INTRODUCTION

The L4973 family is a 3.5A monolithic step-down dc-dc converter, available in POWERDIP18( 12+3+3)

and SO20L (12+4+4) plastic packages. The operating input supply voltage range is from 8V to 55V, and

the output ranges from 3.3V (L4973D3.3) and 5.1V(L4973D5.1) to 40V. Other regulated outputs below

3.3V are also possible (See Application Note AN938).

Using two L4973D is possible to deliver up to 7A with a good sharing between the two sections or a re-

dundant 3.5A. The two devices work at a switching frequency of 200kHz. At Vcc = 24V, Vo = 5.1V at 7A

the efficiency is 87%. At 3.5A output, the efficiency is 90%.

Current Sharing Operating Principle

The current sharing configuration, shown in fig. 1, is based upon two L497x devices U1 and U2. Any de-

vice in the L497x family can be used for this purpose.

The U1 regulator acts as a master which regulates the output voltage.

The second section U2 works as a current follower. Its task is to deliver an output current equal to the

May 1999

®

AN1126

APPLICATION NOTE

CURRENT SHARING OF THE L4973

IN A MULTIPHASE APPLICATION

by Domenico Arrigo & Giuseppe Gattavari

Electrical Specifications

Input Voltage range

8V-30V

Output Voltage

5.1V

±3% (Line, Load and Temperature)

Output Voltage Ripple

47mV (0.92%/Vo)

Output Current range

0 to 7A

Max Output Ripple current

15%

Min Iomax Current limit

8A

Switching frequency

200kHz

Rint

Cint

I +

Vout

L

Cout

OUT

U1

L497x

COMP

GND

OP-AMP

+

-

COMP

OUT

FB

U2

L497x

GND

Rs

Rs

L

I -

FB

Vcc

Vcc

Vcc

Figure 1. Current Sharing Operating Principle

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