R6986 datasheet(50/62 Pages) STMICROELECTRONICS

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Application notes

R6986

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DocID026996 Rev 2

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 11 some inductor part numbers are listed.

Table 10. Input capacitors

Manufacturer

Series

Size

Cap value (

F)

Rated voltage (V)

TDK

C3225X7S1H106M

1210

C3216X5R1H106M

1206

Taiyo Yuden

UMK325BJ106MM-T

1210

Table 11. 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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