LT1576/LT1576-5
U
TYPICAL APPLICATION
Dual Output SEPIC Converter
losses. C4 provides a low impedance path to maintain an
equal voltage swing in L1B, improving regulation. In a
flybackconverter,duringswitchontime,alltheconverter’s
energyisstoredinL1Aonly, sincenocurrentflowsinL1B.
At switch off, energy is transferred by magnetic coupling
into L1B, powering the –5V rail. C4 pulls L1B positive
duringswitchontime, causingcurrenttoflow, andenergy
to build in L1B and C4. At switch off, the energy stored in
both L1B and C4 supply the –5V rail. This reduces the
current in L1A and changes L1B current waveform from
square to triangular. For details on this circuit see Design
Note 100.
The circuit in Figure 17 generates both positive and
negative 5V outputs with a single piece of magnetics. L1
is a 33µH surface mount inductor from Coiltronics. It is
manufactured with two identical windings that can be
connected in series or parallel. The topology 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 the 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
C2
0.33µF
D2
1N914
L1A*
33µH
BOOST
INPUT
OUTPUT
5V
V
IN
V
SW
6V TO 25V
LT1576
BIAS
FB
R1
15.8k
+
R2
4.99k
SHDN
GND
V
C
C1**
100µF
D1
1N5818
10V TANT
+
C3
C
C
22µF
100pF
35V TANT
GND
+
+
C5**
C4**
100µF
* L1 IS A SINGLE CORE WITH TWO WINDINGS
COILTRONICS CTX33-2
100µF
L1B*
D3
1N5818
10V TANT
** AVX TSPD107M010
†
OUTPUT
IF LOAD CAN GO TO ZERO, AN OPTIONAL
†
–5V
PRELOAD OF 1k TO 5k MAY BE USED TO
IMPROVE LOAD REGULATION
1576 F17
Figure 17. Dual Output SEPIC Converter
RELATED PARTS
PART NUMBER
LT1074/LT1076
LTC®1148
DESCRIPTION
COMMENTS
Step-Down Switching Regulators
40V Input, 100kHz, 5A and 2A
External FET Switches
High Efficiency Synchronous Step-Down Switching Regulator
High Efficiency Synchronous Step-Down Switching Regulator
High Efficiency Step-Down and Inverting DC/DC Converter
High Efficiency DC/DC Converter
LTC1149
External FET Switches
LTC1174
0.5A, 150kHz Burst ModeTM Operation
42V, 6A, 500kHz Switch
35V, 3A, 500kHz Switch
Boost Topology
LT1370
LT1371
High Efficiency DC/DC Converter
LT1372/LT1377
LT1374
500kHz and 1MHz High Efficiency 1.5A Switching Regulators
4.5A, 500kHz Step-Down Switching Regulator
1.5A, 500kHz Step-Down Switching Regulator
High Efficiency Step-Down Converter
LT1376
LT1435/LT1436
LT1676/LT1776
LT1777
External Switches, Low Noise
High Efficiency Step-Down Switching Regulators
Low Noise Step-Down Switching Regulator
7.4V to 60V Input, 100kHz/200kHz
48V Input, Internally Limited dv/dt
Burst Mode is a trademark of Linear Technology Corporation.
1576f LT/TP 0999 4K • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
28
●
●
(408)432-1900 FAX:(408)434-0507 www.linear-tech.com
LINEAR TECHNOLOGY CORPORATION 1999
Linear [ Linear ]
