L6985F datasheet(51/65 Pages) STMICROELECTRONICS

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L6985F

Application notes

The peak-to-peak voltage across the input filter can be calculated as:

Equation 46

In case of negligible ESR (MLCC capacitor) the equation of CIN as a function of the target

VPP can be written as follows:

Equation 47

Considering

 = 1 this function has its maximum in D = 0.5:

Equation 48

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

filter in the order of 5% VIN_MAX.

6.6.2

Inductor selection

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

ripple (please refer to Section 6.6.3). 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 49:

Equation 49

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 a minimum duty cycle

(see Section 6.6.1: Input capacitor selection to calculate minimum duty).

Table 11. Input capacitors

Manufacturer

Series

Size

Cap value (

F)

Rated voltage (V)

TDK

C3225X7S1H106M

1210

C3216X5R1H106M

1206

Taiyo Yuden

UMK325BJ106MM-T

1210

VPP

IOUT

CIN fSW



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



----

–



 D



----



ESR

IOUT IL





CIN

IOUT

VPP fSW



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



----

–



 D



----



CINMIN

IOUT

4VPPMAX fSW



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

I

VIN VOUT

–

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



VOUT

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



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