ISL6556A Intersil Corporation, ISL6556A Datasheet - Page 9

• Manufacturer Part Number: ISL6556A
• Description: Optimized Multi-Phase PWM Controller with 6-Bit DAC for VR10.X Application
• Manufacturer: Intersil Corporation

Operation
Multi-Phase Power Conversion
Microprocessor load current profiles have changed to the
point that the advantages of multi-phase 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 multi-phase. The
ISL6556A controller helps simplifying the implementation by
integrating vital functions and requiring minimal output
components. The block diagrams on pages 2 and 3 provide
top level views of multi-phase power conversion using the
ISL65556ACB and ISL6556ACR controllers.
Interleaving
The switching of each channel in a multi-phase 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 currents is reduced
in proportion to the number of phases (Equations 1 and 2).
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 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 terminated 1/3 of a cycle after the PWM pulse of the
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
PWM1, 5V/DIV
IL1 + IL2 + IL3, 7A/DIV
FOR 3-PHASE CONVERTER
IL1, 7A/DIV
PWM3, 5V/DIV
1 s/div
9
IL3, 7A/DIV
PWM2, 5V/DIV
IL2, 7A/DIV
ISL6556A
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 multi-phase 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 multi-phase 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.
Another benefit of interleaving is to reduce input ripple
current. Input capacitance is determined in part by the
maximum input ripple current. Multi-phase 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 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.
I
I
C PP
PP
,
=
S
=
-------------------------------------------------- -
is the switching frequency.
V
-------------------------------------------------------- -
I N
V
I N
-V
L f
-N V
O U T
S
L f
V
IN
S
I N
OUT
V
V
and V
IN
OUT
V
OUT
OUT
are the input and output
(EQ. 2)
(EQ. 1)