28
LT1578/LT1578-2.5
PART NUMBER DESCRIPTION COMMENTS
LT1074/LT1076 Step-Down Switching Regulators 40V Input, 100kHz, 5A and 2A
LTC1174 High Efficiency Step-Down and Inverting DC/DC Converter 0.5A, 150kHz Burst Mode
TM
Operation
LT1370 High Efficiency DC/DC Converter 42V, 6A, 500kHz Switch
LT1371 High Efficiency DC/DC Converter 35V, 3A, 500kHz Switch
LT1372/LT1377 500kHz and 1MHz High Efficiency 1.5A Switching Regulators Boost Topology
LT1376 High Efficiency Step-Down Switching Regulator 25V, 1.5A, 500kHz Switch
LT1507 High Efficiency Step-Down Switching Regulator 15V, 1.5A, 500kHz Switch
LT1676/LT1776 High Efficiency Step-Down Switching Regulators 7.4V to 60V Input, 100kHz/200kHz
LTC1772 SOT-23 Low Voltage Step-Down DC/DC Controller 550kHz, Drives PFET, 6-Lead SOT-23 Package; up to 4.5A Output Current
LTC1735 High Efficiency Step-Down Converter Synchronous Buck Controller Drives External MOSFETs
LT1777 Low Noise Step-Down Switching Regulator 48V Input, Internally Limited dV/dt, Programmable di/dt
Burst Mode is a trademark of Linear Technology Corporation.
1578f LT/TP 0100 4K • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1999
TYPICAL APPLICATION
U
Dual Output SEPIC␣ Converter
The circuit in Figure 17 generates both positive and
negative 5V outputs with a single piece of magnetics. The
inductor 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) to-
pology 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
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear-tech.com
BOOST
LT1578
V
IN
OUTPUT
5V
OUTPUT
–5V
†
* L1 IS A SINGLE CORE WITH TWO WINDINGS
COILTRONICS CTX33-2
** AVX TSPD107M010
†
IF LOAD CAN GO TO ZERO, AN OPTIONAL
PRELOAD OF 1k TO 5k MAY BE USED TO
IMPROVE LOAD REGULATION
INPUT
6V
TO 15V
GND
1578 F17
C2
0.33µF
C
C
100pF
D1
1N5818
C1**
100µF
10V TANT
C5**
100µF
10V TANT
C3
22µF
35V TANT
C4**
100µF
D2
1N914
R1
15.8k
R2
4.99k
D3
1N5818
L1A*
33µH
L1B*
V
SW
FB
GND
SHDN
V
C
+
+
+
+
RELATED PARTS
Figure 17. Dual Output SEPIC Converter
coupling losses. C4 provides a low impedance path to
maintain an equal voltage swing in L1B, improving regu-
lation. 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 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.
