L7985 datasheet(15/42 Pages) STMICROELECTRONICS

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L7985

Application information

6.1

Input capacitor selection

The capacitor connected to the input must be capable of supporting the maximum input

operating voltage and the maximum RMS input current required by the device. The input

capacitor is subject to a pulsed current, the RMS value of which is dissipated over its ESR,

affecting the overall system efficiency.

So, the input capacitor must have an RMS current rating higher than the maximum RMS

input current and an ESR value compliant with the expected efficiency.

The maximum RMS input current flowing through the capacitor can be calculated as:

Equation 6

where IO is the maximum DC output current, D is the duty cycle, is the efficiency.

Considering

, this function has a maximum of D = 0.5 and it is equal to IO/2.

In a specific application, the range of possible duty cycles must be considered in order to

find out the maximum RMS input current. The maximum and minimum duty cycles can be

calculated as:

Equation 7

and

Equation 8

where VF is the forward voltage on the freewheeling diode and VSW is the 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:

IRMS





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

–



-------



DMAX

VOUT VF

VINMIN VSW

–

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

DMIN

VOUT VF

VINMAX VSW

–

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

VPP

CIN FSW



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



----

–



 D D



----

–





ESR IO



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