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FN9223.1
July 28, 2008
Input Capacitor Selection
The important parameters for the bulk input capacitors are
the voltage rating and the RMS current rating. For reliable
operation, select bulk input capacitors with voltage and
current ratings above the maximum input voltage and
largest RMS current required by the circuit. The capacitor
voltage rating should be at least 1.25x greater than the
maximum input voltage. The input RMS current required for
a multiphase converter can be approximated with the aid of
Figure 12.
As the input capacitors are responsible for sourcing the AC
component of the input current flowing into the upper
MOSFETs, their RMS current capacity must be sufficient to
handle the AC component of the current drawn by the upper
MOSFETs. Figure 12 can be used to determine the
input-capacitor RMS current function of duty cycle,
maximum sustained output current (I
O
), and the ratio of the
peak-to-peak inductor current (I
L,P-P
) to the maximum
sustained load current, I
O
. Figure 12 can also be used as a
reference demonstrating the dramatic reduction in input
capacitor RMS current in a 2-phase DC/DC converter, as
compared to a single-phase regulator.
Use a mix of input bypass capacitors to control the input
voltage ripple. Use ceramic capacitance for the high
frequency decoupling and bulk capacitors to supply the
RMS current. Minimize the connection path inductance of
the high frequency decoupling ceramic capacitors (from
drain of upper MOSFET to source of lower MOSFET).
For bulk capacitance, several electrolytic or high-capacity MLC
capacitors may be needed. For surface mount designs, solid
tantalum capacitors can be used, but caution must be
exercised with regard to the capacitor surge current rating.
These capacitors must be capable of handling the
surge-current at power-up.
0.3
0.1
0
0.2
INPUT CAPACITOR CURRENT (I
RMS
/ I
O
)
FIGURE 12. NORMALIZED INPUT RMS CURRENT vs DUTY
CYCLE FOR A 2-PHASE CONVERTER
00.2 0.50.1 0.3 0.4
DUTY CYCLE (V
O
/V
IN
)
I
L,P-P
= 0
I
L,P-P
= 0.5 x I
O
I
L,P-P
= 0.75 x I
O
ISL8101
