L6983 datasheet(33/63 Pages) STMICROELECTRONICS

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IL,PK=IOUT+∆IL2

(44)

So if the inductor value decreases, the peak current (that has to be lower than the current limit of the device)

increases. The higher is the inductor value, the higher is the average output current that can be delivered, without

reaching the current limit.

8.5.3

Output capacitor selection

The triangular shape current ripple (with zero average value) flowing into the output capacitor gives the output

voltage ripple, that depends on the capacitor value and the equivalent resistive component (ESR). Therefore, the

output capacitor has to be selected in order to have a voltage ripple compliant with the application requirements.

The voltage ripple equation can be calculated as:

∆VOUT=ESR∙∆IL,MAX+ ∆IL,MAX

8∙COUT∙FSW

(45)

For a ceramic (MLCC) capacitor, the capacitive component of the ripple dominates the resistive one. While for an

electrolytic capacitor the opposite is true. Neglecting the ESR contribution the minimum value of the output

capacitor is given by:

COUT,min,RIPPLE= ∆IL,MAX

8∙∆VOUT∙FSW

(46)

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. A good rule to obtain a proper dimensioning for the minimum amount of the

output capacitor is set the target system bandwidth equal to FSW/8. The following equation keep into account the

precedent consideration:

COUT,BW,min= 8.04

Fsw8∙VOUT

(47)

The maximum amount of the output capacitor is given by:

COUT,BW,MAX=0.960∙10−3

VOUT

(48)

L6983

Design of the power components

DS13116 - Rev 1

page 33/63

L6983 Arkusz danych(PDF) 33 Page - STMicroelectronics