isl6326b Intersil Corporation, isl6326b Datasheet - Page 28

• Manufacturer Part Number: isl6326b
• Description: 4-phase Pwm Controller With 8-bit Dac Code Capable Of Precision Dcr Differential Current Sensing
• Manufacturer: Intersil Corporation

voltage ripple as outlined in Output Filter Design. Choose the
lowest switching frequency that allows the regulator to meet
the transient-response requirements.
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.
For a two phase design, use Figure 17 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
FIGURE 17. NORMALIZED INPUT-CAPACITOR RMS CURRENT
FIGURE 18. NORMALIZED INPUT-CAPACITOR RMS CURRENT
0.3
0.2
0.1
0.3
0.2
0.1
0
0
0
0
I
I
L,PP
L,PP
I
I
I
L,PP
L,PP
L,PP
= 0
= 0.25 I
= 0
= 0.5 I
= 0.75 I
vs DUTY CYCLE FOR 2-PHASE CONVERTER
vs DUTY CYCLE FOR 3-PHASE CONVERTER
0.2
0.2
O
O
O
DUTY CYCLE (V
DUTY CYCLE (V
0.4
0.4
28
I
I
L,PP
L,PP
= 0.5 I
= 0.75 I
0.6
0.6
O
O/
/V
V
O
IN
IN
O
)
)
O
), and the ratio
0.8
0.8
L,PP
) to I
1.0
1.0
O
ISL6326B
.
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 18 and 19 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 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 20 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 19. NORMALIZED INPUT-CAPACITOR RMS CURRENT
0.3
0.2
0.1
0
0
I
I
L,PP
L,PP
= 0
= 0.25 I
vs DUTY CYCLE FOR 4-PHASE CONVERTER
0.2
O
DUTY CYCLE (V
0.4
I
I
L,PP
L,PP
= 0.5 I
= 0.75 I
0.6
O/
L,PP
V
O
IN
O
)
to I
O
0.8
of 0.5. The
April 21, 2006
FN9286.0
1.0