LM2742 datasheet(17/22 Pages) NSC

17 / 22 page

Example Circuits (Continued)

This circuit shows the LM2742 paired with a cost effective

solution to provide the 5V chip power supply, using no extra

components other than the LM78L05 regulator itself. The

input voltage comes from a ’brick’ power supply which does

not regulate the 12V line tightly. Additional, inexpensive 10uF

ceramic capacitors (Cinx and Cox) help isolate devices with

sensitive databands, such as DSL and cable modems, from

switching noise and harmonics.

In situations where low cost is very important, the LM2742

can also be used as an asynchronous controller, as shown in

the above circuit. Although a a schottky diode in place of the

bottom FET will not be as efficient, it will cost much less than

the FET. The 5V at low current needed to run the LM2742

could come from a zener diode or inexpensive regulator,

such as the one shown in Figure 7. Because the LM2742

senses current in the low side MOSFET, the current limit

feature will not function in an asynchronous design. The

ISEN pin should be left open in this case.

20087508

FIGURE 7. 12V Unregulated to 3.3V, 3A, 750kHz

20087509

FIGURE 8. 12V to 5V, 1.8A, 100kHz

LM2742 Datasheet(PDF) 17 Page - National Semiconductor (TI)