LT1767 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1767' PDF : 20 Pages View PDF
LT1767/LT1767-1.8/
LT1767-2.5/LT1767-3.3/LT1767-5
APPLICATIONS INFORMATION
The ramp is linear and rise times in the order of 100ms are
possible. Since the circuit is voltage controlled, the ramp
rate is unaffected by load characteristics and maximum
output current is unchanged. Variants of this circuit can
be used for sequencing multiple regulator outputs.
Dual Output SEPIC Converter
The circuit in Figure 11 generates both positive and negative
5V outputs with a single piece of magnetics. The two in-
ductors shown are actually just two windings on a standard
B H Electronics 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.
CMDSH-3
REMOVABLE
INPUT
UPS120*
83k
28.5k
0.1µF
BOOST
VIN
VSW
LT1767-3.3
SHDN
FB
SYNC GND VC
2.2µF
1.5nF
4.7k
5µH
UPS120
3.3V, 1A ALTERNATE
SUPPLY
10µF
1767 F09
* ONLY REQUIRED IF INPUT CAN SINK >10mA
Figure 9. Dual Source Supply with 6µA Reverse Leakage
D2
CMDSH-3
C2
0.1µF
VIN
12V
VIN
C3
2.2µF
BOOST
LT1767-5
VSW
D1
SHDN
FB
SYNC GND VC
CC
330pF
Q1
D1: UPS120
Q1: 2N3904
L1
5µH
C1 +
100µF
OUTPUT
5V
1A
R3
2k
CSS
15nF
R4
47k
1767 F10
C2
0.1µF
VIN
6V TO 15V
BOOST
VIN
VSW
LT1767-5
SHDN
FB
SYNC GND VC
C3
2.2µF
16V
CERAMIC
CC
330pF
GND
C4
* L1 IS A SINGLE CORE WITH TWO WINDINGS 2.2µF
BH ELECTRONICS #511-1013
† IF LOAD CAN GO TO ZERO,
16V
CERAMIC
AN OPTIONAL PRELOAD OF 1k TO 5k
MAY BE USED TO IMPROVE LOAD REGULATION
D1, D3: UPS120
D2
CMDSH-3
L1A*
9µH
OUTPUT
5V
+ C1
100µF
10V TANT
D1
C5 +
L1B* 100µF
10V TANT
D3
OUTPUT
–5V†
1767 F11
Figure 10. Buck Converter with Adjustable Soft-Start
Figure 11. Dual Output SEPIC Converter
16
For more information www.linear.com/LT1767
1767fb
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