DN122 - Dual Regulators Power Pentium® Processor or Upgrade CPU

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Dual Regulators Power Pentium Processor or Upgrade CPU
Design Note 122
Craig Varga
Many manufacturers of Pentium processor-based
motherboards have been searching for an economical
solution to the problem of powering the present generation Pentium P54C and accommodating the upgrade
processors that will soon become available. The existing
processor uses a single supply for both the processor
core and the I/O. For the highest frequency offerings,
the supply required is 3.5V ±100mV (VRE specification).
For the lower performance end of the clock frequency
spectrum, a supply voltage of 3.3V ±5% is adequate.
Recently, Intel respecified the standard 3.3V CPUs for
operation at 3.5V. This allows designs for any clock
frequency to be operated from a single 3.5V supply. The
I/O ring and chipset should be powered by the same
voltage as the CPU core, whether that is 3.3V or 3.5V.
The P55C upgrade processor, which will soon be available, requires separate supplies for the core and the I/O.
The nominal core voltage is targeted at 2.500V ±5%,
whereas the I/O supply is still nominally 3.3V. There is
also a processor pin, VCC2DET, at location AL1, that is
bonded to ground on the P55C, but is open on the P54C.
A significant complication is introduced by the core
and I/O power pins of the P54C being shorted together
on-chip. Figure 1 shows the system block diagram. If
the core and I/O supplies don’t deliver proportional currents, damage to the P54 metallization may occur. The
LT®1580/LT1587-based circuit shown in Figure 2 will
automatically supply the required voltages to the CPU
and the I/O circuitry based on the status of the VCC2DET
pin and share the load between the two regulators.
I/O
REG
I/O
LOADS
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R12
0.0075Ω*
5V
+
12V
R1
10k
C1
220μF
10V
C8
0.1μF
U1
LT1006
–
C9
220μF
10V
+
U2
LT1587
INTERNAL
METAL,
P54 ONLY
CPU
DN122 F01
Figure 1. System Configuration
+
ADJ
R3
110Ω
1%
R4
178Ω
1%
C6
0.01μF
C4
0.33μF
5V
D2
1N4148
12V
C11 +
22μF
R13
0.005Ω* 35V
+
I/O
SUPPLY
3.3V
C3
220μF
10V
VOUT
D1
R2
1N4148 470Ω
+
R10
10k
VIN
4
VCONT
VS
1
5
VIN
VOUT
ADJ
CORE
SUPPLY
3.5V/2.5V
C10
1μF
U3
LT1580
3
+
2
C5
0.33μF
C2
220μF
10V
R7
107Ω
0.25%
R6
89.8Ω
0.5%
Q1
ZVN4206
R11
10k
R14 2Ω
R8
5V
107Ω
0.35%
R5
10k
Q2
2N3904
CORE
REG
02/96/122_conv
A Simple Solution
This dual linear regulator circuit employs an LT1580 for
the CPU core supply and an LT1587 for the I/O supply.
The LT1580 has a precision reference, remote sense
and exceptionally low dropout voltage. It is capable of
meeting the stringent VRE voltage specification when
subjected to the scrutiny of worst-case analysis. The
LT1587 is rated at 3.0A maximum current and is adequate
to power the I/O supply of most desktop systems. If
more than 3A of I/O current is required—your design
has a very large L2 cache, for example—an LT1585A,
which is capable of 5.0A, may be substituted for the
*RESISTORS ARE IMPLEMENTED AS COPPER TRACES ON PCB
IF 1 OZ COPPER, TRACE WIDTHS ARE 0.05 INCH
IF 2 OZ COPPER, TRACE WIDTHS ARE 0.025 INCH
R13 IS 0.83 INCHES LONG, R12 IS 1.24 INCHES LONG
C7
330μF
6.3V
Q3
2N7002
R9
10k
E3
TO CPU
VOLTAGE
SELECT PIN
E3 CPU TYPE
0
P55C
1
P54C
DN122 F02
Figure 2. Power Supply Schematic Diagram
LT1587 by changing one resistor value (R12). See the
Design Equations for details.
Op amp U1 forces the two regulators to share the load
current when the outputs are shorted together by the
CPU metalization. The load current is sensed by the
two low value current sense resistors R12 and R13.
These resistors are actually implemented as short
traces on the PC board. The design does not depend
on the sense resistors’ absolute values being accurate;
only ratiometric matching is required for the circuit to
function properly. The resistance ratio will be very wellcontrolled across PC board production lots.
Amplifier U1 pulls up on the Adjust pin of U2, raising
the output voltage of the I/O regulator until the proper
current ratio between the two regulators is established.
This condition is met when the voltage drop across
the sense resistors is equal. The regulator currents
are inversely proportional to the sense resistor values,
and hence, to the resistor trace lengths. If a different
current ratio is desired, just refigure the trace lengths
per the equations given. The voltage drop across the
resistors at full load is approximately 25mV. Of course,
discrete resistors may be used if desired, but they are
quite costly compared to a PC board trace.
Nonideal components will translate into errors in the
current sharing ratio. With the components shown,
the largest contributor to current-sharing errors is the
error amplifier offset voltage. The very low offset of
the economical LT1006CS8 (400μV max) ensures a
worst-case share error of only 1.6%. If the through
hole version of the LT1006 is used, this error drops by
a factor of five. It is possible to further reduce the value
of the sense resistors with this op amp.
If a user should upgrade to a P55C, E3 is now connected
to ground. This turns off Q2, allowing Q1 and Q3 to
turn on. Q1 shorts out part of the feedback divider of
the LT1580, lowering its output to 2.500V. Q3 pulls
the noninverting input of U1 low, forcing the op amp
output to ground. D1 is now back biased, effectively
disconnecting the op amp from the circuit, which causes
the I/O regulator’s output to drop to 3.3V. Resistor R11
pulls up the cathode of D2 when powering a P54 so
that diode leakage current does not cause an error in
current sharing.
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Linear Technology Corporation
The LT1580 permits remote sensing of the load voltage
at the CPU. Also, by inserting a low value resistor in
the sense line, a small intentional load regulation error is introduced, which, it can be shown, will reduce
the peak-to-peak transient response of the regulator.
The regulation error is well-controlled at the load and
is not a function of any trace resistance or parasitics.
This technique realizes a reduction in the amount of
output capacitance required to control the core voltage transients.
Conclusion
With a small number of low cost components, it is
possible to eliminate the need for replaceable power
supply modules and still accommodate the desire to
upgrade the microprocessor to improved technology.
Moreover, the solution results in an “idiot proof” design,
preventing the application of an inappropriate supply
voltage, which could damage an expensive CPU.
Design Equations:
Assume VS of approximately 25mV,
R13 =
VS
ICORE
I
R12 = CORE (R13)
II/O
1oz copper thickness is 0.0036cm
2oz copper thickness is 0.0071cm
for 1oz copper PC board, use 0.127cm (0.050") wide
traces
for 2oz copper PC board, use 0.064cm (0.025") wide
traces
L = R (t)(w)
RS
Where L is the trace length in cm
R is the desired resistance
RS is the specific resistivity of copper: 1.72μΩ cm
t is the copper thickness of the PC board in cm
For applications help,
call (408) 432-1900
dn122f_conv LT/GP 0596 REV A 2K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1996
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