LT1576_15 Datasheet, PDF (28/28 Pages) Linear Technology – 1.5A, 200kHz Step-Down Switching Regulator

LT1576/LT1576-5
TYPICAL APPLICATION
Dual Output SEPIC␣ Converter
The circuit in Figure 17 generates both positive and
negative 5V outputs with a single piece of magnetics. 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.
INPUT
6V TO 25V
C2
BOOST
0.33µF
V_IN
V_SW
LT1576 BIAS
SHDN
GND
FB
V_C
+ 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
C_C
100pF
C4** ⁺
100µF
L1A*
33µH
D2
1N914
D1
1N5818
R1
15.8k
+
R2
4.99k
C1**
100µF
10V TANT
L1B*
D3
1N5818
+ C5**
100µF
10V TANT
Figure 17. Dual Output SEPIC Converter
OUTPUT
5V
OUTPUT
–5V^†
1576 F17
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Burst Mode is a trademark of Linear Technology Corporation.
COMMENTS
40V Input, 100kHz, 5A and 2A
External FET Switches
External FET Switches
0.5A, 150kHz Burst Mode^TM Operation
42V, 6A, 500kHz Switch
35V, 3A, 500kHz Switch
Boost Topology
External Switches, Low Noise
7.4V to 60V Input, 100kHz/200kHz
48V Input, Internally Limited dv/dt
28 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
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sn1576 1576fs LT/TP 0999 4K • PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 1999