A6986F datasheet(53/72 Pages) STMICROELECTRONICS

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A6986F

Application notes

7.6.2

Inductor selection

The inductor current ripple flowing into the output capacitor determines the output voltage

ripple (please refer to Section 7.6.3). Usually the inductor value is selected in order to keep

the current ripple lower than 20% - 40% of the output current over the input voltage range.

The inductance value can be calculated by Equation 50:

Equation 50

Where TON and TOFF are the on and off time of the internal power switch. The maximum

current ripple, at fixed VOUT, is obtained at maximum TOFF that is at minimum duty cycle

(see Section 7.6.1: Input capacitor selection to calculate minimum duty). So fixing

IL = 20%

to 40% of the maximum output current, the minimum inductance value can be calculated:

Equation 51

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

For example for VOUT = 3.3 V, VIN = 12 V, IO = 2 A and FSW = 500 kHz the minimum

inductance value to have

IL = 30% of IO is about 8.2 µH.

The peak current through the inductor is given by:

Equation 52

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 14 some inductor part numbers are listed.

7.6.3

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.

Table 14. Inductors

Manufacturer

Series

Inductor value (

H)

Saturation current (A)

Coilcraft

XAL50xx

2.2 to 22

6.5 to 2.7

XAL60xx

2.2 to 22

12.5 to 4

I

VIN VOUT

–

------------------------------ TON



VOUT

-------------- TOFF



LMIN

VOUT

I

LMAX

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

1DMIN

–

FSW

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



ILPK



IOUT

I

--------

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