Example Circuits (Continued)
20049408
FIGURE 7. 12V Unregulated to 3.3V, 3A, 750kHz
This circuit shows the LM27x7 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.
20049409
FIGURE 8. 12V to 5V, 1.8A, 100kHz
In situations where low cost is very important, the LM27x7
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 LM27x7
could come from a zener diode or inexpensive regulator,
such as the one shown in Figure 7. Because the LM27x7
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.
17
www.national.com
NSC [ National Semiconductor ]
