Datasheet

V
OUT
(V
PVIN
- V
OUT
)
V
PVIN
I
RMS_CIN
= I
OUT
(A)
VOUT 1V/DIV
EN 1V/DIV
PGOOD 1V/DIV
2 ms/DIV
IOUT 1A/DIV
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Startup, Pre-Biased Load Switching Waveform at No Load (DCM Mode)
Switching Waveform With 5A Load
9 Component Selection
This section provides a simplified design procedure necessary to select the external components to build
a fully functional efficient step-down power supply. As with any DC-DC converter, numerous tradeoffs are
possible to optimize the design for efficiency, size, and performance. Unless otherwise indicated, all
formulae assume units of Amps (A) for current, Farads (F) for capacitance, Henries (H) for inductance,
Volts (V) for voltages and Hertz (Hz) for frequencies. For more details, please refer to the LM21305
datasheet.
9.1 Input Capacitors
PVIN is the supply voltage for the switcher power stage. It is the supply that delivers the output power.
The input capacitors supply the large AC switching current drawn by the switching action of the internal
MOSFETs. The input current of a buck converter is discontinuous, so the ripple current supplied by the
input capacitor is large. The input capacitor must be rated to handle this current. To prevent large voltage
transients, a low ESR input capacitor sized for the maximum RMS current should be used. The maximum
RMS current is given by:
(1)
The power dissipated in the input capacitor is given by:
P
D_CIN
= I
RMS
2
R
ESR_CIN
7
SNVA432C–March 2010–Revised May 2013 AN-2042 LM21305 Evaluation Board
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