DN82 - 5V to 3.3V Regulator with Fail-Safe Switchover

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5V to 3.3V Regulator with Fail-Safe Switchover – Design Note 82
Mitchell Lee
Newer microprocessors designed for replacing existing
5V units operate from lower voltage supplies. 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 transistor Q2 which grounds the gate of a bypassing
switch (Q1). Q1 is turned off leaving the LT®1085 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 microprocessor.
No service intervention is required to ensure correct
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 is an
open circuit or a shorting link, and in other cases it is
an open-drain output.
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respective owners.
5V
12V
5V
R5
100k
R1
20k
R4
100k
R4
100k
VS
G1
Q2
2N2222
R1
3.3M
Q1
MTB30N06EL
IN1
1/2
LTC1157
Q1
MTB30N06EL
VOLDET
(H = 5V, L = 3.45V)
VOLDET
(H = 5V, L = 3.45V)
MICROPROCESSOR
3.45V, 5V
5V
+
IN
LT1085 OUT
C1
10μF
ADJ
+
R2
115Ω
1%
C2
22μF
TA
VCC
*
MICROPROCESSOR
3.45V, 5V
5V
DN82 • F01
+
IN
LT1085 OUT
C1
10μF
ADJ
R3
200Ω *FOR BEST PERFORMANCE, PLACE 4 EACH 47μF, 9 EACH 0.1μF AND
9 EACH 0.01μF BYPASS CAPACITORS ON THE VCC PINS OF THE
1%
+
R2
115Ω
1%
C2
22μF
TA
VCC
*
DN82 • F02
R3
200Ω
1%
INTEL 486 MICROPROCESSOR. ESR OF THE 47μF < 0.1Ω
Figure 1. Bypass Circuit for 3.xxV and 5V Microprocessor
Swaps Using Transistor Buffer
05/94/82_conv
Figure 2. Bypass Circuit for 3.xxV and 5V Microprocessor
Swaps Using High-Side Gate Driver (No 12V Supply
Required)
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 required.
The circuits shown in Figures 1 and 2 are fully compatible with 3.3V applications by simply substituting 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
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
LT1084
LT1085
High Current, Low Quiescent
Current at High Loads
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.
5V
INPUT
D1
MBRS140T3
Q1
Si9430
+
C1: Ta
C3: SANYO (OS-CON) 20SA100M,
ESR = 0.037Ω, IRMS = 2.25A
C7: SANYO (OS-CON) 10SA220M,
ESR = 0.035Ω, IRMS = 2.36A
Q1: SILICONIX PMOS BVDSS = 20V,
DCRON = 0.100Ω, Qg = 50nC
Q2: SILICONIX NMOS BVDSS = 30V,
DCRON = 0.050Ω, Qg = 30nC
D1: MOTOROLA SCHOTTKY VBR = 30V
R2: KRL NP-2A-C1-0R020J, PD = 1W
L1: COILTRONICS CTX10-4
C3
100μF
20V w2
Q2
Si9410
C1
1μF
2
+
3
4
C5
200pF
NPO
5
6
C4
3300pF
X7R
R1
470Ω
7
P-DRIVE
NC
VIN
N-DRIVE
NC
LTC1148
P-GND
CT
INT VCC
ITH
SENSE –
S-GND
SHDN
VFB
SENSE +
14
13
L1
10μH
12
11
10
5V
R4
20k
1%
SHUTDOWN
9
8
C6
0.01μF
R2
0.033Ω
+
1
C2
0.1μF
C7
220μF
6.3V w2
5V
Q3
2N2222
VOLDET
3.45V
R3
35.7k
1%
DN82 • F03
100k
3.45V
3A
Figure 3
Data Sheet Download
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