ISL6334D_09 Datasheet, PDF (26/28 Pages) Intersil Corporation – VR11.1, 4-Phase PWM Controller with Phase Dropping, Droop Disabled and Load Current Monitoring Features

0.3 _{IL(P-P) = 0}
I_L(P-P) = 0.25 I_O
0.2
I_L(P-P) = 0.5 I_O
I_L(P-P) = 0.75 I_O
ISL6334D
0.6
0.4
0.1
0
0
0.2
0.4
0.6
0.8
1.0
DUTY CYCLE (V_O/V_IN)
FIGURE 20. NORMALIZED INPUT-CAPACITOR RMS CURRENT
vs DUTY CYCLE FOR 4-PHASE CONVERTER
For a 2-phase design, use Figure 18 to determine the input
capacitor RMS current requirement given the duty cycle,
maximum sustained output current (I_O), and the ratio of the
per-phase 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 19 and 20 provide the same input RMS current
information for 3- and 4-phase designs respectively. Use the
same approach to selecting the bulk capacitor type and
number, as previously described.
Low capacitance, high-frequency ceramic capacitors are
needed in addition to the bulk capacitors to suppress leading
and falling edge voltage spikes. The result from the high
current slew rates produced by the upper MOSFETs turn on
and off. Select low ESL ceramic capacitors and place one as
close as possible to each upper MOSFET drain to minimize
board parasitic impedances and maximize suppression.
0.2
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_O/V_IN)
FIGURE 21. NORMALIZED INPUT-CAPACITOR RMS
CURRENT vs DUTY CYCLE FOR SINGLE-PHASE
CONVERTER
MULTIPHASE RMS IMPROVEMENT
Figure 21 is provided as a reference to demonstrate the
dramatic reductions in input-capacitor RMS current upon the
implementation of the multiphase topology. For example,
compare the input RMS current requirements of a 2-phase
converter versus that of a single phase. Assume both
converters have a duty cycle of 0.25, a maximum sustained
output current of 40A, and a ratio of I_L(P-P) to I_O of 0.5. The
single phase converter would require 17.3A_RMS current
capacity while the two-phase converter would only require
10.9A_RMS. The advantages become even more pronounced
when output current is increased and additional phases are
added to keep the component cost down relative to the
single phase approach.
Layout Considerations
The following layout strategies are intended to minimize the
impact of board parasitic impedances on converter
performance and to optimize the heat-dissipating capabilities
of the printed-circuit board. These sections highlight some
important practices which should not be overlooked during the
layout process.
26
FN6802.1
May 28, 2009