UltraFast Linear Regulator Eliminates All Bulk Tantalum
and Electrolytic Output Capacitors
Design Note 157
Anthony Bonte
New LTC Regulator Controllers
The LT1575/LT1577 family of controller ICs drives discrete
N-channel MOSFETs and produces low dropout,
UltraFast™ 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 operates with only twenty-four
high frequency decoupling, 1μF ceramic capacitors
required for the microprocessor core.
To improve profi t margins, some manufacturers
reduce output capacitance and ignore the true regulation requirements. Many power supplies are deemed
reliable if Windows95 boots up more than once.
Most motherboards are only warranted for 90 days.
LTC believes that many system crashes (blamed on
software) are attributable to poor power supply
regulation. To address these issues, Linear Technology
introduces the LT ®1575/LT1577.
Adjustable and fixed voltage versions accommodate
any microprocessor voltage. MOSFET RDS(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 package is an 8-pin SO or PDIP and the
dual LT1577 package is a 16-pin narrow body SO.
12V
RESET
Introduction
Powering 200+MHz microprocessors requires high
current, tight tolerance, fast transient response power
supplies. Fast load transients mandate bulk output
capacitance to maintain regulation and thus, cost
increases. Surface mount tantalum capacitors are
expensive and require voltage derating for reliable
performance. Electrolytic capacitors are physically
large and exhibit increased ESR with age. Therefore,
transient response and regulation performance degrade.
1
2
C2A
0.1μF
R9 = TRACE
RESISTOR
1/2 LT1577
SHDN1 IPOS1
VIN1
INEG1
16
15
3
14
GND1 GATE1
4 OUT
13
COMP1
– 3.3
C3
0.22μF
R5
3.9Ω
12V
RESET
C4
10pF
1/2 LT1577
5
C2B
0.1μF
6
7
8
SHDN2 IPOS2
VIN2
GND2
FB
INEG2
GATE2
COMP2
R9
0.004Ω
+
Q2
IRFZ14
R7
3.9k
C8
1500pF
C9-C11
1200μF
10V
×3
5V
C1
0.1μF
VCC2DET’s signal determines circuit operation. In a
P54C circuit, VCC2DET is open and the core and I/O
VIO
C20-C31
1μF
X7R
CERAMIC
0805 CASE
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/O operates from
3.3V and the core operates from 2.8V.
3.3V
3.5V
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and UltraFast is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
IF VCC2DET = OPEN, THEN VIO = VCORE = 3.5V
IF VCC2DET = GND, THEN VIO = 3.3V AND VCORE = 2.8V
12
C5
0.1μF
11
R6
10 3.9Ω
9
C6
10pF
50mV/DIV
Q3
IRFZ34
R8
7.5k
C7
1000pF
C32-C55
1μF
X7R
CERAMIC
0805 CASE
R2
1.21k
VCORE
R1
2.8V
1.58k 3.5V
R4
10k
2A/DIV
R3
2.67k
Q1
2N7002
TO CPU
VCC2DET
DN157 F01
Figure 1. LT1577 P54C/P55C Pentium Processor Autoselect Circuit
06/97/157_conv
0
I = 0.2A TO 5A
200μs/DIV
DN157 F02
Figure 2. Transient Response for 0.2A
to 5A Output Load Step
1
2
12V
3
4
C6
0.1μF
IPOS
SHDN
8
7
U1
INEG
LT1575
6
GND
GATE
VIN
FB
COMP
Q1
IRLZ44
5
C21, 10pF
12V
+
C11
150μF
16V
+
C12
150μF
16V
+
C13
150μF
16V
+
C14
150μF
16V
EXTVCC
1
C8, 68pF
C9
1500pF
R5
16.5k
ITH
4
5
6
INTVCC
11
BG
SGND
S+
8
VOS
S–
7
10
R1
2.1k, 1%
C16
1μF
R2
1.21k
1%
L1
4μH
C3, 0.1μF
D1, CMDSH-3
Q3
3.3V
VCORE
C1, 470pF
1μF
X7R
CERAMIC
0805 CASE
×40
Q2
12
SFB
PGND
C15
1μF
16
14
U2
SW
LTC1435
15
RUN/SS
BOOST
3
C10, 1000pF
TG
COSC
2
C7, 0.1μF
C17
1μF
13
VIN
9
R9
2k
C22, 1000pF
R6
0.0075
R3
100
R4
100
+
C18
1000μF
10V
C20
1000μF
10V
+
+
C5
0.1μF
D2
MBRS330T3
C23
1μF
C19
1000μF
10V
C2, 1000pF
C4, 4.7μF
+
R7
35.7k
DN157 F03
R8
15K
L1 = COILTRONICS CTX02-13199
Q2, Q3 = SILICONIX SUD50N03-10
supply planes connect together. Q1 turns on and the
Q3 (IRFZ34) regulator 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, VCC2DET 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
common 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
tolerance 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
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50mV/DIV
Figure 3. 12V to 3.3V/9A (14A Peak) Hybrid Regulator
200μs/DIV
DN157 F04
Figure 4. Transient Response for Figure 3's
Circuit to a 10A Load Step
efficiency of 72%. The LT1575 uses an IRLZ44 as the
pass transistor, allowing < 550mV dropout voltage. The
switching regulator’s output is set to 4V.
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
performance level required by motherboard designers.
For applications help,
call (408) 432-1900
dn157f_conv LT/TP 0697 155K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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© LINEAR TECHNOLOGY CORPORATION 1997