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ST1S31 データシート(PDF) 16 Page - STMicroelectronics

部品番号 ST1S31
部品情報  3 A DC step-down switching regulator
PDF  35 Pages
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メーカー  STMICROELECTRONICS [STMicroelectronics]
ホームページ  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

ST1S31 データシート(HTML) 16 Page - STMicroelectronics

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Application information
ST1S31
16/35
DocID022998 Rev 4
5
Application information
5.1
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 15
where Io is the maximum DC output current, D is the duty cycle, and
is the efficiency.
Considering
= 1, this function has a maximum at D = 0.5 and is equal to Io/2.
The peak-to-peak voltage across the input capacitor can be calculated as:
Equation 16
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 peak-to-peak voltage ripple (VPP) can be
written as follows:
Equation 17
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 18
Typically, CIN is dimensioned to keep the maximum peak-to-peak voltage ripple in the order
of 1% of VINMAX.
IRMS
IO
D
2D
2
---------------
–
D
2
2
-------
+
=
VPP
IO
CIN FSW
-------------------------
1
D
----
–

 D D
----
1D
–

+
ESR IO
+
=
CIN
IO
VPP FSW
---------------------------
1
D
----
–

 D D
----
1D
–

+
=
CIN_MIN
IO
2VPP_MAX FSW

------------------------------------------------
=



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