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05/94/82
5V to 3.3V Regulator with Fail-Safe Switchover – Design Note 82
Mitchell Lee
Newer microprocessors in personal computers have
low voltage chips which are designed to replace existing
5V units. In the past a processor swap was simply a
matter of removing one IC and replacing it with an
updated version. But now the upgrade path involves
switching from a 5V chip to one that requires 3.xxV.
One means of changing supply voltage from 5V to 3.xxV
is to clip a jumper that bypasses a local 3.xxV regulator.
This is not a good solution since it leaves too much to
chance. Failure to remove the jumper can result in the
instant destruction of the new microprocessor upon
application of power. A means of automatically sensing
the presence of a 3.xxV or 5V processor is necessary.
Intel microprocessors include a special pin called
“VOLDET” which can be used to determine whether or
not a particular chip needs 3.xxV or 5V. Figure 1 shows
a simple circuit that takes advantage of this pin to
automatically “jumper out” a 3.xxV regulator whenever
a 5V processor is inserted into the socket. VOLDET is
pulled low on 3.xx processors; it is buffered by transis-
tor Q2 which grounds the gate of a bypassing switch
(Q1). Q1 is turned off leaving the LT1085 to regulate the
microprocessor’s VCC line.
For 5V microprocessors the VOLDET pin is high; Q2 is
turned off allowing Q1’s gate to pull up to 12V, turning
itself on. With the LT1085 shorted from input to output
by the MOSFET, 5V flows directly to the microproces-
sor. No service intervention is required to ensure cor-
rect VCC potential.
The circuit in Figure 1 is fine for cases where 12V is
available to enhance the MOSFET switch. However, in
portable applications,12V is frequently not available or
available only on an intermittent basis. Figure 2 shows
a second solution using a high-side gate driver to
control the MOSFET. A VOLDET pull-up resistor is
required in both figures because in some cases VOLDET
Figure 1. Bypass Circuit for 3.xxV and 5V Microprocessor
Swaps Using Transistor Buffer Figure 2. Bypass Circuit for 3.xxV and 5V Microprocessor
Swaps Using High-Side Gate Driver (No 12V Supply Required)
+
LT1085 3.45V, 5V
MICROPROCESSOR
*
Q1
MTB30N06EL
Q2
2N2222
12V 5V
ADJ
IN V
CC
DN82 • F01
VOLDET
(H = 5V, L = 3.45V)
C1
10µF
+
C2
22µF
T
A
R2
115Ω
1%
R1
20k
R5
100k R4
100k
R3
200Ω
1%
OUT5V
+
LT1085 3.45V, 5V
MICROPROCESSOR
*
Q1
MTB30N06EL
ADJ
IN V
CC
DN82 • F02
VOLDET
(H = 5V, L = 3.45V)
C1
10µF
+
C2
22µF
T
A
R2
115Ω
1%
R1
3.3M
R4
100k
R3
200Ω
1%
OUT5V
1/2
LTC1157
V
S
5V
G1 IN1
* FOR BEST PERFORMANCE, PLACE 4 EACH 47µF, 9 EACH 0.1µF AND
9 EACH 0.01µF BYPASS CAPACITORS ON THE V
CC
PINS OF THE
INTEL 486
TM
MICROPROCESSOR. ESR OF THE 47µF < 0.1Ω.
INTEL 486 IS A TRADEMARK OF INTEL CORPORATION
LINEAR TECHNOLOGY CORPORATION 1994
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900
●
FAX
: (408) 434-0507
●
TELEX
: 499-3977
LT/GP 0594 190K • PRINTED IN THE USA
For literature on our Linear Regulators,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 361
is an open circuit or a shorting link, and in other cases
it is an open-drain output.
VOLDET is pulled up to 5V in both circuits. This could
pose a problem for 3.xxV processors with open-drain
VOLDET pins, but for 3.xxV devices VOLDET is always
pulled low and 5V never reaches it. The 5V reaches
VOLDET only on 5V devices.
For certain families of microprocessors, 3.3V is re-
quired. The circuits shown in Figures 1 and 2 are fully
compatible with 3.3V applications by simply substitut-
ing a fixed 3.3V version of the regulator (use an LT1085-
3.3). Higher current operation is also possible. The
LT1085 is suitable for 3A applications; use an LT1084
and an MTB50N06EL for up to 5A. Table 1 shows the
wide range of linear regulators available at currents of
up to 10A.
In some applications the complexity of a high efficiency
switching regulator may be justified for reasons of
battery life. Figure 3 shows a switcher that not only
converts 5V to 3.45V but also acts as its own bypass
switch for applications where a 5V output is required.
An open drain or collector pulling the VFB pin low causes
1
2
3
4
5
6
7
14
13
12
11
10
9
8
P-DRIVE
NC
V
IN
C
T
INT V
CC
I
TH
SENSE
–
N-DRIVE
NC
P-GND
S-GND
SHDN
V
FB
SENSE
+
LTC1148
C2
0.1µF
C6
0.01µFR2
0.033 Ω
R4
20k
1%
Q3
2N2222
R3
35.7k
1%
5V
3.45V
SHUTDOWN
L1
10µH
Q2
Si9410
D1
MBRS140T3
C3
100µF
20V × 2
5V
INPUT
C7
220µF
6.3V × 2
3.45V
3A
C4
3300pF
X7R
R1
470Ω
C5
200pF
NPO
C1: Ta
C3: SANYO (OS-CON) 20SA100M,
ESR = 0.037Ω, I
RMS
= 2.25A
C7: SANYO (OS-CON) 10SA220M,
ESR = 0.035Ω, I
RMS
= 2.36A
Q1, Q2: SILICONIX PMOS BV
DSS
= 20V,
DCR
ON
= 0.100Ω, Qg = 50nC
Q3: SILICONIX NMOS BV
DSS
= 30V,
DCR
ON
= 0.050Ω, Qg = 30nC
D1: MOTOROLA SCHOTTKY VBR = 30V
R2: KRL NP-2A-C1-0R020J, P
D
= 1W
L1: COILTRONICS CTX10-4
DN82 • F03
+
C1
1µF
Q1
Si9430
+
+
Figure 3
the top side P-channel MOSFET to turn on 100%,
effectively shorting the output to the 5V input. If the
open collector is turned off, the LTC1148 operates as a
high efficiency buck mode power converter, delivering
a regulated 3.45V to the load. For 3.3V applications a
fixed 3.3V version of the LTC1148 is available.
Table 1. Linear Regulators for 5V to 3.3V Conversion
LOAD CURRENT DEVICE FEATURES
150mA LT1121-3.3 Shutdown, Small Capacitors
700mA LT1129-3.3 Shutdown, Small Capacitors
800mA LT1117-3.3 SOT-223
1.5A LT1086 DD Package
3A to 7.5A LT1083 High Current, Low Quiescent
LT1084 Current at High Loads
LT1085
10A 2 × LT1087 Parallel, Kelvin Sensed
The topic of powering low voltage microprocessors in
a 5V environment is covered extensively in Application
Note 58, available on request. Both linear and switching
solutions are discussed.
