Datasheet
V
OUT
=
-V
IN
1-D
D
+ L1 -
- L2 +
+ L1 -
- L2 +
V
IN
V
IN
C
OUT
C
OUT
V
OUT
+ C
CUK
-
+
-
V
OUT
+ C
CUK
-
+
-
a
b
PWM COMPARATOR
¦
Q1
Q2
x10
R3
30k
R4
140k
R5 R6
R
C
C
C
RAMP
GENERATOR
1.4MHz
OSCILLATOR
Q
R
s
R
0.05
CURRENT LIMIT
COMPARATOR
DRIVER
+
-
-
+
g
m
-
+
R1
EXTERNAL
R2
EXTERNAL
C
FF
(OPTIONAL)
1 SW
2 GND
5
3 NFB
4
V
IN
SHDN
SHUTDOWN
FF
V
O
R
THERMAL
SHUTDOWN
LM2611
SNOS965I –JUNE 2001–REVISED APRIL 2013
www.ti.com
Block Diagram
Operation
Cuk Converter
Figure 13. Operating Cycles of a Cuk Converter
The LM2611 is a current mode, fixed frequency PWM switching regulator with a −1.23V reference that makes it
ideal for use in a Cuk converter. The Cuk converter inverts the input and can step up or step down the absolute
value. Using inductors on both the input and output, the Cuk converter produces very little input and output
current ripple. This is a significant advantage over other inverting topologies such as the buck-boost and flyback.
The operating states of the Cuk converter are shown in Figure 13. During the first cycle, the transistor switch is
closed and the diode is open. L1 is charged by the source and L2 is charged by C
CUK
, while the output current is
provided by L2. In the second cycle, L1 charges C
CUK
and L2 discharges through the load. By applying the volt-
second balance to either of the inductors, the relationship of V
OUT
to the duty cycle (D) is found to be:
(1)
The following sections review the steady-state design of the LM2611 Cuk converter.
Output and Input Inductor
Figure 14 and Figure 15 show the steady-state voltage and current waveforms for L1 and L2, respectively.
Referring to Figure 13 (a), when the switch is closed, V
IN
is applied across L1. In the next cycle, the switch opens
and the diode becomes forward biased, and V
OUT
is applied across L1 (the voltage across C
CUK
is V
IN
− V
OUT
.
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