Datasheet

Application information ST1S14
22/46 DocID17977 Rev 2
7 Application information
7.1 Component selection
7.1.1 Input capacitor
The input capacitor must be able to support the maximum input operating voltage and the
maximum RMS input current.
Since step-down converters draw current from the input in pulses, the input current is
squared and the height of each pulse is equal to the output current. The input capacitor has
to absorb all this switching current, whose RMS value can be up to the load current divided
by two (worst case, with duty cycle of 50 %). For this reason, the quality of these capacitors
must be very high to minimize the power dissipation generated by the internal ESR, thereby
improving system reliability and efficiency. The critical parameter is usually the RMS current
rating, which must be higher than the RMS current flowing through the capacitor. The
maximum RMS input current (flowing through the input capacitor) is:
Equation 17
where η is the expected system efficiency, D is the duty cycle, and I
O
is the output DC
current. Considering η = 1 this function reaches its maximum value at D = 0.5 and the
equivalent RMS current is equal to I
O
divided by 2. The maximum and minimum duty cycles
are:
Equation 18
and
Equation 19
Where V
F
is the freewheeling diode forward voltage and V
SW
the voltage drop across the
internal PDMOS. Considering the range D
MIN
to D
MAX
, it is possible to determine the
maximum IRMS going through the input capacitor. Capacitors that may be considered are:
Electrolytic capacitors:
These are widely used due to their low cost and their availability in a wide range of
RMS current ratings.
The only drawback is that, considering ripple current rating requirements, they are
physically larger than other capacitors.
I
RMS
I
O
D
2D
2
⋅
η
-----------------–
D
2
η
2
------ -+⋅=
D
MAX
V
OUT
V
F
+
V
INMIN
V
SW
–
------------------------------------ -=
D
MIN
V
OUT
V
F
+
V
INMAX
V
SW
–
--------------------------------------=