Datasheet

Design Procedure (Continued)
In some applications, the ESR zero F
z1
can not be cancelled
by F
p2
. Then, C
c2
is needed to introduce F
pc2
to cancel the
ESR zero, F
p2
= 1/(2πC
c2
R
o
\R
c
).
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, C
c1
= 2.2nF, and R
c
= 15KΩ are
good choices for most applications. If the ESR zero is too
low to be cancelled by F
p2
, add C
c2
.
If the transient response to a step load is important, choose
R
C
to be higher than 10kΩ.
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:
R
FB1
=R
FB2
*(V
OUT
-V
REF
)/V
REF
where V
REF
= 1.238V.
Select resistors between 10kΩ and 100kΩ. (1% or higher
accuracy metal film resistors for R
FB1
and R
FB2
.)
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
powered by a separate source, the only requirement of the
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.
LM2655
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