LM3578AM datasheet(18/20 Pages) NSC

18 / 20 page

Typical Applications (Continued)

A minimum value for an ideal output capacitor C3, could be

calculated asC=I

o xt/

∆V where I

o is the load current, t is

the transistor on time (typically 0.4/f

osc), and

∆V is the peak-

to-peak output voltage ripple. A larger output capacitor than

this theoretical value should be used since electrolytics have

poor high frequency performance. Experience has shown

that a value from 5 to 10 times the calculated value should

be used.

For good efficiency, the diodes must have a low forward

voltage drop and be fast switching. 1N5819 Schottky diodes

work well.

Transformer selection should be picked for an output tran-

sistor “on” time of 0.4/f

osc, and a primary inductance high

enough to prevent the output transistor switch from ramping

higher than the transistor’s rating of 750 mA. Pulse Engi-

neering (San Diego, Calif.) and Renco Electronics, Inc.

(Deer Park, N.Y.) can provide further assistance in selecting

the proper transformer for a specific application need. The

transformer used in Figure 23 was a Pulse Engineering

PE-64287.

LM3578AM Datenblatt(PDF) 18 Page - National Semiconductor (TI)