LM2611AMF datasheet(7/13 Pages) NSC

7 / 13 page

Operation

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 con-

verter produces very little input and output current ripple.

This is a significant advantage over other inverting topolo-

gies such as the buck-boost and flyback.

The operating states of the Cuk converter are shown in

Figure 1. 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:

The following sections review the steady-state design of the

LM2611 Cuk converter.

Output and Input Inductor

Figure 2 and Figure 3 show the steady-state voltage and

current waveforms for L1 and L2, respectively. Referring to

Figure 1 (a), when the switch is closed, V

IN is applied across

L1. In the next cycle, the switch opens and the diode be-

comes forward biased, and V

OUT is applied across L1 (the

voltage across C

CUK is VIN −VOUT.

The voltage and current waveforms of inductor L2 are shown

Figure 3. During the first cycle of operation, when the

switch is closed, V

IN is applied across L2. When the switch

opens, V

OUT is applied across L2.

20018105

FIGURE 1. Operating Cycles of a Cuk Converter

20018103

FIGURE 2. Voltage and Current Waveforms in Inductor

L1 of a Cuk Converter

20018104

FIGURE 3. Voltage and Current Waveforms in Inductor

L2 of a Cuk Converter

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