DocID026968 Rev 2

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AST1S31HF

Application information

33

The peak current through the inductor is given by Equation 31:

Equation 31

So if the inductor value decreases, the peak current (which 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 some inductor part numbers are listed.

7.3

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 32

For a ceramic (MLCC) capacitor, the capacitive component of the ripple dominates the

resistive one. While for an electrolytic capacitor the opposite is true.

As 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 Section 6.3 on page 11 help to check loop stability given the application

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

In Table 8 some capacitor series are listed.

Table 7. Inductors

Manufacturer

Series

Inductor value (

Saturation current (A)

COILTRONICS

DRA73

0.6 to 2.2

5.5 to 7.9

COILCRAFT

XAL40XX

0.6 to 2.2

5.4 to 8.35



I

L

2

--------

+

=

Table 8. Output capacitors

Manufacturer

Series

Cap value (

Rated voltage (V)

ESR (m

)

MURATA

GCM

22 to 470

10

5

TDK

CGA6

22 to 470

16

10

V

OUT

ESR

I

MAX



I

MAX



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

+

=