lt1765es8-trpbf Linear Technology Corporation, lt1765es8-trpbf Datasheet - Page 15

• Manufacturer Part Number: lt1765es8-trpbf
• Description: Monolithic 3a, 1.25mhz Step-down Switching Regulators
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Using the values shown in Figure 10,
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.
INPUT
RiseTime
RiseTime
2.2µF
12V
C3
Figure 10. Buck Converter with Adjustable Soft Start
+
=
=
V
SHDN
IN
SYNC
( )(
(
R C
47 10 15 10
4
LT1765-5
330pF
BOOST
U
•
GND V
C
(
C
V
SS
REMOVABLE
3
BE
V
)( •
C
)(
SW
FB
0 7
U
)
0.18µF
V
.
INPUT
OUT
Q1
C2
)
–
D1
Figure 9. Dual Source Supply with 6µA Reverse Leakage
9
MBRS330T3*
W
)( )
CMDSH-3
83k
28.5k
* ONLY REQUIRED IF ADDITIONAL LOADS ON THE INPUT CAN SINK >10mA
5µH
L1
R4
47k
5
100µF
D2
R3
2k
C1
=
D1: B220A
Q1: 2N3904
15nF
5
C
SS
ms
2.2µF
V
SHDN
U
IN
SYNC
1765 F10
OUTPUT
5V
2.5A
LT1765-3.3
BOOST
GND
V
C
V
SW
FB
2.2nF
Dual Output Converter
The circuit in Figure 11 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 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.
LT1765/LT1765-1.8/LT1765-2.5/
0.18µF
B220A
CMDSH-3
5µH
3.3V, 2A
4.7µF
LT1765-3.3/LT1765-5
ALTERNATE
1765 F09
SUPPLY
15
1765fb