LINEAR High Efficiency, High Density, PolyPhase Converters for High Current Applications AN77 Data Sheet User Manual

Application Note 77 September 1999

High Efficiency, High Density, Poly Phase Converters for High Current Applications Wei Chen

INTRODUCTION low impedance gate drives, current-sharing, overvoltage

protection, optional overcurrent latch-off and foldback

As logic systems get larger and more complex, their

current limit. Additionally, the LTC1629 can be configured

supply current requirements continue to rise. Systems

for 2-, 3-, 4-, 6- and 12-phase operation with a simple

requiring 100A are fairly common. A high current power

phase selection signal (high, low or open). Optimizing the

supply to meet such requirements usually requires paral-

number of phases can help achieve the smallest and the

leling several power regulators to alleviate the thermal

most cost-effective power supply design.

stress on the individual power components. A power supply designer is left with the choice of how to drive these This application note analyzes the performance of paralleled regulators: brute-force single-phase or smart Poly Phase converters and provides guidelines for select- Poly Phase TM. ing the phase number and designing a Poly Phase con-

verter using the LTC1629. The following questions will be

A Poly Phase converter interleaves the clock signals of the

answered as the discussion goes on:

paralleled power stages, reducing input and output ripple current without increasing the switching frequency. The How much do I gain by using a Poly Phase architecture? decreased power loss from the ESR of the input capacitor

How many phases do I need for my application?

and the low switching losses associated with MOSFETs at

relatively low switching frequencies help achieve high How do I design a Poly Phase converter?

power conversion efficiency. The size and cost of the input capacitors are also greatly reduced as a result of input HOW DO POLYPHASE TECHNIQUES EFFECT CIRCUIT ripple current cancellation. Since output ripple current PERFORMANCE? cancellation also occurs, lower value inductors can be

In general, Poly Phase operation improves the large signal

used. This results in improved dynamic response to load

performance of a switched mode power converter, by

transients. A combination of lower current rating and

such means as reducing ripple current and ripple voltage.

decreased inductance also allows the use of smaller-

A synchronous buck converter is used as an example in

sized, low profile, surface mounted inductors. For

this application note to analyze the effects of Poly Phase

multioutput applications, Poly Phase converters may also

techniques on circuit performance.

provide the benefit of smaller input capacitors.

High current outputs usually require paralleling several

Previously, the implementation of multiphase designs

regulators. The single-regulator approach is not feasible

was difficult and expensive because of complex timing and

because of the unacceptable thermal stress on the indi-

current-sharing requirements. The newly developed

vidual power components. Paralleled regulators are syn-

LTC1629 solves these problems for high current, single

chronized to have the same switching frequency to eliminate

output designs, while the LTC1628 addresses dual-output

beat frequency noise at both the input and output termi-

applications. Both ICs are dual, current mode, Poly Phase

nals. Based on the phase relationship between the paral-

controllers that can drive two synchronous buck stages simultaneously. The features of the LTC1629 include a , LTC and LT are registered trademarks of Linear Technology Corporation.

Poly Phase is a trademark of Linear Technology Corporation.

unity-gain differential amplifier for true remote sensing,

AN77-1