ST1S14 STMicroelectronics, ST1S14 Datasheet - Page 20

• Manufacturer Part Number: ST1S14
• Description: Up to 3 A step down switching regulator
• Manufacturer: STMicroelectronics

Application information
7
7.1
7.1.1
20/42
Application information
Component selection
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
has to 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
current. Considering η = 1 this function reaches its maximum value at D = 0.5 and the
equivalent RMS current is equal to I
are:
Equation 18
and
Equation 19
Where V
internal PDMOS. Considering the range D
IRMS going through the input capacitor. Capacitors that can be considered are:
Electrolytic capacitors:
These are widely used due to their low price 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.
F
is the free wheeling diode forward voltage and V
Doc ID 17977 Rev 1
I
RMS
D
D
MAX
O
MIN
divided by 2. The maximum and minimum duty cycles
=
=
I
=
O
------------------------------------ -
V
MIN
⋅
------------------------------------- -
V
INMIN
V
INMAX
V
D 2 D
OUT
to D
OUT
–
-------------- -
–
+
+
MAX
⋅
–
η
V
V
V
SW
V
F
2
F
SW
, it is possible to determine the max
+
D
------ -
η
2
2
SW
the voltage drop across the
O
is the output DC
ST1S14