isl6307b Intersil Corporation, isl6307b Datasheet - Page 12

• Manufacturer Part Number: isl6307b
• Description: 6-phase Vr11 Pwm Controller With 8-bit Vid Code Capable Of Precision Rds On Or Dcr Differential Current Sensing For Applications In Which Supply Voltage Is Higher Than 5v
• Manufacturer: Intersil Corporation

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 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
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’s peak-to-peak inductor current.
In Equation 1, V
voltages respectively, L is the single-channel inductor value,
and f
I
P-P
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS
=
S
(
----------------------------------------------------- -
is the switching frequency.
V
PWM1, 5V/DIV
IN
–
L f
IL1 + IL2 + IL3, 7A/DIV
V
FOR 3-PHASE CONVERTER
OUT
S
IN
V
IL1, 7A/DIV
IN
) V
and V
OUT
PWM3, 5V/DIV
OUT
12
1µs/DIV
are the input and output
IL3, 7A/DIV
PWM2, 5V/DIV
IL2, 7A/DIV
(EQ. 1)
ISL6307B
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.
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 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 22, 23 and 24 in the section entitled Input Capacitor
Selection 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. Figure 25 shows the single
phase input-capacitor RMS current for comparison.
I
C P-P
,
FIGURE 2. CHANNEL INPUT CURRENTS AND INPUT-
=
(
----------------------------------------------------------- -
INPUT-CAPACITOR CURRENT, 10A/DIV
V
IN
–
CAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
N V
CHANNEL 1
INPUT CURRENT
10A/DIV
L f
S
OUT
V
IN
CHANNEL 2
INPUT CURRENT
10A/DIV
) V
OUT
CHANNEL 3
INPUT CURRENT
10A/DIV
1µs/DIV
March 9, 2006
(EQ. 2)
FN9225.0