LT1578 Linear Technology, LT1578 Datasheet - Page 28

• Manufacturer Part Number: LT1578
• Description: 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

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