L6984 datasheet(24/34 Pages) STMICROELECTRONICS

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Design of the power components

L6984

24/34

DocID025378 Rev 3

Considering

this function has its maximum in D = 0.5:

Equation 18

Typically CIN is dimensioned to keep the maximum peak-to-peak voltage across the input

filter in the order of 5% VIN_MAX.

4.2

Inductor selection

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

ripple (please refer to Section 4.3: Output capacitor selection). 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 the following equation:

Equation 19

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 4.1 to calculate minimum duty). So fixing

IL = 20 % to 40 % of the maximum

output current, the minimum inductance value can be calculated:

Equation 20

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

For example for VOUT = 3.3 V, VIN = 12 V, IO = 0.4 A and FSW = 600 kHz the minimum

inductance value to have

IL = 30 % of IO is about 33 µH.

The peak current through the inductor is given by:

Equation 21

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.

Table 6. Input capacitors

Manufacture

Series

Size

Cap value (

F) Rated voltage (V)

TDK

C3225X7S1H106M

1210

C3216X5R1H106M

1206

Taiyo Yuden

UMK325BJ106MM-T

1210

CIN_MIN



PP_MAX fSW



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

I

VIN VOUT

–

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



VOUT

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



LMIN

VOUT

I

MAX

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

1DMIN

–

FSW

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



ILPK



I

--------

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