L5988D datasheet(25/52 Pages) STMICROELECTRONICS

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L5988D

Application information

6.1

Input capacitor selection

The capacitor connected to the input has to be capable to support 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 a 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 10

Where Io is the maximum DC output current, D is the duty cycles,

is the efficiency. This

function has a maximum at D = 0.5 and, considering

= 1, it is equal to Io/2.

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

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

calculated as:

Equation 11

and

Equation 12

Where

V

HIGH_SIDE and VLOW_SIDE are the voltage drops across the embedded switches.

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

Equation 13

Given a physical dimension, ceramic capacitors can met well the requirements of the input

filter substaining an higher input 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 14

IRMS





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

–



-------



DMAX

VOUT VLOW_SIDE

VINMIN VLOW_SIDE VHIGH_SIDE

–

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

DMIN

VOUT VLOW_SIDE

VINMAX VLOW_SIDE VHIGH_SIDE

–

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

VPP

CIN fSW



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



----

–



 D





----

–





ESR IO



CIN

VPP fSW



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



----

–



 D





----

–





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