LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
APPLICATIONS INFORMATION
CMDSH-3
0.18μF
MBRS330T3*
REMOVABLE
5μH
BOOST
3.3V, 2A
4.7μF
ALTERNATE
SUPPLY
V
V
IN
SW
FB
INPUT
83k
LT1765-3.3
SHDN
SYNC GND
V
C
28.5k
2.2μF
2.2nF
B220A
1765 F09
* ONLY REQUIRED IF ADDITIONAL LOADS ON THE INPUT CAN SINK >10mA
Figure 9. Dual Source Supply with 6μA Reverse Leakage
D2
CMDSH-3
Dual Output Converter
ThecircuitinFigure11generatesbothpositiveandnegative
5Voutputswithasinglepieceofmagnetics.Thetwoinduc-
tors 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 cur-
rents, see Design Note 100
C2
0.18μF
L1
5μH
OUTPUT
5V
BOOST
INPUT
12V
V
V
SW
IN
2.5A
+
C1
LT1765-5
D1
C3
100μF
2.2μF
SHDN
SYNC GND
FB
C
V
C
SS
R3
2k
15nF
Q1
C
C
1765 F10
330pF
R4
47k
D1: B220A
Q1: 2N3904
Figure 10. Buck Converter with Adjustable Soft Start
(R4)(CSS)(VOUT
(VBE)
)
RiseTime =
Using the values shown in Figure 10,
(47 •103)(15•10–9)(5)
RiseTime =
= 5ms
0.7
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.
1765fd
15
Linear [ Linear ]
