ISL6327A Datasheet, PDF (10/29 Pages) Intersil Corporation – Enhanced 6-Phase PWM Controller with 8-Bit VID Code and Differential Inductor DCR or Resistor Current Sensing

ISL6327A
Operation
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 which is
both cost-effective and thermally viable, have forced a
change to the cost-saving approach of multiphase. The
ISL6327A controller helps reduce the complexity of
implementation by integrating vital functions and requiring
minimal output components. The block diagrams on page 3,
page 4 and page 5 provide top level views of multiphase
power conversion using the ISL6327A 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 three-phase
converter has a combined ripple frequency three times
greater than the ripple frequency of any one phase. In
addition, the peak-to-peak amplitude of the combined
inductor current is reduced in proportion to the number of
phases (Equations 1 and 2). The increased ripple frequency
and the 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 three channel currents (IL1, IL2, and IL3)
combine to form the AC ripple current and the DC load
current. The ripple component has three times the ripple
frequency of each individual channel current. Each PWM
pulse is triggered 1/3 of a cycle after the start of the PWM
pulse of the previous phase. The DC components of the
inductor currents combine to feed the load.
IL1 + IL2 + IL3, 7A/DIV
IL3, 7A/DIV
PWM3, 5V/DIV
IL2, 7A/DIV
IL1, 7A/DIV
PWM2, 5V/DIV
PWM1, 5V/DIV
1µs/DIV
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
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.
I_{PP =}
(_{---V----I--N-----}–_{-----V----O----U-----T---})_{----V----O----U-----T-}
L f_S V_IN
(EQ. 1)
In Equation 1, V_IN and V_OUT are the input and the output
voltages respectively, L is the single-channel inductor value,
and f_S is the switching frequency.
INPUT-CAPACITOR CURRENT 10A/DIV
CHANNEL 3
INPUT CURRENT
10A/DIV
CHANNEL 2
INPUT CURRENT
10A/DIV
CHANNEL 1
INPUT CURRENT
10A/DIV
1µs/DIV
FIGURE 2. CHANNEL INPUT CURRENTS AND
INPUT-CAPACITOR RMS CURRENT FOR
3-PHASE CONVERTER
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
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.
I_{C(P – P)}=
(_{---V----I--N-----}–_{-----N------V----O-----U----T---})_{----V----O----U-----T-}
L f_S V_IN
(EQ. 2)
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
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FN6833.0
February 17, 2009