L7981 datasheet(16/44 Pages) STMICROELECTRONICS

16 / 44 page

Application informations

L7981

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Doc ID 15182 Rev 3

Given the physical dimension, ceramic capacitors can meet well the requirements of the

input filter sustaining an higher input RMS current than electrolytic / tantalum types. In this

case the equation of CIN as a function of the target VPP can be written as follows:

Equation 7

neglecting the small ESR of ceramic capacitors.

Considering

η=1, this function has its maximum in D=0.5, thus, given the maximum peak to

peak input voltage (VPP_MAX), the minimum input capacitor (CIN_MIN) value is:

Equation 8

Typically CIN is dimensioned to keep the maximum peak-peak voltage in the order of 1% of

VINMAX

Table 6. some multi layer ceramic capacitors suitable for this device are reported

A ceramic bypass capacitor, as close to the VCC and GND pins as possible, so that

additional parasitic ESR and ESL are minimized, is suggested in order to prevent instability

on the output voltage due to noise. The value of the bypass capacitor can go from 100 nF to

1 µF.

6.2

Inductor selection

The inductance value fixes the current ripple flowing through the output capacitor. So the

minimum inductance value in order to have the expected current ripple has to be selected.

The rule to fix the current ripple value is to have a ripple at 20%-40% of the output current.

In the continuos current mode (CCM), the inductance value can be calculated by the

following equation:

Table 6.

Input MLCC capacitors

Manufacture

Series

Cap value (

μF)

Rated voltage (V)

Taiyo Yuden

UMK325BJ106MM-T

GMK325BJ106MN-T

Murata

GRM32ER71H475K

4.7

⋅

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

----

–

⎝⎠

⎛⎞

----

–

()

⋅

IN_MIN

PP_MAX

⋅⋅

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