Datasheet

LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
15
1765fd
APPLICATIONS INFORMATION
Figure 10. Buck Converter with Adjustable Soft Start
BOOST
LT1765-5
V
IN
OUTPUT
5V
2.5A
INPUT
12V
1765 F10
C2
0.18μF
C1
100μF
C
SS
15nF
C
C
330pF
D1
C3
2.2μF
D2
CMDSH-3
L1
5μH
R3
2k
V
SW
FB
SHDN
GND
V
C
SYNC
+
R4
47k
Q1
D1: B220A
Q1: 2N3904
RiseTime
RC V
V
SS OUT
BE
=
()( )( )
()
4
Using the values shown in Figure 10,
RiseTime ms==
(• )(• )()
.
–
47 10 15 10 5
07
5
39
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 Converter
The circuit in Figure 11 generates both positive and negative
5V outputs with a single piece of magnetics. The two induc-
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 fl yback 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 fl yback converter, during
switch on time, all the converter’s energy is stored in L1A
only, since no current fl ows 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 fl ow, 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
Figure 9. Dual Source Supply with 6μA Reverse Leakage
3.3V, 2A
* ONLY REQUIRED IF ADDITIONAL LOADS ON THE INPUT CAN SINK >10mA
REMOVABLE
INPUT
0.18μF
2.2nF
83k
B220A
1765 F09
2.2μF
MBRS330T3*
CMDSH-3
5μH
4.7μF
BOOST
LT1765-3.3
V
IN
V
SW
FBSHDN
GND
V
C
SYNC
ALTERNATE
SUPPLY
28.5k