A general rule of thumb is to keep f

Cuk designs. Low ESR tantalum capacitors will usually be

rated for at least 180mA in a voltage rating of 10V or above.

However the ESR in a tantalum capacitor (even in a low ESR

tantalum capacitor) is much higher than in a ceramic capaci-

tor and could place f

run unstable.

Improving Transient Response/Compensation

The compensator in the LM2611 is internal. However, a

zero-pole pair can be added to the open loop frequency

response by inserting a feed forward capacitor, C

allel to the top feedback resistor (R

bandwidth can be improved with the added zero-pole pair.

This inturn will improve the transient response to a step load

change (see

Figure 6 and Figure 7). The position of the

zero-pole pair is a function of the feedback resistors and the

capacitor value:

(1)

(2)

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

circuit parameters such as D, I

For most cases, placing the zero at 34 krad/s (5.4 kHz) is

effective (this corresponds to the values on the front page

schematic). Notice how the pole position,

ω

on the feedback resistors R

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.

Hysteric 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 8 and Figure 9.

20018120

) LEVEL 3

FIGURE 6. 130mA to 400mA Transient Response

of the circuit in

Figure 10 with C

20018122

) LEVEL 3

FIGURE 7. 130mA to 400mA Transient Response

of the circuit in

Figure 10 with C

20018121

FIGURE 8. The LM2611 in PWM mode has very low

ripple

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