ISL6323EVAL1Z Intersil, ISL6323EVAL1Z Datasheet - Page 11

• Manufacturer Part Number: ISL6323EVAL1Z
• Description: EVAL BOARD 1 FOR ISL6323
• Manufacturer: Intersil

Specifications of ISL6323EVAL1Z

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

shoot-through protection circuitry to determine when the
lower MOSFET has turned off.
Operation
The ISL6323 utilizes a multiphase architecture to provide a
low cost, space saving power conversion solution for the
processor core voltage. The controller also implements a
simple single phase architecture to provide the Northbridge
voltage on the same chip.
Multiphase Power Conversion
Microprocessor load current profiles have changed to the
point that the advantages of multiphase power conversion
are impossible to ignore. The technical challenges
associated with producing a single-phase converter that is
both cost-effective and thermally viable have forced a
change to the cost-saving approach of multiphase. The
ISL6323 controller helps simplify implementation by
integrating vital functions and requiring minimal external
components. The “Controller Block Diagram” on page 3
provides a top level view of the multiphase power conversion
using the ISL6323 controller.
Interleaving
The switching of each channel in a multiphase converter is
timed to be symmetrically out-of-phase with each of the other
channels. In a 3-phase converter, each channel switches 1/3
cycle after the previous channel and 1/3 cycle before the
following channel. As a result, the 3-phase converter has a
combined ripple frequency 3x greater than the ripple frequency
of any one phase. In addition, the peak-to-peak amplitude of
the combined inductor currents is reduced in proportion to the
number of phases (Equations 2 and 3). Increased ripple
frequency and lower ripple amplitude mean that the designer
can use less per-channel inductance and lower total output
capacitance for any performance specification.
Figure 1 illustrates the multiplicative effect on output ripple
frequency. The 3-channel currents (IL1, IL2, and IL3)
combine to form the AC ripple current and the DC load
current. The ripple component has 3x the ripple frequency of
each individual channel current. Each PWM pulse is
terminated 1/3 of a cycle after the PWM pulse of the previous
phase. The peak-to-peak current for each phase is about 7A,
and the DC components of the inductor currents combine to
feed the load.
To understand the reduction of ripple current amplitude in the
multiphase circuit, examine the equation representing an
individual channel peak-to-peak inductor current.
In Equation 2, V
voltages respectively, L is the single-channel inductor value,
and f
I
P P
–
S
=
is the switching frequency.
(
----------------------------------------------------- -
V
IN
–
L f
V
OUT
S
IN
V
IN
and V
) V
OUT
OUT
11
are the input and output
(EQ. 2)
ISL6323
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 2 to the
expression for the peak-to-peak current after the summation
of N symmetrically phase-shifted inductor currents in
Equation 3. Peak-to-peak ripple current decreases by an
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 input ripple
current. Input capacitance is determined in part by the
maximum input ripple current. Multiphase topologies can
improve overall system cost and size by lowering input ripple
current and allowing the designer to reduce the cost of input
capacitance. The example in Figure 2 illustrates input
currents from a 3-phase converter combining to reduce the
total input ripple current.
The converter depicted in Figure 2 delivers 1.5V to a 36A 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
must use an input capacitor bank with twice the RMS current
capacity as the equivalent 3-phase converter.
Figures 25, 26 and 27 in the section entitled “Input Capacitor
Selection” on page 31 can be used to determine the input
capacitor RMS current based on load current, duty cycle,
and the number of channels. They are provided as aids in
determining the optimal input capacitor solution.
I
C P-P
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
(
RMS
)
=
(
----------------------------------------------------------- -
PWM1, 5V/DIV
V
input capacitor current. The single-phase converter
IN
IL1 + IL2 + IL3, 7A/DIV
FOR 3-PHASE CONVERTER
–
N V
L f
IL1, 7A/DIV
S
OUT
V
IN
) V
PWM3, 5V/DIV
OUT
1µs/DIV
IL3, 7A/DIV
PWM2, 5V/DIV
IL2, 7A/DIV
October 21, 2008
(EQ. 3)
FN9278.4