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ISL6219A Datasheet(PDF) 7 Page - Intersil Corporation

Part No. ISL6219A
Description  Microprocessor CORE Voltage Regulator Precision Multi-Phase BUCK PWM Controller for Mobile Applications
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home
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ISL6219A Datasheet(HTML) 7 Page - Intersil Corporation

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7
Operation
Multi-Phase Power Conversion
Multi-phase power conversion provides the most cost-
effective power solution when load currents are no longer
easily supported by single-phase converters. Although its
greater complexity presents additional technical challenges,
the multi-phase approach offers cost-saving advantages
with improved response time, superior ripple cancellation,
and excellent thermal distribution.
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 2 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 waveforms 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, VIN and VOUT are the input and output
voltages respectively, L is the single-channel inductor value,
and fS is the switching frequency.
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 Equa-
tion 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 resis-
tance (ESR), and inductor ripple current. Reducing the induc-
tor 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 3 illustrates input
currents from a three-phase converter combining to reduce
the total input ripple current.
The converter depicted in Figure 3 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 down
12V to 1.5V at 36A. The single-phase converter has 11.9A
rms input capacitor current. The single-phase converter
FIGURE 2. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
1
µs/div
PWM2, 5V/DIV
PWM1, 5V/DIV
IL2, 7A/DIV
IL1, 7A/DIV
IL1 + IL2 + IL3, 7A/DIV
IL3, 7A/DIV
PWM3, 5V/DIV
I
LPP
,
V
IN
V
OUT
() V
OUT
Lf
S VIN
------------------------------------------------------
=
(EQ. 1)
I
PP
V
IN
NV
OUT
() V
OUT
Lf
S VIN
------------------------------------------------------------
=
(EQ. 2)
FIGURE 3. CHANNEL INPUT CURRENTS AND INPUT-
CAPACITOR RMS CURRENT FOR 3-PHASE
CONVERTER
Channel 1
input current
10A/DIV
Channel 2
input current
10A/DIV
Channel 3
input current
10A/DIV
Input-capacitor current, 10A/DIV
1
µs/div
ISL6219A


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