LM2611AMF datasheet(8/15 Pages) NSC

8 / 15 page

Operation (Continued)

(1)

(2)

The optimal position for this zero-pole pair will vary with

circuit parameters such as D, I

OUT,COUT, L1, L2, and CCUK.

For most cases, the value for the zero frequency is between

5 kHz to 20 kHz. Notice how the pole position,

p, is depen-

dant on the feedback resistors R

FB1 and RFB2, and therefore

also dependant on the output voltage. As the output voltage

becomes closer to −1.26V, the pole moves towards the zero,

tending to cancel it out. If the absolute magnitude of the

output voltage is less than 3.3V, adding the zero-pole pair

will not have much effect on the response.

Hysteretic Mode

As the output current decreases, there will come a point

when the energy stored in the Cuk capacitor is more than the

energy required by the load. The excess energy is absorbed

by the output capacitor, causing the output voltage to in-

crease out of regulation. The LM2611 detects when this

happens and enters a pulse skipping, or hysteretic mode. In

hysteretic mode, the output voltage ripple will increase, as

illustrated in Figure 7 and Figure 8.

20018120

) LEVEL 3

FIGURE 5. 130mA to 400mA Transient Response

of the circuit in Figure 9 with C

FF= 1nF

20018122

) LEVEL 3

FIGURE 6. 130mA to 400mA Transient Response

of the circuit in Figure 9 with C

FF disconnected

20018121

FIGURE 7. The LM2611 in PWM mode has very low

ripple

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