ISL6334DIRZ datasheet(26/28 Pages) INTERSIL

26 / 28 page

FN6802.2

August 31, 2010

For a 2-phase design, use Figure 18 to determine the input

capacitor RMS current requirement given the duty cycle,

maximum sustained output current (IO), and the ratio of the

per-phase peak-to-peak inductor current (IL(P-P)) to IO.

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.

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 IL(P-P) to IO of 0.5. The

single phase converter would require 17.3ARMS current

capacity while the two-phase converter would only require

10.9ARMS. 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.

FIGURE 20. NORMALIZED INPUT-CAPACITOR RMS CURRENT

vs DUTY CYCLE FOR 4-PHASE CONVERTER

00.4

1.0

0.2

0.6

0.8

DUTY CYCLE (VO/VIN)

0.3

0.1

0.2

IL(P-P) = 0

IL(P-P) = 0.25 IO

IL(P-P) = 0.5 IO

IL(P-P) = 0.75 IO

FIGURE 21. NORMALIZED INPUT-CAPACITOR RMS

CURRENT vs DUTY CYCLE FOR SINGLE-PHASE

CONVERTER

00.4

1.0

0.2

0.6

0.8

DUTY CYCLE (VO/VIN)

0.6

0.2

0.4

IL(P-P) = 0

IL(P-P) = 0.5 IO

IL(P-P) = 0.75 IO

ISL6334D

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