ISL6323 Datasheet, PDF (31/34 Pages) Intersil Corporation – Hybrid SVI/PVI

ISL6323
Input Capacitor Selection
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 which is related to duty cycle and the number of
active phases.
0.3
I_L(P-P) = 0
I_L(P-P) = 0.25 I_O
I_L(P-P) = 0.5 I_O
I_L(P-P) = 0.75 I_O
0.3
I_L(P-P) = 0
I_L(P-P) = 0.25 I_O
0.2
0.1
I_L(P-P) = 0.5 I_O
I_L(P-P)= 0.75 I_O
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 25. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 4-PHASE CONVERTER
For a four-phase design, use Figure 25 to determine the
input-capacitor RMS current requirement set by the duty
cycle, maximum sustained output current (I_O), and the ratio
of the peak-to-peak inductor current (I_L(P-P)) to I_O. Select a
bulk capacitor with a ripple current rating which will minimize
the total number of input capacitors required to support the
RMS current calculated.
The voltage rating of the capacitors should also be at least
1.25x greater than the maximum input voltage. Figures 26
and 27 provide the same input RMS current information for
three-phase and two-phase designs respectively. Use the
same approach for selecting the bulk capacitor type and
number.
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_IN/V_O)
FIGURE 26. NORMALIZED INPUT-CAPACITOR RMS
CURRENT FOR 3-PHASE CONVERTER
Low capacitance, high-frequency ceramic capacitors are
needed in addition to the input bulk capacitors to suppress
leading and falling edge voltage spikes. The spikes result from
the high current slew rate produced by the upper MOSFET
turn on and off. Select low ESL ceramic capacitors and place
one as close as possible to each upper MOSFET drain to
minimize board parasitics and maximize suppression.
0.3
0.2
0.1
I_L(P-P) = 0
I_L(P-P) = 0.5 I_O
I_L(P-P) = 0.75 I_O
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_IN/V_O)
FIGURE 27. NORMALIZED INPUT-CAPACITOR RMS
CURRENT FOR 2-PHASE CONVERTER
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31
FN9278.2
April 7, 2008