ISL6336D Datasheet, PDF (13/30 Pages) Intersil Corporation – VR11.1, 6-Phase PWM Controller with Phase Dropping,Droop Disabled and Load Current Monitoring Features

ISL6336D
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 are both cost-effective
and thermally viable), have forced a change to the cost-saving
approach of multiphase. The ISL6336D controller helps reduce
the complexity of implementation by integrating vital functions
and requiring minimal output components. The block diagrams
on pages 7, 8, and 9 provide top level views of multiphase power
conversion using the ISL6336D 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 3-phase converter has a combined ripple frequency
3x 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 3x 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 DC components of
the inductor currents combine to feed the load.
IL1 + IL2 + IL3, 7A/DIV
IL1, 7A/DIV
PWM1, 5V/DIV
IL2, 7A/DIV
IL3, 7A/DIV
PWM2, 5V/DIV
PWM3, 5V/DIV
1µs/DIV
FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS FOR
3-PHASE CONVERTER
To understand the reduction of ripple current amplitude in the
multiphase circuit, examine Equation 1, which represents an
individual channel’s peak-to-peak inductor current.
I_{PP =}
_{---V----I--N-----}–_{-----V----O-----U----T---}_{----V----O----U-----T-}
L f_SW 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_SW is the
switching frequency.
INPUT-CAPACITOR CURRENT, 10A/DIV
CHANNEL 1
INPUT CURRENT
10A/DIV
CHANNEL 2
INPUT CURRENT
10A/DIV
CHANNEL 3
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, PP}=
_{---V----I--N-----}–_{-----N------V----O-----U----T---}_{----V----O----U-----T-}
L f_SW V_IN
(EQ. 2)
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 3-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 11.9ARMS
input capacitor current. The single-phase converter must use an
input capacitor bank with twice the RMS current capacity as the
equivalent 3-phase converter.
Figures 23, 24 and 25 in the section entitled “Input Capacitor
Selection” on page 27 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 26 shows the single
phase input-capacitor RMS current for comparison.
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FN8320.0
October 6, 2014