

0.32

Coilcraft

LPS3314-333ML

33

0.92

0.58

0.38

Coilcraft

MSS5121-333ML

33

0.48

0.76

0.64

Table 11.

Input capacitor part numbers

Manufacturer

Part number

Capacitor value (

µF)

Rated voltage

TAYIO YUDEN

TMK212BJ225MG-T

2.2

25

I

CinRMS

I

()

D1

D

–

()

⋅

⋅

=

C

OUT

18

α

⋅

2

Π f

sw

k

⋅⋅

⋅

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

>

α

0.9V

V

OUT

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

=