Example Circuits (Continued)
20049407
FIGURE 6. 1.8V and 3.3V, 1A, 1.4MHz, Simultaneous
The circuits in Figure 6 are intended for ADSL applications,
where the high switching frequency keeps noise out of the
data transmission range. In this design, the 1.8 and 3.3V
outputs come up simultaneously by using the same softstart
capacitor. Because two current sources now charge the
same capacitor, the capacitance must be doubled to achieve
the same softstart time. (Here, 40nF is used to achieve a
5ms softstart time.) A common softstart capacitor means
that, should one circuit enter current limit, the other circuit
will also enter current limit. In addition, if both circuits are
built with the LM2727, a UVP or OVP fault on one circuit will
cause both circuits to latch off. The additional compensation
components Rc2 and Cc3 are needed for the low ESR, all
ceramic output capacitors, and the wide (3x) range of Vin.
www.national.com
16
NSC [ National Semiconductor ]
