ISL6336DIRZ datasheet(13/30 Pages) RENESAS

13 / 30 page

ISL6336D

FN8320 Rev 0.00

Page 13 of 30

Oct 6, 2014

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.

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.

In Equation 1, VIN and VOUT are the input and output voltages

respectively, L is the single-channel inductor value, and fSW is the

switching frequency.

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 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.

FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS FOR

3-PHASE CONVERTER

1µs/DIV

PWM2, 5V/DIV

PWM3, 5V/DIV

IL2, 7A/DIV

IL3, 7A/DIV

IL1 + IL2 + IL3, 7A/DIV

IL1, 7A/DIV

PWM1, 5V/DIV

IPP

VIN VOUT

–

 V

OUT

LfSW VIN

------------------------------------------------------

(EQ. 1)

FIGURE 2. CHANNEL INPUT CURRENTS AND INPUT-CAPACITOR

RMS CURRENT FOR 3-PHASE CONVERTER

CHANNEL 3

INPUT CURRENT

10A/DIV

CHANNEL 2

INPUT CURRENT

10A/DIV

CHANNEL 1

INPUT CURRENT

10A/DIV

INPUT-CAPACITOR CURRENT, 10A/DIV

1µs/DIV

ICPP

VIN NVOUT

–

 V

OUT

LfSW VIN

------------------------------------------------------------

(EQ. 2)

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