DN87 - Fast Regulator Paces High Performance Processors

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Fast Regulator Paces High Performance Processors
Design Note 87
Mitchell Lee and Craig Varga
New high performance microprocessors require a
fresh look at power supply transient response. Pentium
processors, for example, have current demands that go
from a low idle mode of 200mA to a full load current of
4A in 20ns. A transition of the same magnitude occurs
as the processor reenters its power saving mode. In
addition, the overall supply tolerances have been narrowed significantly from the traditional ±5% for 5V
supplies and include transient conditions. When all
possible DC error terms are accounted for, the transient
response of the power supply when subject to the load
step mentioned above must be within ±46mV!
To address this problem Linear Technology has developed the LT®1585 linear regulator. It features 1% initial
accuracy, excellent temperature drift and load regulation,
and virtually perfect line regulation. Complementing superb DC characteristics, the LT1585 exhibits extremely
fast response to transients. The regulator is offered as
an adjustable regulator requiring two resistors to set the
operating point, as well as fixed versions which have
been trimmed and optimized for 3.3V, 3.38V, 3.45V,
and 3.6V outputs. Fixed versions are fully specified for
worst-case DC error bounds; in adjustable designs the
effects of the external voltage-setting resistors must
be taken into account.
Transient response is affected by more than the regulator itself. Stray inductances in the layout and bypass
capacitors, as well as capacitor ESR dominate the
response during the first 400ns of transient. Figure 1
shows a bypassing scheme developed to meet all of
the requirements for the Intel P54C-VR microprocessor.
Multiple capacitors are required to reduce the total ESR
and ESL, which affect the transient response.
Input capacitors C1 and C2 function primarily to
decouple load transients from the 5V logic supply. The
values used here are optimized for a typical 5V desktop
computer “silver box” power supply input. C5 to C10
provide bulk capacitance at low ESR and ESL, and C11
to C20 keep the ESR and ESL low at high (>100kHz)
frequencies. C4 is a damper and it minimizes ringing
during settling.
A good place to locate the surface mount decoupling
components is in the center of the Pentium socket
cavity on the top side of the circuit board. Consider
using concentric rings of power and ground plane on
the top layer of the board within the socket center for
bussing the capacitors together. Tie the main power and
ground planes to these cavity planes with a minimum
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
THERMALLOY
7020B-MT
4.75V TO
5.25V
+
IN
C1, C2
220μF
10V
SANYO OS-CON
2w
OUT
LT1585CT-3.38
GND
PLACE IN SOCKET CAVITY
+
C4
1200μF
16V
NICHICON PL
+
C5 TO C10
220μF
10V
AVX TPS
6w
C11 TO C20
1μF
16V
AVX Y5V 0805
10w
DN87 F01
AVX CORPORATION: (803) 448-9411
NICHICON (AMERICA) CORPORATION: (708) 843-7500
SANYO VIDEO COMPONENTS (USA) CORPORATION: (619) 661-6322
THERMALLOY INCORPORATED: (214) 243-4321
FOR CORRECT OPERATION OF MICROPROCESSOR, DO NOT SUBSTITUTE COMPONENTS
Figure 1. Recommended Bypassing Scheme for Correct Transient Response
10/94/87_conv
MICROPROCESSOR
LOAD
of two vias per capacitor. This will minimize parasitic
inductance. The regulator and damper capacitor should
be located close to (<1") the microprocessor socket to
minimize circuit trace inductance.
Verifying the regulator and microprocessor layout can
be accomplished with a controlled load such as the
Power Validator manufactured by Intel. This device plugs
directly into the microprocessor socket and simulates
worst-case load transients conditions.
An oscilloscope photograph of the LT1585’s response to
a worst-case 200mA to 4A load step is shown in Figure
2. Trace C is the load current step, which is essentially
flat at 4A with a 20ns rise time. Trace A is the output
settling response at 20mV per division. Cursor trace B
marks –46mV relative to the initial output voltage. At
the onset of load current, the microprocessor socket
voltage dips to –38mV as a result of inductive effects in
the board and capacitors, and the ESR of the capacitors.
The inductive effects persist for approximately 400ns.
For the next 3μs the output droops as load current
drains the bypass capacitors. The trend then reverses
as the LT1585 catches up with the load demand, and
the output settles after approximately 50μs.
Running 4A with a 1.7V drop, the regulator dissipates
6.8W. The heat sink shown in Figure 1, with 100ft/min
air flow is adequate for worst-case operating conditions.
The adjustable version of the LT1585 makes it relatively
easy to accommodate multiple mircoprocessor power
supply voltage specifications (see Figure 3). To retain the
tight tolerance of the LT1585 internal reference, 0.5%
adjustment resistors are recommended. R1 is sized to
carry approximately 10mA idling current (≤124Ω), and
R2 is calculated from:
R2 =
20mV/DIV
A
B
C
1A/DIV
VO – VREF
VREF
+ IADJ
R1
where:
IADJ = 60μA and VREF = 1.250V
DN87 F02
1μs/DIV
Figure 2. Transient Response at Onset of 4A Load
Current Step
4.75V TO
5.25V
IN
+
C1, C2
220μF
10V
SANYO OS-CON
2w
Figure 3 shows the connections for R1 and R2. Note that
C5 to C10 are reduced in value from Figure 1 without
compromising the transient response. The addition of C3
makes this possible and also eliminates the need for C4.
3.38V
4A
OUT
LT1585CT
ADJ
C3
330nF
16V
AVX X7R 0805
R1
110Ω
0.5%
+
C5 TO C10
100μF
10V
AVX TPS
6w
C11 TO C20
1μF
16V
AVX Y5V 0805
10w
R2
189Ω
0.5%
DN87 F03
Figure 3. Recommended Adjustable Circuit
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900
dn87f_conv LT/GP 1094 190K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 1994
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