IN

CIN(RMS)

i

s 1

o

V

1

I

2 3

V

f L

1

V

=

æ

ö

+

ç

÷

ç

÷

è

ø

iSW(A)

IL1+ IL2

t

ICL

ISW

'iL1 +'iL2

iSW

SW(PEAK )

L1

L2

L1

L2

L1

L2

IN

S

OUT

1

2

i

i

i

I

I

i

i

V

D T

D

1

1

I

1

1-D

2

L

L

=

+

=

+

+ D

+ D

æ

ö

´ ´

æ

ö

=

´

+

+

´

+

ç

÷

ç

÷

è

ø

è

ø

IN

S

L1

1

V

D

T

i

2

L

´ ´

D

=

´

L1

L2

OUT

D

D

I

I

I

1-D

1-D

=

=

IN

S

L2

2

V

D

T

i

2

L

´ ´

D

=

´

LM2611

SNOS965J – JUNE 2001 – REVISED DECEMBER 2015

www.ti.com

Feature Description (continued)

(3)

(4)

(5)

Use these equations to choose correct core sizes for the inductors. The design of the LM2611's internal

compensation assumes L1 and L2 are equal to 10 to 22 µH, thus TI recommends staying within this range.

7.3.3 Switch Current Limit

The LM2611 incorporates a separate current limit comparator, making current limit independent of any other

variables. The current limit comparator measures the switch current versus a reference that represents current

limit. If at any time the switch current surpasses the current limit, the switch opens until the next switching period.

To determine the maximum load for a given set of conditions, both the input and output inductor currents must be

considered. The switch current is equal to iL1 + iL2, and is drawn in Figure 16. In summary, Equation 6 shows:

(6)

ISW(PEAK) must be less than the current limit (1.2 A typical), but will also be limited by the thermal resistivity of the

LM2611 device's 5-pin, SOT-23 package (

θJA = 265°C/W).

The peak value is equal to the sum of the average currents through L1 and L2 and the average-to-peak current

ripples through L1 and L2.

Figure 16. Switch Current Waveform in a Cuk Converter.

7.3.4 Input Capacitor

The input current waveform to a Cuk converter is continuous and triangular, as shown in Figure 14. The input

inductor insures that the input capacitor sees fairly low ripple currents. However, as the input inductor gets

smaller, the input ripple goes up. The RMS current in the input capacitor is shown in Equation 7.

(7)

The input capacitor should be capable of handling the RMS current. Although the input capacitor is not so critical

in a Cuk converter, a 10-µF or higher value good quality capacitor prevents any impedance interactions with the

input supply. TI recommends connecting a 0.1-µF or 1-µF ceramic bypass capacitor on the VIN pin (pin 5) of the

IC. This capacitor must be connected very close to pin 5 (within 0.2 inches).

10

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