R7986A datasheet(16/37 Pages) STMICROELECTRONICS

16 / 37 page

Application information

R7986A

16/37

DocID026997 Rev 1

and:

Equation 8

where VF is the forward voltage on the freewheeling diode and VSW is voltage drop across

the internal PDMOS.

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

Equation 9

where ESR is the equivalent series resistance of the capacitor.

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

= 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 recommended 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.

Table 6. Input MLCC capacitors

Manufacture

Series

Cap value (

F)

Rated voltage (V)

Taiyo Yuden

UMK325BJ106MM-T

GMK325BJ106MN-T

Murata

GRM32ER71H475K

4.7

DMIN

VOUT VF

VINMAX VSW

–

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

VPP

CIN FSW



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



----

–



 D D



----

–





ESR IO



CIN

VPP FSW



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



----

–



 D D



----

–





CIN_MIN

2VPP_MAX FSW



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

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