LM2655 datasheet(12/16 Pages) NSC

12 / 16 page

APPLICATION CIRCUITS (Continued)

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 ex-

plained in the INPUT CAPACITOR section.

OBTAINING OUTPUT VOLTAGES OF LESS THAN 1.25V

Some applications require output voltages less than 1.25V.

The circuit shown in

Figure 5 will allow the LM2655 to do

such a conversion. By referencing the two feedback resis-

tors to V

ADJ (VADJ > 1.24V), VOUT can be adjusted from 0V

to V

ADJ by the equation:

OUT =(VREF-VADJ)*(RFB1+RFB2)/RFB2 +VADJ

where V

REF = 1.24V. VADJ can be any voltage higher than

REF (1.24V). In Figure 5,VADJ is produced by an LMV431

adjustable reference following the equation:

ADJ = 1.24*(RADJ1/RADJ2 + 1).

DS101284-25

FIGURE 3. Programmable output voltage.

DS101284-23

FIGURE 4. Extended input voltage range.

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