Application information

ST1S40

16/30

Doc ID 17928 Rev 4

Equation 20

so if the inductor value decreases, the peak current (that must be lower than the current limit

of the device) increases. The higher the inductor value, the higher the average output

current that can be delivered, without reaching the current limit.

In Table 7 below some inductor part numbers are listed.

6.3

Output capacitor selection

The current in the output capacitor has a triangular waveform which generates a voltage

ripple across it. This ripple is due to the capacitive component (charge or discharge of the

output capacitor) and the resistive component (due to the voltage drop across its ESR). So

the output capacitor must be selected in order to have a voltage ripple compliant with the

application requirements.

The amount of the voltage ripple can be calculated starting from the current ripple obtained

by the inductor selection.

Equation 21

For ceramic (MLCC) capacitors the capacitive component of the ripple dominates the

resistive one. Whilst for electrolytic capacitors the opposite is true.

Since the compensation network is internal, the output capacitor should be selected in order

to have a proper phase margin and then a stable control loop.

The equations of Chapter 5.2 help to check loop stability given the application conditions,

the value of the inductor, and of the output capacitor.

In Table 8 below some capacitor series are listed.

Table 7.

Inductors

Manufacturer

Series

Inductor value (

μH)

Saturation current (A)

Coilcraft

XPL7030

2.2 to 4.7

6.8 to 10.5

MSS1048

2.2 to 6.8

4.14 to 6.62

MSS1260

10

5.5

Wurth

WE-HC/HCA

3.3 to 4.7

7 to 11

WE-TPC typ XLH

3.6 to 6.2

4.5 to 6.4

WE-PD type L

10

5.6

TDK

RLF7030T

2.2 to 4.7

4 to 6

I

LPK

,

I

O

ΔI

L

2

--------

+

=

ΔV

OUT

ESR

ΔI

MAX

⋅

ΔI

MAX

8C

OUT

f

SW

⋅⋅

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

+

=