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ISL6244

Multi-Phase PWM Controller

Intersil 

ISL6244

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.

IL1 + IL2 + IL3, 7A/DIV

IL3, 7A/DIV

PWM3, 5V/DIV

IL2, 7A/DIV

IL1, 7A/DIV

PWM2, 5V/DIV

PWM1, 5V/DIV

1µs/DIV

FIGURE 13. PWM AND INDUCTOR-CURRENT WAVEFORMS

FOR 3-PHASE CONVERTER

Figure 13 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, or 1.33µs for fS = 250kHz,

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.

IPP =

(---V----I--N-----–-----V----O-----U----T---)----V----O----U-----T-

L fS VIN

(EQ. 1)

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

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.

IC, PP=

(---V----I--N-----–-----N------V----O-----U----T---)----V----O----U-----T-

L fS VIN

(EQ. 2)

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 14 illustrates input

currents from a three-phase converter combining to reduce

the total input ripple current.

INPUT-CAPACITOR CURRENT, 15A/DIV

CHANNEL 3

INPUT CURRENT

15A/DIV

CHANNEL 2

INPUT CURRENT

15A/DIV

CHANNEL 1

INPUT CURRENT

15A/DIV

1µs/DIV

FIGURE 14. CHANNEL INPUT CURRENTS AND INPUT-

CAPACITOR RMS CURRENT FOR 3-PHASE

CONVERTER

The converter depicted in Figure 14 delivers 52A to a 1.20V

load from a 19V input. The RMS input capacitor current is

6.5A. Compare this to a single-phase converter also

stepping down 19V to 1.20V at 52A. The single-phase

converter has 11.96A 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 28, 29 and 30 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 31 shows the single

phase input-capacitor RMS current for comparison.

10

FN9106.3

December 28, 2004

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