LM2655 datasheet(13/18 Pages) TI

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Design Procedure (Continued)

In some applications, the ESR zero F

z1 can not be cancelled

by F

p2. Then, Cc2 is needed to introduce Fpc2 to cancel the

ESR zero, F

p2 = 1/(2

πC

c2Ro\Rc).

The rule of thumb is to have more than 45˚ phase margin at

the crossover frequency (G=1).

If C

OUT is higher than 68µF, Cc1 = 2.2nF, and Rc = 15K

Ω are

good choices for most applications. If the ESR zero is too

low to be cancelled by F

p2, add Cc2.

If the transient response to a step load is important, choose

C to be higher than 10k

Ω.

Application Circuits

PROGRAMMABLE OUTPUT VOLTAGE

Using the adjustable output version of the LM2655 as shown

in Figure 3, output voltages between 1.24V and 13V can be

achieved. Use the following formula to select the appropriate

resistor values:

FB1 =RFB2*(VOUT -VREF)/VREF

where V

REF = 1.238V.

Select resistors between 10k

Ω and 100kΩ. (1% or higher

accuracy metal film resistors for R

FB1 and RFB2.)

EXTENDING INPUT VOLTAGE RANGE

Figure 4 shows a way to configure the LM2655 so that input

voltages of less than 4V can be converted. This circuit

makes use of the separate analog and power V

IN pins. All

the supervisory circuits of the LM2655 are powered through

the AV

IN pin, while the source voltage that is to be converted

is input to the PV

IN pins. The internal circuitry of the LM2655

has an operating range of 4V < V

CC < 14V, so a voltage

within this range must be applied to AV

IN. This source may

be low power because it only needs to supply 5mA. An input

capacitor should be connected across this source, and a

small bypass capacitor should be placed physically close to

the AV

IN pin to ground. With all the internal circuitry being

voltage at PV

IN is that it be slightly higher (

∼500mV) than the

desired output voltage. The source connected to PV

IN will

also need an input capacitor and bypass capacitor, but the

input capacitor must be selected following the guidelines

explained in the INPUT CAPACITOR section.

10128425

FIGURE 3. Programmable output voltage.

10128423

FIGURE 4. Extended input voltage range.

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