L7986TA datasheet(16/43 Pages) STMICROELECTRONICS

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Application information

L7986TA

16/43

Doc ID 022098 Rev 3

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

input filter sustaining a 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 10

neglecting the small ESR of ceramic capacitors.

Considering

η=1, this function has its maximum in D=0.5, therefore, given the maximum

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

Equation 11

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

of VINMAX

In 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, must be selected.

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

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

following equation:

Equation 12

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

⋅⋅

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

ΔI

OUT

–

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

⋅

OUT

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

OFF

⋅

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