SYSTEM DESCRIPTION

The actual generation of Pentium Pro

® microprocessors, requires a power supply with a programmable

output voltage that decreases with the increasing of the processor internal clock. At the same time

higher output currents are required and superior load transient performance has to be supplied due to

very high dI/dt requests.

The VRM (Voltage Regulator Module) proposed (Fig 1) is a step down converter based on the L4990A,

very fast current mode controller. A complete system solution is given and described. A connector with a

standard VRM pinout is used, with 4 bit for Voltage Identify system, Power Good, Enable and constant

limiting current functions are available. Output Over Voltage Protection is also available.

July 1999

®

AN908

APPLICATION NOTE

HIGH PERFORMANCE VRM USING L4990A,

FOR Pentium Pro

® PROCESSOR

by Nicola Tricomi

ELECTRICAL SPECIFICATION

Input Characteristics

Symbol

Parameter

Test Condition

Min.

Typ.

Max.

Unit

Vi

Input Voltage

4.75

5

5.25

V

di/dt

Slew Rate

Load Transient

0.04

A/

µs

Vbias

Bias Voltage

11.4

12

14

V

Output Characteristics

Symbol

Parameter

Test Condition

Min.

Typ.

Max.

Unit

Output Voltage range

By 4 bits VID

2.1

3.5

V

Vonom

Nominal Output Voltage

2.9

V

Io

Output Current

0.4

12.5

A

Peak Current

Pulse by pulse mode

13

A

Regulation

Including variation due to:

- Io = min-max

- Vo imitial set point accuracy

- Line regulation

- Output ripple and noise

- Load transient dIo/dt = 30A/

µs

- Temperature

±5%

η

Efficiency

Vo = min-max; Io = min-max

70

85

93

%

Output Over Voltage Protection

Vo = min-max

1.18

Foldback Current

Short circuit condition

12

A

fsw

Switching frequency

300

kHz

1/9