DN74 - Techniques for Deriving 3.3V from 5V Supplies

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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
5
4.5V TO 5.5V
IN
OUT
1
+
LT1129-3.3
4
SHUTDOWN
SENSE
2
GND
3.3V OUT
700mA
3.3μF
SOLID TANTALUM
of only 4.5V. Figure 1 shows a circuit that converts a
4.5V minimum input to 3.3V with an output tolerance
of only 3% (100mV). The LT®1129-3.3 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
%/t'
3
Figure 1. Low Dropout Regulators Delivers
3.3V from 5V Logic Supply
L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered
trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
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 w 3
25V
C5
470pF
NPO
CT
N DRIVE
SGND
PGND
11
12
14
Q2
Si9410DY
D1
MBRS140T3
C7
220μF w 2
10V
%/t'
Figure 2. 94% Efficiency Synchronous Buck Regulator Pumps Out 2A at 3.3V from 5V Logic Supply
09/93/74_conv
an LTC1148-3.3 converter. The LTC1148 uses both
Burst Mode® operation and continuous, constant offtime control to regulate the output voltage, and maintain
high efficiency across a wide range of output loading
conditions. Efficiency as a function of output current
is plotted in Figure 3.
100
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.
Table 1. Linear Regulators for 5V to 3.3V Conversion
VIN = 5V
LOAD CURRENT
90
EFFICIENCY (%)
Table 2 summarizes the practical current range of a
number of switching regulators for 5V to 3.3V applications, along with their typical efficiencies.
80
70
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
10mA
100mA
FEATURES
150mA
60
50
1mA
DEVICE
2 × LT1087
Parallel, Kelvin Sensed
1A 2A
OUTPUT CURRENT
%/t'
Figure 3. LTC1148-3.3: Measured Efficiency
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.
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.
Data Sheet Download
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Linear Technology Corporation
Table 2. Switching Regulators for 5V to 3.3V Conversion
LOAD
CURRENT
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 applications help,
call (408) 432-1900
dn74f_conv LT/GP 0993 190K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 1993
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