Datasheet

Application information ST1CC40
22/37 DocID18279 Rev 5
7.1.3 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
must 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 24
where  is the expected system efficiency, D is the duty cycle and I
O
is the output DC
current. Considering  = 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 25
and
Equation 26
Table 6. Inductor selection
Manufacturer Series Inductor value (µH) Saturation current (A)
Würth Elektronik
WE-HCI 7040 1 to 4.7 20 to 7
WE-HCI 7050 4.9 to 10 20 to 4.0
Coilcraft XPL 7030 2.2 to 10 29 to 7.2
I
RMS
I
O
D
2D
2


-------------- -–
D
2

2
------ -+=
D
MAX
V
OUT
V
F
+
V
INMIN
V
SW
–
------------------------------------ -=
D
MIN
V
OUT
V
F
+
V
INMAX
V
SW
–
--------------------------------------=