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/
s
Vbias
Bias Voltage
I
12
= 45mA
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/
s
- Temperature
5
%
Efficiency
Vo = min-max; Io = min-max
70
85
93
%
Output Over Voltage Protection
Vo = min-max
1.18
V
O
Foldback Current
Short circuit condition
12
A
fsw
Switching frequency
300
kHz
1/9
Supply Voltages
The VRM is supplied by the 5V and the 12V
standard outputs available from the conventional
PC power supplies. The 12V is used here to sup-
ply the L4990A and the 5V power bus is used to
supply the output power on the regulated DC out-
put. An input inductor, L3, is necessary to limit the
dI/dt of the input current during load transients
conditions.
This value must not exceed 0.1 A/us to prevent
the intervention of protections on the main power
supply.
Control Inputs
Enable: The module accept an open collector sig-
nal that disable the power supply in the low state.
This function is realized by discharging the soft
start capacitor.
This solution allows the module to restart with
the soft start sequence.
Voltage Identification: The module is able to ac-
cept 4 open collectors input signals coming from
the microprocessor.
Table 1 shows the VID0-3 code. The voltage
identification is implemented with R23-26, R9 and
R14 resistors,
to allow to programm the con-
verter output voltage at a value lower than the
L4990A internal feedback reference, of 2.5V.
The four
voltage identification bit are buffered
with a 3-State buffer, 74HCT125.
Table 1.
VID3
VID2
VID1
VID0
Vo
1
1
1
1
no def
1
1
1
0
2.1
1
1
0
1
2.2
1
1
0
0
2.3
1
0
1
1
2.4
1
0
1
0
2.5
1
0
0
1
2.6
1
0
0
0
2.7
0
1
1
1
2.8
0
1
1
0
2.9
0
1
0
1
3
0
1
0
0
3.1
0
0
1
1
3.2
0
0
1
0
3.3
0
0
0
1
3.4
0
0
0
0
3.5
0 = Processor pin connected to ground
1 = Processor pin Open
Output Control
Power Good Output: A PWRG signal is available.
It's in the Low State when the programmed out-
put voltage is inside the +/-10% of the nominal
voltage, and in the High state outside the men-
tioned voltage range. This function is realized by
using two comparators inside the quad LM339.
Internal Controls
Current limiting: The current limiting intervention
is fixed at 13A to guarantee a good functionality
at 12A continuous output current.
Behind 13A, the controller is going into pulse-by-
pulse constant limiting current. A foldback type
limiting current is also available.
In case a hiccup limiting current is accepted, the
L4990A controller has this function on board.
Over Voltage Protection: a circuit is detecting an
Overvoltage condition
when output voltage is
18% above the nominal output voltage. In this
case, the VRM is disabled.
Connector description
The connector is standard 40 pin, with a current
capability of 3A/pin (type AMPMOD2 part number
532956-7 or equivalent) and the pinout is shown
in Tab2.
Table 2. Connector pinout.
Pin #
ROW A
ROW B
1
Vin (5V)
Vin (5V)
2
Vin (5V)
Vin (5V)
3
Vin (5V)
Vin (5V)
4
Vin (12V)
Vin (12V)
5
Reserved
Reserved
6
Reserved
OUTEN
7
VID0
VID1
8
VID2
VID3
9
Reserved
PWRGD
10
Vo
GND
11
GND
Vo
12
Vo
GND
13
GND
Vo
14
Vo
GND
15
GND
Vo
16
Vo
GND
17
GND
Vo
18
Vo
GND
19
GND
Vo
20
Vo
GND
AN908 APPLICATION NOTE
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0
2
4
6
8
10
12
14
78
80
82
84
86
88
90
92
94
Output Current
%
Vo=3.3V
Vo=2.9V
Figure 2. Efficiency vs. Output Current
Figure 5. Load Transient
Io = 12A to 0.5A
CH1: Vo = 2.9V (50mV/div)
CH2: Io (5A/div)
Time: 5
s/div
Figure 4. Load Transient
Io = 0.5A to 12A
CH1: Vo = 2.9V (50mV/div)
CH2: Io (5A/div)
Time: 5
s/div
Figure 3. Input Current
CH1: Iinput (5V) (1A/div)
CH2: Vo = 2.9V (50mV/div)
Time: 10
s/div
dIi/dt = 0.04A/
s
dIo/dt = 30A/
s
dIo/dt = 30A/
s
Vo = 2.9V
Vo = 2.9V
(A)
AN908 APPLICATION NOTE
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