LT1766 Linear Technology, LT1766 Datasheet - Page 23

• Manufacturer Part Number: LT1766
• Description: 5.5V to 60V 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

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.
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.
RiseTime
Rise Time
INPUT
40V
Figure 13. Buck Converter with Adjustable Soft-Start
C3
2.2 F
50V
CER
V
SHDN
SYNC
IN
0.022 F
BOOST
LT1766
2.2k
C
R
GND
C
47 10
R
4
C
U
BIAS
•
C
V
SW
SS
FB
C
V
BE
3
C
220pF
F
0.33 F
U
0 7
V
15 10
.
C2
OUT
Q1
•
D1
1N4148W
–
47 H
D2
9
W
R4
47k
L1
100 F
R3
2k
C1
5
15nF
C
+
SS
5
ms
R1
15.4k
R2
4.99k
U
1766 F13
OUTPUT
5V
1A
* L1 IS A SINGLE CORE WITH TWO WINDINGS
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.
†
TO 60V
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 LT1766 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:
GND
COILTRONICS #CTX50-3A
IF LOAD CAN GO TO ZERO, AN OPTIONAL
PRELOAD OF 1k TO 5k MAY BE USED TO
IMPROVE LOAD REGULATION
D1, D3: 10MQ060N
2.2 F
100V
7.5V
CER
V
C3
I
IN
MAX
Figure 14. Dual Output SEPIC Converter
I
P
V
SHDN
SYNC
GND
0.022 F
IN
–
2
LT1766
2.2k
C
BOOST
R
C
(
(
C
V
V
OUT
OUT
(
V
V
IN
SW
C
FB
100 F
TANT
0.33 F
)(
10V
C
220pF
C4
F
V
LT1766/LT1766-5
V
V
C2
OUT
IN
IN
+
)( )( )
– . )(
)
f L
0 3
L1B*
D1
1N4148W
D3
50 H
D2
L1A*
15.4k
4.99k
V
(
V
OUT
R1
R2
OUT
100 F
TANT
10V
C5
)(
+
V
V
+
F
1766 F14
IN
)
C1
100 F
10V
TANT
V
5V
(SEE DN100
FOR MAX I
OUT1
23
– . )
V
–5V
0 3
OUT2
1766fa
†
OUT
)