ISL6327 Intersil Corporation, ISL6327 Datasheet - Page 11

• Manufacturer Part Number: ISL6327
• Description: Enhanced 6-Phase PWM Controller
• Manufacturer: Intersil Corporation

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pulse of the previous phase. The DC components of the
inductor currents combine to feed the load.
To understand the reduction of the ripple current amplitude in
the multiphase circuit, examine the equation representing an
individual channel’s peak-to-peak inductor current.
In Equation 1, V
voltages respectively, L is the single-channel inductor value,
and f
The output capacitors conduct the ripple component of the
inductor current. In the case of multiphase converters, the
capacitor current is the sum of the ripple currents from each
of the individual channels. Compare Equation 1 to the
expression for the peak-to-peak current after the summation
of N symmetrically phase-shifted inductor currents in
Equation 2. Peak-to-peak ripple current decreases by an
I
PP
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
FIGURE 2. CHANNEL INPUT CURRENTS AND INPUT-
=
S
(
----------------------------------------------------- -
V
is the switching frequency.
IN
INPUT-CAPACITOR CURRENT 10A/DIV
PWM1, 5V/DIV
–
L f
V
IL1 + IL2 + IL3, 7A/DIV
FOR 3-PHASE CONVERTER
CAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
OUT
S
CHANNEL 1
INPUT CURRENT
10A/DIV
V
IN
IL1, 7A/DIV
IN
) V
and V
OUT
CHANNEL 2
INPUT CURRENT
10A/DIV
PWM3, 5V/DIV
OUT
CHANNEL 3
INPUT CURRENT
10A/DIV
1µs/DIV
11
1µs/DIV
are the input and the output
IL3, 7A/DIV
PWM2, 5V/DIV
IL2, 7A/DIV
(EQ. 1)
ISL6327
amount proportional to the number of channels. Output-
voltage ripple is a function of capacitance, capacitor
equivalent series resistance (ESR), and inductor ripple
current. Reducing the inductor ripple current allows the
designer to use fewer or less costly output capacitors.
Another benefit of interleaving is to reduce the input ripple
current. The input capacitance is determined in part by the
maximum input ripple current. Multiphase topologies can
improve the overall system cost and size by lowering the
input ripple current and allowing the designer to reduce the
cost of input capacitance. The example in Figure 2 illustrates
the input currents from a three-phase converter combining to
reduce the total input ripple current.
The converter depicted in Figure 2 delivers 36A to a 1.5V load
from a 12V input. The RMS input capacitor current is 5.9A.
Compare this to a single-phase converter also stepping down
12V to 1.5V at 36A. The single-phase converter has 11.9A
RMS input capacitor current. The single-phase converter
must use an input capacitor bank with twice the RMS current
capacity as the equivalent three-phase converter.
Figures 19, 20 and 21 in the section titled Input Capacitor
Selection can be used to determine the input-capacitor RMS
current based on the load current, the duty cycle, and the
number of channels. They are provided as aids in
determining the optimal input capacitor solution. Figure 22
shows the single phase input-capacitor RMS current for
comparison.
PWM Modulation Scheme
The ISL6327 adopts Intersil's proprietary Active Pulse
Positioning (APP) modulation scheme to improve the
transient performance. APP control is a unique dual-edge
PWM modulation scheme with both PWM leading and
trailing edges being independently moved to provide the
best response to the transient loads. The PWM frequency,
however, is constant and set by the external resistor
between the FS pin and GND.
To further improve the transient response, the ISL6327 also
implements Intersil's proprietary Adaptive Phase Alignment
(APA) technique. APA, with sufficiently large load step
currents, can turn on all phases together.
With both APP and APA control, ISL6327 can achieve
excellent transient performance and reduce the demand on
the output capacitors.
Under the steady state conditions the operation of the
ISL6327 PWM modulator appears to be that of a
conventional trailing edge modulator. Conventional analysis
and design methods can therefore be used for steady state
and small signal operation.
I
C PP
,
=
(
----------------------------------------------------------- -
V
IN
–
N V
L f
S
OUT
V
IN
) V
OUT
June 5, 2006
(EQ. 2)
FN9276.1