advertisement
06/97/157
Circle No. 198
UltraFast Linear Regulator Eliminates All Bulk Tantalum
and Electrolytic Output Capacitors
Design Note 157
Anthony Bonte
Figure 1. LT1577 P54C/P55C Pentium Processor Autoselect Circuit
1
2
3
4
16
15
14
13
1/2 LT1577
IPOS1
INEG1
GATE1
COMP1
SHDN1
V
IN1
GND1
OUT
–3.3 R7
3.9k
R9
0.004Ω
R9 = TRACE
RESISTOR
V
IO
Q2
IRFZ14
R5
3.9Ω
C8
1500pF
C1
0.1µF
3.3V
3.5V
C20-C31
1µF
X7R
CERAMIC
0805 CASE
C2A
0.1µF
C3
0.22µF
12V
12V
RESET
RESET
C4
10pF
C9-C11
1200µF
10V
×3
+
5
6
7
8
12
11
10
9
1/2 LT1577
IPOS2
INEG2
GATE2
COMP2
SHDN2
V
IN2
GND2
FB R8
7.5k R1
1.58k R4
10k
R2
1.21k
R3
2.67k
Q1
2N7002
V
CORE
IF V
CC2DET
= OPEN, THEN V
IO
= V
CORE
= 3.5V
IF V
CC2DET
= GND, THEN V
IO
= 3.3V AND V
CORE
= 2.8V
V
CC2DET
Q3
IRFZ34
R6
3.9Ω
C7
1000pF
C5
0.1µF
2.8V
3.5V
TO CPU
DN157 F01
C32-C55
1µF
X7R
CERAMIC
0805 CASE
C2B
0.1µF
C6
10pF
5V
I = 0.2A TO 5A 200µs/DIV
DN157 F02
0
2A/DIV
50mV/DIV
Figure 2. Transient Response for
0.2A to 5A Output Load Step
Introduction
Powering 200+MHz microprocessors requires high current,
tight tolerance, fast transient response power supplies. Fast
load transients mandate bulk output capacitance to main-
tain regulation and thus, cost increases. Surface mount tan-
talum capacitors are expensive and require voltage derating
for reliable performance. Electrolytic capacitors are physi-
cally large and exhibit increased ESR with age. Therefore,
transient response and regulation performance degrade.
To improve profit margins, some manufacturers reduce
output capacitance and ignore the true regulation require-
ments. Many power supplies are deemed reliable if
Windows
®
95 boots up more than once. Most motherboards
are only warranted for 90 days. LTC believes that many sys-
tem crashes (blamed on software) are attributable to poor
power supply regulation. To address these issues, Linear
Technology introduces the LT
®
1575/LT1577.
New LTC Regulator Controllers
The LT1575/LT1577 family of controller ICs drives discrete
N-channel MOSFETs and produces low dropout, UltraFast
TM
transient response regulators. These ICs feature 1% typical
performance over all DC tolerances. Superior transient load
performance eliminates all bulk output capacitors. An
LT1577 based P55C Pentium
®
processor power supply op-
erates with only twenty-four high frequency decoupling, 1µF
ceramic capacitors required for the microprocessor core.
Adjustable and fixed voltage versions accommodate any
microprocessor voltage. MOSFET R
DS(ON)
selection allows
custom dropout voltage performance. The controllers also
provide current limiting, on/off control and overvoltage
protection or thermal shutdown. The single LT1575 pack-
age is an 8-pin SO or PDIP and the dual LT1577 package is
a 16-pin narrow body SO.
Figure 1 illustrates an LT1577 application with a fixed 3.3V
and an adjustable voltage regulator for a P54C/P55C
Pentium processor autoselect circuit. The P54C Pentium
processor core and I/O circuitry operate from 3.5V. The
P55C Pentium processor I/0 operates from 3.3V and the
core operates from 2.8V.
V
CC2DET
’s signal determines circuit operation. In a P54C
circuit, V
CC2DET
is open and the core and I/O supply planes
, LTC and LT are registered trademarks of Linear Technology Corporation.
UltraFast is a trademark of Linear Technology Corporation.
Windows is a registered trademark of Microsoft Corporation.
Pentium is a registered trademark of Intel Corporation.
LINEAR TECHNOLOGY CORPORATION 1997
dn157f LT/TP 0697 155K • PRINTED IN THE USA
Linear Technology Cor poration
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
(408) 432-1900
FAX: (408) 434-0507
●
TELEX: 499-3977
●
www.linear-tech.com
For literature on our Linear Regulators,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 2360
+
+ + + +
DN157 F03
TG
SW
BOOST
INTV
CC
BG
S
+
S
–
EXTV
CC
C
OSC
RUN/SS
I
TH
SFB
SGND
V
OS
9
1
2
3
4
5
6
16
14
15
12
11
8
7
13
10 C4, 4.7µFC5
0.1µF
D2
MBRS330T3
R8
15K
R3
100 R4
100
C18
1000µF
10V
C20
1000µF
10V
C19
1000µF
10V
R6
0.0075Ω
L1
4µH
C2, 1000pF
V
IN
U2
LTC1435
IPOS
INEG
GATE
COMP
SHDN
V
IN
GND
FB
C21, 10pF
C22, 1000pF
R2
1.21k
1%
C1, 470pF
R9
2k
Q1
IRLZ44
R1
2.1k, 1% V
CORE
3.3V
1
2
3
4
8
7
6
5
U1
LT1575
PGND
D1, CMDSH-3 Q3
Q2
C16
1µF
C14
150µF
16V
C15
1µF
C17
1µF
12V
C11
150µF
16V
C12
150µF
16V
C13
150µF
16V
C3, 0.1µF
C9
1500pF
R5
16.5k
C10, 1000pF
C8, 68pF
C7, 0.1µF
R7
35.7k
+
C23
1µF
C6
0.1µF
12V
1µF
X7R
CERAMIC
0805 CASE
×40
+
+
L1=COILTRONICS CTX02-13199
Q2, Q3 =SILICONIX SUD50N03-10
Figure 4 shows the transient response for a 10A, 50ns rise/
fall time load step. The only output capacitors are 40, 1µF
surface mount ceramic capacitors. The circuit eliminates
about a dozen low ESR tantalum capacitors, which would
be required without the linear regulator.
Conclusion
The LT1575/LT1577 combine the benefits of low dropout
voltage, precision performance, UltraFast transient
response and significant output capacitance cost savings.
The LT1575/LT1577 controller ICs step to the next perfor-
mance level required by motherboard designers.
connect together. Q1 turns on and the Q3 (IRFZ34) regu-
lator controls its output to 3.5V. The Q2 (IRFZ14) regulator
attempts to control its output to 3.3V, but its feedback pin
(Pin 4) senses 3.5V and turns Q2 off. Q3 supplies all core
and I/O power.
In a P55C circuit, V
CC2DET
is grounded and the core and
I/O supply planes are separate. Q2 controls the I/O voltage
to 3.3V and Q3 controls the core voltage to 2.8V. The I/O
circuitry’s lower current requirement permits a lower cost
MOSFET for Q2 and reduced output capacitance.
The current limit sense resistor is made of “free” PCB trace.
Q2’s and Q3’s common-drain connection permits com-
mon heat sink mounting. The COMP pin components
adjust frequency compensation for each regulator relative
to the MOSFET and output capacitors used.
Figure 2 shows the core regulator transient response for a
4.8A load current step in a P55C setup. Compensation
limits overshoot/undershoot to 50mV. The ±100mV toler-
ance for a VRE processor is easily met. The autoselect
concept is easily extended to the multiplicity of voltages
required by various processors. Consult LTC for details.
Figure 3 shows a 3.3V, 14A logic supply that uses an
LT1575 as a post-regulator on an LTC
®
1435 synchronous
buck regulator, generating 3.3V from 12V with an overall
efficiency of 72%. The LT1575 uses an IRLZ44 as the pass
transistor, allowing < 550mV dropout voltage. The switch-
ing regulator’s output is set to 4V.
50mV/DIV
DN157 F04
200µs/DIV
Figure 4. Transient Response for Figure 3’s Circuit
to a 10A Load Step
Figure 3. 12V to 3.3V/9A (14A Peak) Hybrid Regulator
