Datasheet

LM2578A, LM3578A
SNVS767E –AUGUST 2000–REVISED APRIL 2013 www.ti.com
A minimum value for an ideal output capacitor C3, could be calculated as C = I
o
× t/ΔV where I
o
is the load
current, t is the transistor on time (typically 0.4/f
osc
), and ΔV 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 transistor “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 Engineering (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 36
was a Pulse Engineering PE-64287.
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