In this lab we will use the LM3578
switching regulator to implement step-down (buck)
dc-dc converters. This is a general purpose regulator
which allows the switching frequency to be varied by a timing
capacitor, and allows various different modes of operation. It
also has a current-limiting feature. Document your design formulations,
schematics, and results in your lab notebook, and explain and
comment upon your results.
Step-down (buck) converter.
Design, build, and test a buck converter that produces 10 V dc output from a 15 Vdc input, using the LM3578 switching regulator. Choose the switching frequency such that:
Load the regulator with a resistive
load which will draw 50 mA, and use a 100 uF filter capacitor
to reduce the ripple. Make sure the load resistors will handle
Note that the specification sheet
for the LM3578 gives cookbook design procedures for using the
regulator, but that the procedures provide little understanding
of what you are doing. The best design approach will be a hybrid
one using the general formulations discussed in class, and the
chip-specific needs of the LM3578. The way in which the LM3578
is used as a buck converter is shown in Figure 15 of the specification
sheets; explain briefly the purpose of the different components.
(Note that the LM3578 switches on and off large amounts of current.
This creates transients on the power line which can affect the
operation of the chip. Putting a capacitor (1 uF or 0.1 uF) between
pin 8 and ground will reduce the size of the transients and help
the chip to operate correctly.)
Test and experiment with the regulator
in the following way (For each test look at the waveforms at Vo, pin 5 and pin 6):