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Techniques for Deriving 3.3V from 5V Supplies – Design Note 74
Mitchell Lee
Microprocessor chip sets and logic families that operate
from 3.3V supplies are gaining acceptance in both desktop and portable computers. Computing rates, and in
most cases, energy consumed by these circuits show a
strong improvement over 5V technology. The main power
supply in most systems is still 5V, necessitating a local 5V
to 3.3V regulator.
Linear regulators are viable solutions at lower (IO ≤ 1A)
currents, but they must have a low dropout voltage in order
to maintain regulation with a worst case input of only 4.5V.
5
4.5V TO 5.5V
IN
1
OUT
3.3V OUT
700mA
+
LT1129-3.3
4
SHUTDOWN
2
SENSE
GND
3.3µF
SOLID TANTALUM
DN74 • F01
3
Figure 1. Low Dropout Regulator Delivers
3.3V from 5V Logic Supply
Figure 1 shows a circuit that converts a 4.5V minimum
input to 3.3V with an output tolerance of only 3% (100mV).
The LT1129-3.3 can handle up to 700mA in surface mount
configurations, and includes both 16µA shutdown and
50µA standby currents for system sleep modes. Unlike
other linear regulators, the LT1129-3.3 combines both low
dropout and low voltage operation. Small input and output
capacitors facilitate compact, surface mount designs.
For the LT1129-3.3, dissipation amounts to a little under
1.5W at full output current. The 5-lead surface mount DD
package handles this without the aid of a heat sink, provided the device is mounted over at least 2500mm2 of
ground or power supply plane. Efficiency is around 62%.
Dissipation in linear regulators becomes prohibitive at
higher current levels where they are supplanted by high
efficiency switching regulators. A 2A, 5V to 3.3V switching
regulator is shown in Figure 2. This synchronous buck
converter is implemented with an LTC1148-3.3 converter.
The LTC1148 uses both Burst ModeTM operation and continuous, constant off-time control to regulate the output
Burst ModeTM is a trademark of Linear Technology Corporation
4.5V TO 5.5V
INPUT
+
C1
1µF
+
C2
100nF
3
VIN
10
SHUTDOWN P DRIVE
1
Q1
Si9430DY
L1
50µH
LT1148-3.3
6
ITH
R1
1k
SENSE +
8
SENSE –
7
R2
0.05Ω
3.3V
2A
C6
10nF
+
4
C4
3.3nF
X7R
C3
22µF × 3
25V
C5
470pF
NPO
CT
N DRIVE
SGND
PGND
11
12
14
Q2
Si9410DY
D1
MBRS140T3
C7
220µF × 2
10V
DN74 • F02
Figure 2. 94% Efficiency Synchronous Buck Regulator Pumps Out 2A at 3.3V from 5V Logic Supply
09/93/74
www.BDTIC.com/Linear
voltage, and maintain high efficiency across a wide range Table 2 summarizes the practical current range of a number
of output loading conditions. Efficiency as a function of of switching regulators for 5V to 3.3V applications, along
with their typical efficiencies.
output current is plotted in Figure 3.
A 5V to 3.3V converter circuit collection is presented in
Application Note 55, covering the entire range of currents
listed in Tables 1 and 2.
100
VIN = 5V
EFFICIENCY (%)
90
Table 1. Linear Regulators for 5V to 3.3V Conversion
80
LOAD CURRENT
70
60
50
1mA
10mA
100mA
DEVICE
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
1A 2A
OUTPUT CURRENT
DN74 • F03
FEATURES
150mA
2 × LT1087
10A
Parallel, Kelvin Sensed
Figure 3. LTC1148-3.3: Measured Efficiency
Table 2. Switching Regulators for 5V to 3.3V Conversion
All of the components used in the Figure 2 switching
regulator are surface mount types, including the inductor
and shunt resistor, which are traditionally associated with
through hole assembly techniques.
LOAD
CURRENT
Depending on the application, a variety of linear and
switching regulator circuits are available for output currents ranging from 150mA to 20A. Choices in linear regulators are summarized in Table 1. There are some cases,
such as in minicomputers and workstations, where higher
dissipations may be an acceptable compromise against the
circuit complexity and cost of a switching regulator, hence
the >1A entries. Heat sinks are required.
DEVICE
EFFICIENCY
FEATURES
200mA
to 400mA
LTC1174-3.3
90%
Internal P-Channel
Switch, 1µA Shutdown
0.5A to 2A
LTC1147-3.3
92%
8-Pin SO,
High Efficiency Converter
1A to 5A
LTC1148-3.3
94%
Ultra-High Efficiency
Synchronous Converter
5A to 20A
LT1158
91%
Ultra-High Current
Synchronous Converter
For literature on our Voltage Regulators,
call (800) 637-5545. For applications help,
call (408) 432-1900, Ext. 361
Linear Technology Corporation
LT/GP 0993 190K
1630 McCarthy Blvd., Milpitas, CA 95035-7487
www.BDTIC.com/Linear
(408) 432-1900 ● FAX: (408) 434-0507 ● TELEX: 499-3977
 LINEAR TECHNOLOGY CORPORATION 1993