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ISL6308

Three-Phase Buck PWM Controller with High Current Integrated MOSFET Drivers

Renesas Electronics 

ISL6308

PHASE1, PHASE2, and PHASE3 (Pins 29, 28, 22)

Connect these pins to the sources of the upper MOSFETs.

These pins are the return path for the upper MOSFETs’

drives.

LGATE1, LGATE2, and LGATE3 (Pins 34, 23, 17)

These pins are used to control the lower MOSFETs and are

monitored for shoot-through prevention purposes. Connect

these pins to the lower MOSFETs’ gates. Do not use external

series gate resistors as this might lead to shoot-through.

PGOOD (Pin 35)

PGOOD is used as an indication of the end of soft-start. It is

an open-drain logic output that is low impedance until the

soft-start is completed and VOUT is equal to the VID setting.

Once in normal operation, PGOOD indicates whether the

output voltage is within specified overvoltage and

undervoltage limits. If the output voltage exceeds these limits

or a reset event occurs (such as an overcurrent event),

PGOOD becomes high impedance again. The potential at

this pin should not exceed that of the potential at VCC pin by

more than a typical forward diode drop at any time.

OVP (Pin 38)

Overvoltage protection pin. This pin pulls to VCC when an

overvoltage condition is detected. Connect this pin to the

gate of an SCR or MOSFET tied across VIN and ground to

prevent damage to a load device.

Operation

Multi-Phase Power Conversion

Modern low voltage DC/DC converter load current profiles

have changed to the point that the advantages of multi-phase

power conversion are impossible to ignore. The technical

challenges associated with producing a single-phase

converter that is both cost-effective and thermally viable have

forced a change to the cost-saving approach of multi-phase.

The ISL6308 controller helps simplify implementation by

integrating vital functions and requiring minimal output

components. The “Block Diagram” on page 3 provides a top

level view of multi-phase power conversion using the ISL6308

controller.

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 one third cycle after the previous channel and one

third 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

FN9208 Rev 4.00

Sep 30, 2008

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 three times the ripple

frequency of each individual channel current. Each PWM

pulse is terminated one third of a cycle after the PWM pulse of

the previous phase. The peak-to-peak current for each phase

is about 7A, and the DC components of the inductor currents

combine to feed the load.

IL1 + IL2 + IL3, 7A/DIV

IL3, 7A/DIV

PWM3, 5V/DIV

IL2, 7A/DIV

PWM2, 5V/DIV

IL1, 7A/DIV

PWM1, 5V/DIV

FIGURE 1. PWM AND INDUCTOR-CURRENT WAVEFORMS

FOR 3-PHASE CONVERTER

To understand the reduction of ripple current amplitude in the

multi-phase circuit, examine Equation 1, which represents

an individual channel peak-to-peak inductor current.

IPP = ---V----I--N---L--–-----V-F---O-S---W-U----T------V----I-V-N---O----U-----T-

(EQ. 1)

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 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-----–---L--N------F----SV----WO-----U----T-V----I--N----V----O----U-----T-

(EQ. 2)

Page 9 of 28

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