L6985F datasheet(52/65 Pages) STMICROELECTRONICS

52 / 65 page

Application notes

L6985F

52/65

DocID027836 Rev 2

So fixing

IL = 20% to 40% of the maximum output current, the minimum inductance value

can be calculated:

Equation 50

where fSW is the switching frequency 1/(TON + TOFF).

For example for VOUT = 3.3 V, VIN = 12 V, IOUT = 0.5 A and FSW = 2 MHz the minimum

inductance value to have

IL = 30% of IOUT is about 8.2 µH.

The peak current through the inductor is given by:

Equation 51

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.

In Table 12 some inductor part numbers are listed.

6.6.3

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). As a consequence 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:

Equation 52

Usually the resistive component of the ripple can be neglected if the selected output

capacitor is a multi layer ceramic capacitor (MLCC).

The output capacitor is important also for loop stability: it determines the main pole and the

zero due to its ESR. (See Section 5: Closing the loop on page 36 to consider its effect in the

system stability).

Table 12. Inductors

Manufacturer

Series

Inductor value (

H)

Saturation current (A)

Coilcraft

XAL40xx

2.2 to 15

5.6 to 2.8

XAL50xx

2.2 to 22

9.2 to 3.6

XFL40xx

2.2 to 4.7

3.7 to 2.7

LMIN

VOUT

I

LMAX

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

1DMIN

–

FSW

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



ILPK



IOUT

I

--------

V

OUT

ESR



 I

LMAX

I

LMAX

8COUT fSW



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