LT1578 データシートの表示（PDF） - Linear Technology
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LT1578 Datasheet PDF : 28 Pages
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LT1578/LT1578-2.5
TYPICAL APPLICATION
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
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.
INPUT
6V TO 15V
C2
BOOST
0.33µF
VIN
VSW
LT1578
SHDN
GND
FB
VC
+ C3
22µF
35V TANT
GND
* 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
CC
100pF
C4** +
100µF
D2
1N914
L1A*
33µH R1
15.8k
+
D1
1N5818
R2
4.99k
+
L1B* D3
1N5818
Figure 17. Dual Output SEPIC Converter
OUTPUT
5V
C1**
100µF
10V TANT
C5**
100µF
10V TANT
OUTPUT
–5V†
1578 F17
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Burst Mode is a trademark of Linear Technology Corporation.
28 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX:(408)434-0507 q www.linear-tech.com
1578f LT/TP 0100 4K • PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 1999
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