28
LT1576/LT1576-5
sn1576 1576fs LT/TP 0999 4K • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1999
TYPICAL APPLICATION
U
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear-tech.com
BOOST
LT1576
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 25V
GND
1576 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
BIAS
GND
SHDN
V
C
+
+
+
+
RELATED PARTS
PART NUMBER DESCRIPTION COMMENTS
LT1074/LT1076 Step-Down Switching Regulators 40V Input, 100kHz, 5A and 2A
LTC
®
1148 High Efficiency Synchronous Step-Down Switching Regulator External FET Switches
LTC1149 High Efficiency Synchronous Step-Down Switching Regulator External FET Switches
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
LT1374 4.5A, 500kHz Step-Down Switching Regulator
LT1376 1.5A, 500kHz Step-Down Switching Regulator
LT1435/LT1436 High Efficiency Step-Down Converter External Switches, Low Noise
LT1676/LT1776 High Efficiency Step-Down Switching Regulators 7.4V to 60V Input, 100kHz/200kHz
LT1777 Low Noise Step-Down Switching Regulator 48V Input, Internally Limited dv/dt
Burst Mode is a trademark of Linear Technology Corporation.
Figure 17. Dual Output SEPIC Converter
Dual Output SEPIC␣ Converter
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
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 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.
