LM2665 datasheet(7/8 Pages) NSC

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Other Applications (Continued)

Cascading Devices

Cascading the LM2665s is an easy way to produce a greater

voltage (A two-stage cascade circuit is shown in Figure 4).

The effective output resistance is equal to the weighted sum

of each individual device:

out = 1.5Rout_1 +Rout_2

Note that, the increasing of the number of cascading stages

is pracitically limited since it significantly reduces the effi-

ciency, increases the output resistance and output voltage

ripple.

Regulating V

OUT

It is possible to regulate the output of the LM2665 by use of

a low dropout regulator (such as LP2980-5.0). The whole

converter is depicted in Figure 5.

A different output voltage is possible by use of LP2980-3.3,

LP2980-3.0, or LP2980-adj.

Note that, the following conditions must be satisfied simulta-

neously for worst case design:

in_min

out_min +Vdrop_max (LP2980) + Iout_max xRout_max (LM2665)

in_max < Vout_max +Vdrop_min (LP2980) + Iout_min xRout_min (LM2665)

DS100049-19

FIGURE 3. Lowering Output Resistance by Paralleling Devices

DS100049-20

FIGURE 4. Increasing Output Voltage by Cascading Devices

DS100049-21

FIGURE 5. Generate a Regulated +5V from +3V Input Voltage

LM2665 Fiches technique(PDF) 7 Page - National Semiconductor (TI)