Datasheet

DocID023654 Rev 2 15/27
ST1S41 Application information
6 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 15
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 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 meet well 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 17
neglecting the small ESR of ceramic capacitors.
Considering η =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 18
I
RMS
I
O
D
2D
2
⋅
η
-----------------–
D
2
η
2
------ -+⋅=
V
PP
I
O
C
IN
F
SW
⋅
----------------------------
1
D
η
----–
⎝⎠
⎛⎞
D
D
η
----
1D–()⋅+⋅ ESR I
O
⋅+⋅=
C
IN
I
O
V
PP
F
SW
⋅
-----------------------------
1
D
η
----–
⎝⎠
⎛⎞
D
D
η
----
1D–()⋅+⋅⋅=
C
IN_MIN
I
O
2V
PP_MAX
F
SW
⋅⋅
-----------------------------------------------------=