LT3430 Linear Technology, LT3430 Datasheet - Page 24

• Manufacturer Part Number: LT3430
• Description: High Voltage/ 3A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT3430
APPLICATIO S I FOR ATIO
DUAL OUTPUT SEPIC CONVERTER
The circuit in Figure 14 generates both positive and
negative 5V outputs with a single piece of magnetics. The
two inductors shown are actually just two windings on a
standard Coiltronics inductor. 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 a 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, including
maximum output currents, see Design Note 100.
24
U
U
* L1 IS A SINGLE CORE WITH TWO WINDINGS
†
7.5V TO 60V
COILTRONICS #CTX25-4A
IF LOAD CAN GO TO ZERO, AN OPTIONAL
PRELOAD OF 1k TO 5k MAY BE USED TO
IMPROVE LOAD REGULATION
D1, D3: 30BQ060
CERAMIC
GND
V
IN
4.7 F
100V
W
C3
Figure 14. Dual Output SEPIC Converter
U
V
SHDN
SYNC
GND
0.022 F
IN
LT3430
3.3k
C
BOOST
C
R
C
V
SW
C
FB
100 F
TANT
0.68 F
C
220pF
10V
C4
F
POSITIVE-TO-NEGATIVE CONVERTER
The circuit in Figure 15 is a positive-to-negative topology
using a grounded inductor. It differs from the standard
approach in the way the IC chip derives its feedback signal
because the LT3430 accepts only positive feedback sig-
nals. The ground pin must be tied to the regulated negative
output. A resistor divider to the FB pin then provides the
proper feedback voltage for the chip.
The following equation can be used to calculate maximum
load current for the positive-to-negative converter:
Example: with V
V
C2
F
+
I
V
V
V
0.15 = Switch voltage drop at 3A
I
= 0.52V, I
P
MAX
IN
OUT
F
= Maximum rated switch current
= Catch diode forward voltage
= Minimum input voltage
= Output voltage
L1B*
MMSD914TI
D1
D3
25 H
D2
I
L1A*
P
15.4k
4.99k
P
R1
R2
–
= 3A: I
10V TANT
2
IN(MIN)
100 F
(
(
V
V
OUT
C5
OUT
(
+
V
MAX
IN
+
)(
= 5.5V, V
C1
100 F
10V TANT
3430 F14
V
V
= 0.6A.
V
OUT
IN
IN
)( )( )
– . )(
V
5V
V
–5V
OUT
OUT
)
f L
0 15
†
OUT
(
V
V
= 12V, L = 10 H,
OUT
OUT
)(
V
IN
V
F
– . )
)
sn3430 3430is
0 15