AN908 Datasheet, PDF (1/9 Pages) STMicroelectronics – APPLICATION NOTE

AN908
®
APPLICATION NOTE
HIGH PERFORMANCE VRM USING L4990A,
FOR Pentium Pro® PROCESSOR
by Nicola Tricomi
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.
ELECTRICAL SPECIFICATION
Input Characteristics
Symbol
Vi
di/dt
Vbias
Parameter
Input Voltage
Slew Rate
Bias Voltage
Test Condition
Load Transient
I₁₂ = 45mA
Min.
4.75
11.4
Typ.
5
12
Max.
5.25
0.04
14
Unit
V
A/µs
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
Foldback Current
Short circuit condition
1.18 ⋅ V_O
12
A
fsw
Switching frequency
300
kHz
July 1999
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