LM25005 datasheet(18/21 Pages) NSC

18 / 21 page

Application Information (Continued)

BIAS POWER DISSIPATION REDUCTION

Buck regulators operating with high input voltage can dissi-

pate an appreciable amount of power for the bias of the IC.

The V

CC regulator must step-down the input voltage VIN to a

nominal V

CC level of 7V. The large voltage drop across the

CC regulator translates into a large power dissipation within

the Vcc regulator. There are several techniques that can

significantly reduce this bias regulator power dissipation.

Figures 11 and 12 depict two methods to bias the IC from the

output voltage. In each case the internal Vcc regulator is

used to initially bias the VCC pin. After the output voltage is

established, the VCC pin potential is raised above the nomi-

nal 7V regulation level, which effectively disables the internal

CC regulator. The voltage applied to the VCC pin should

never exceed 14V. The V

CC voltage should never be larger

than the V

IN voltage.

20170018

FIGURE 11. VCC Bias from VOUT for 8V < VOUT < 14V

20170019

FIGURE 12. VCC Bias with Additional Winding on the Output Inductor

LM25005 Datasheet(PDF) 18 Page - National Semiconductor (TI)