isl6307b Intersil Corporation, isl6307b Datasheet - Page 31

• Manufacturer Part Number: isl6307b
• Description: 6-phase Vr11 Pwm Controller With 8-bit Vid Code Capable Of Precision Rds On Or Dcr Differential Current Sensing For Applications In Which Supply Voltage Is Higher Than 5v
• Manufacturer: Intersil Corporation

For a two phase design, use Figure 22 to determine the
input-capacitor RMS current requirement given the duty
cycle, maximum sustained output current (I
of the per-phase peak-to-peak inductor current (I
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 1.25 times greater
than the maximum input voltage.
Figures 23 and 24 provide the same input RMS current
information for three and four phase designs respectively.
Use the same approach to selecting the bulk capacitor type
and number as described above.
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 turning
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.
FIGURE 23. NORMALIZED INPUT-CAPACITOR RMS CURRENT
0.3
0.2
0.1
0
0
I
I
L,P-P
L,P-P
= 0
= 0.25 I
vs DUTY CYCLE FOR 3-PHASE CONVERTER
0.2
O
DUTY CYCLE (V
0.4
31
I
I
L,P-P
L,P-P
= 0.5 I
= 0.75 I
0.6
O/
V
IN
O
O
)
O
), and the ratio
0.8
L,P-P
) to I
1.0
O
ISL6307B
.
MULTIPHASE RMS IMPROVEMENT
Figure 25 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 two-phase
converter versus that of a single phase. Assume both
converters have a duty cycle of 0.25, maximum sustained
output current of 40A, and a ratio of I
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.
FIGURE 25. NORMALIZED INPUT-CAPACITOR RMS
FIGURE 24. NORMALIZED INPUT-CAPACITOR RMS CURRENT
0.3
0.2
0.1
0.6
0.4
0.2
0
0
0
0
I
I
L,P-P
L,P-P
= 0
= 0.25 I
vs DUTY CYCLE FOR 4-PHASE CONVERTER
I
I
I
CURRENT vs DUTY CYCLE FOR SINGLE-PHASE
CONVERTER
L,P-P
L,P-P
L,P-P
0.2
0.2
O
= 0
= 0.5 I
= 0.75 I
DUTY CYCLE (V
DUTY CYCLE (V
O
O
0.4
0.4
I
I
L,P-P
L,P-P
= 0.5 I
= 0.75 I
0.6
0.6
O/
O/
L,P-P
V
V
IN
IN
O
O
)
)
to I
O
0.8
0.8
of 0.5. The
March 9, 2006
FN9225.0
1.0
1.0