DN181 - A High Efficiency 500kHz, 4.5A Step-Down Converter in an SO-8 Package

advertisement
A High Efficiency 500kHz, 4.5A Step-Down Converter in an
SO-8 Package – Design Note 181
Karl Edwards
Reducing board space and improving efficiency are key
requirements in many systems, especially at higher currents, where component size and power losses generally
increase. Linear Technology has addressed these issues
with the new LT®1374, a 500kHz, 4.5A monolithic buck
converter designed to meet the needs of higher current
applications. The LT1374 contains the power switch,
logic, oscillator and all the control circuitry necessary
to make a compact, high efficiency buck converter. The
topology is current mode for fast transient response
and good loop stability, with the added benefit of full
cycle-by-cycle current limit.
The device is available in three package options: SO-8,
DD and TO-220. For the most space-sensitive applications, the SO-8 retains the full 4.5A switch rating and
is ideal for medium power applications with high peak
loads. The DD package is intended for surface mount
applications with continuous high current; the TO-220
is for high power, high ambient temperature systems.
A switching frequency of 500kHz allows the use of small,
low value surface mount components to reduce board
area. To further reduce power consumption, the LT1374
has two shutdown modes. A precise 2.38V threshold
on the shutdown (SHDN) pin keeps the internal reference alive but disables switching. This mode can be
used as an accurate input undervoltage lockout, as
shown in Figure 1. Grounding the SHDN pin takes the
part into complete shutdown, reducing supply current
to only 20μA.
For noise-sensitive applications, the SHDN pin can be
replaced by SYNC (LT1374-SYNC), enabling the internal
oscillator to be synchronized to an external system clock
in the range of 580kHz to 1MHz. Both adjustable and
fixed 5V output voltage parts are available. The LT1374,
together with a minimum of small surface mount components, produces a 4.5A step-down regulator that is
efficient in both power and board space.
High Efficiency, 25V, 0.07Ω Switch
High efficiency is the result of a fast bipolar process
and a unique transistor layout that produces a high
voltage switch with only 0.07Ω typical on-resistance.
This permits the LT1374 to operate over an input voltage
range of 5.5V to 25V with switch currents up to 4.5A.
Figure 1 shows an example of the LT1374-5 in a typical
5V output step-down application. Efficiency for a 10V
input is shown in Figure 2. Note that efficiency remains
at over 88% from 0.5A up to the circuit’s maximum 4A
load current.
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.
100
D2
1N914
C3*
10μF TO
50μF
VIN
+
R1
88.7k
R2
59k
C2
0.27μF
BOOST
OUTPUT**
5V
VSW
LT1374-5 BIAS
SHDN
GND
L1**
20μH
FB
VC
CC
1.5nF
D1
MBRS330T3
+
* RIPPLE CURRENT RATING ≥ IOUT/2
** L1 SHOULD BE 5μH FOR LOAD CURRENTS TO 3.5A, 10μH TO 4A, 20μH ABOVE 4A
Figure 1. 5V Buck Converter
C1
100μF, 10V
SOLID
TANTALUM
EFFICIENCY (%)
INPUT
6V TO 25V
VOUT = 5V
VIN = 10V
L = 10μH
95
90
85
80
75
DN181 F01
70
0
0.5
1.0 1.5 2.0 2.5 3.0
OUTPUT CURRENT (A)
3.5
4.0
DN181 F02
Figure 2. 5V Efficiency vs Output Current
05/98/181_conv
4.5A in an SO-8
The output switch of the LT1374 is designed to minimize power dissipation from both switch resistance
and switch drive current. This allows the use of the
SO-8 packaged LT1374 in applications that would have
previously required a power package, especially when
selection is defined by high dynamic load currents.
Typical static and dynamic thermal characteristics for
various load currents are shown in Figures 3 and 4.
These measurements were made in still air with the
LT1374 SO-8 placed on a 4in2 double-sided circuit board.
Multiple vias conduct heat from the board’s topside to a
continuous copper plane on the bottom side. A typical
application for the SO-8 package is supplying a motor
driver. The motor may require 4A at start-up but only
2.5A when running. With a 60°C ambient temperature,
the SO-8 package can provide 4A of load current for
up to seven seconds, followed by 2.5A of continuous
current. If 4A of continuous current were required,
the surface mount DD package (θJA = 30°C/W) could
be used; for even higher power, use the TO-220 (θJC
= 4°C/W).
100
TEMPERATURE RISE (°C)
D2
1N914
VIN = 10V
VOUT = 5V
90
Dual Output SEPIC Converter
The circuit in Figure 5 generates both positive and negative 5V outputs from two windings on a single core. The
converter for the 5V output is a standard buck converter.
The –5V topology would be a simple flyback winding
coupled to the buck converter if C4 were not present.
C4 creates a SEPIC (single-ended primary inductance
converter) topology, which improves regulation and
reduces ripple current in L1. Without C4, the voltage
swing on L1B compared to L1A would vary due to relative loading and coupling losses. C4 provides a low
impedance path to maintain an equal voltage swing in
L1B, improving regulation. In a flyback converter, during
switch on-time, all the converter’s energy is stored in
L1A only, since no current flows in L1B. At switch off,
energy is transferred by magnetic coupling into L1B,
powering the –5V rail. C4 pulls L1B positive during
switch on-time, causing current to flow and energy to
build in L1B and C4. At switch off, the energy stored
in both L1B and C4 supplies the –5V rail. This reduces
the current in L1A and changes L1B’s current waveform
from square to triangular.
IOUT = 4A
80
70
C2
0.27μF
BOOST
INPUT
6V TO 25V
VIN
L1A*
6.8μH
LT1374-5 BIAS
60
50
SHDN
GND
IOUT = 3A
40
30
+
IOUT = 2A
20
C3
22μF
35V TANT
FB
VC
RC
470Ω
GND
0
0
5
10
15
20
TIME (SEC)
25
30
C4
4.7μF
+
DN181 F03
Figure 3. Temperature Rise vs Time
90
VIN = 10V
VOUT = 5V
TEMPERATURE RISE (°C)
80
+
C1**
100μF
10V TANT
+
C5**
100μF
10V TANT
D1
MBRD340
CC
0.01μF
IOUT = 1A
10
OUTPUT
5V
VSW
* L1 IS A SINGLE CORE WITH TWO WINDINGS
BH ELECTRONICS #501-0726 (612) 894-9590
** TOKIN IE475ZY5U-C304 (408) 432-8020
† IF LOAD CAN GO TO ZERO, AN OPTIONAL
PRELOAD OF 1k TO 5k MAY BE USED TO
IMPROVE LOAD REGULATION
L1B* D3
MBRD340
DN181 F05
OUTPUT
–5V†
70
Figure 5. Dual Output SEPIC Converter
60
50
40
30
20
10
0
0
0.5
1.0
1.5
2.0
LOAD CURRENT (A)
2.5
3.0
DN181 F04
Figure 4. Temperature Rise vs Load Current
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn181f_conv LT/TP 0598 340K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1998
Similar pages