ISL6219A_07 Datasheet, PDF (7/17 Pages) Intersil Corporation – Microprocessor CORE Voltage Regulator Precision Multi-Phase BUCK PWM Controller for Mobile Applications

ISL6219A
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.
IL1 + IL2 + IL3, 7A/DIV
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.
I_{L, 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 output
voltages respectively, L is the single-channel inductor value,
and f_S is the switching frequency.
I_PP=
(_---V_{----I--N-----–-----}N_------V_{----O-----U----T---})_----V_{----O----U-----T-}
L f_S V_IN
(EQ. 2)
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 3 illustrates input
currents from a three-phase converter combining to reduce
the total input ripple current.
Input-capacitor current, 10A/DIV
IL3, 7A/DIV
PWM3, 5V/DIV
IL2, 7A/DIV
IL1, 7A/DIV
PWM2, 5V/DIV
PWM1, 5V/DIV
1μs/div
FIGURE 2. PWM AND INDUCTOR-CURRENT WAVEFORMS
FOR 3-PHASE CONVERTER
Channel 3
input current
10A/DIV
Channel 2
input current
10A/DIV
Channel 1
input current
10A/DIV
1μs/div
FIGURE 3. CHANNEL INPUT CURRENTS AND INPUT-
CAPACITOR RMS CURRENT FOR 3-PHASE
7
FN9093.1
March 20, 2007