DN48 - No Design Switching Regulator 5V, 5A Buck (Step Down) Regulator

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No Design Switching Regulator
5V, 5A Buck (Step-Down) Regulator – Design Note 48
Ron Vinsant
Introduction
This simple, no design regulator, is a step-down
DC/DC converter designed to convert an 8V to 40V input
to a regulated 5V output. The 5V output is capable of
sourcing up to 5A of output current.
This converter is based on the Linear Technology
LT®1074 switching regulator IC. This device needs only
a few external parts to make up a complete regulator
including thermal protection and current limit. This
design uses off-the-shelf parts for low cost and easy
availability of components. Specifications for the circuit
are in Table 1.
Circuit Description
Figure 1 shows the schematic of the circuit. For the
purpose of this explanation assume that the output
is at a constant +5V DC and that the input voltage is
greater than +8V DC.
At intervals of ≈10μs (100kHz) the control portion of the
LT1074 turns on the switch transistor between the VIN
and VSW pins impressing a voltage across the inductor,
L1. This causes current to build up in the inductor while
also supplying current to the load and capacitor C1.
The control circuit determines when to turn off the
switch during the 10μs interval to keep the output voltage at +5V DC. When the switch transistor turns off, the
magnetic field in the inductor collapses and the polarity
of the voltage across the inductor changes to try and
maintain the current in the inductor. This current in the
inductor is now directed (due to the change in voltage
polarity across the inductor) by the diode, D1, to the
load. The current will flow from the inductor until the
switch turns on again, (continuous operation) or until the
inductor runs out of energy (discontinuous operation).
Referring back to Figure 1, the divider circuit of R1 and
R2 is used to set the output voltage of the supply against
an internal voltage reference of 2.21V DC.
R3 and C2 make up the frequency compensation network
used to stabilize the feedback loop.
Conclusion
This Design Note demonstrates a fully characterized
step-down converter circuit that is both simple and
low cost. This design can be taken and reliably used
in a production environment without the need for any
custom components. A PC board layout and FAB drawing are available from Linear Technology.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
Table 1. Performance Summary (Operating Temperature Range 0°C to 50°C)
Input Voltage Range
Output
+ 8.0V to +40.0V DC
Output Voltage (± 0.15V DC)
+ 5.00V DC
Max Output Current VIN = 8.0V to 40.0V
08/91/48_conv
5.0A DC
Typical Output Ripple at IOUT = 4.0A DC
at Switching Frequency
With Optional Filter (L2 and C4)
Without Optional Filter (L2 and C4)
5mVp-p
50mVp-p
Load Regulation VIN = 8V
At IOUT = 0.5A DC to IOUT = 5.0A DC
0.5%
Line Regulation IOUT = 5A
At VIN = + 8.0V DC to VIN = +40.0V DC
0.5%
A Part of EDN Magazine
HS1
+VIN
+8V TO +40V 5
L1
50μH
VIN
VSW
+5.00V
4
LT1074CT
FRONT VIEW
5
VIN
4
VSW
3
GND
2
VC
1
FB
GND
3
L3
5μH
VFB
1
VC
2
L2
R3
2k
+
T PACKAGE
5-LEAD PLASTIC TO-220
LT1074CT
+VOUT
R1
2.80k
C3
470μF
50V
D1
MBR745
C2
0.1μF
+VOUT*
R2
2.21k
+
HS2
C1
330μF
50V
+
C4*
39μF
35V
RTN
RTN
* OPTIONAL
+VOUT = 2.21 1+ R1
R2
[
%$t4$)
]
Figure 1. Package and Schematic Diagrams
Table 2. Parts List
REFERENCE
DESIGNATOR
QUANTITY
PART NUMBER
DESCRIPTION
VENDOR
PCB
1
001A
PCB FAB, Buck Switching Regulator
LTC
D1
1
MBR745
Diode, Schottky, 7A, 45V
Motorola
HS2
1
6038B-TT
Heat Sink
Thermalloy
L2
1
2664000101
Shield Bead
Fair-Rite
VR1
1
LT1074CT
Switching Regulator, 100kHz
LTC
HS1
1
7020B-MT
Heat Sink
Thermalloy
C1
1
SXE50VB331M12X20LL
Cap, Alum Elect, 330μF, 50V
United Chemicon
C2
1
CKO6BX104K
Cap, Ceramic, 0.1μF, 50V
AVX
C3
1
UPL1H471MRH
Cap, Alum Elect, 470μF, 50V
Nichicon
C4
1
UPL1V390MAH
Cap, Alum Elect, 39μF, 35V
Nichicon
L1
1
CTX50-5-MP
Inductor, 50μH, 5A
Coiltronics
L3
1
CTX5-5-FR
Inductor, 5μH, 5A
Coiltronics
R1
1
MF 1/8W 2.80kΩ
Res, MF, 1/8W, 1%, 2.80k
R2
1
MF 1/8W 2.21kΩ
Res, MF, 1/8W, 1%, 2.21k
R3
1
CF 1/4W 2kΩ
Res, CF, 1/4W, 5%, 2k
Data Sheet Download
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Linear Technology Corporation
For applications help,
call (408) 432-1900
dn48f_conv BA/GP 0891 180K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 1991
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