L7981 datasheet(15/44 Pages) STMICROELECTRONICS

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L7981

Application informations

Doc ID 15182 Rev 3

15/44

Application informations

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 3

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

η is the efficiency.

Considering

η=1, this function has a maximum at D=0.5 and 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 4

and

Equation 5

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 6

where ESR is the equivalent series resistance of the capacitor.

RMS

⋅

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

–

-------

⋅

MAX

OUT

INMIN

–

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

MIN

OUT

INMAX

–

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

⋅

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

----

–

⎝⎠

⎛⎞

----

–

()

⋅

ESR I

⋅

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