AST1S31HF Datasheet, PDF (19/33 Pages) STMicroelectronics – Internal soft-start and enable

AST1S31HF
7
Application information
Application information
7.1
Input capacitor selection
The capacitor connected to the input must be capable to support the maximum input
operating voltage and the maximum RMS input current required by the device. The input
capacitor is a subject of 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 24
I_RMS = I_O 
D
–
-²----^---^D-----²-

+
-^D_----₂₂-
where I_O 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 I_O/2.
The peak-to-peak voltage across the input capacitor can be calculated as:
Equation 25
V_PP =
-_C----_I--_N----^I-_-^O--_F---_S----_W--- 


1
–
D_{---}

D
+
D_---


1
–
D
+ ESR  I_O
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 C_IN as a function of the target peak-to-peak voltage ripple (V_PP) can be
written as follows:
Equation 26
C_IN
=
-_V----_P---_P---^I--_^O---_F----_S---_W--- 


1
–
D_{---}

D
+
D_---

1
–
D
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 (V_{PP_MAX}), the minimum input capacitor (C_{IN_MIN}) value is:
Equation 27
C_{IN_MIN} = -₂----_---_V-----_P---_P----_{_}--^I_M--^O--_A---_X-----_---_F----_S---_W---
Typically, C_IN is dimensioned to keep the maximum peak-to-peak voltage ripple in the order
of 1% of V_INMAX.
DocID026968 Rev 2
19/33
33