EE 332 Laboratory

Switching Regulators

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 8 V dc output from a 15 Vdc input, using the LM3578 switching regulator. Choose the switching frequency such that:

  1. the regulator operates in continuous mode down to an output load current Io = 50 mA,
  2. the inductor value is no more than a few mH. (The range of values in stock in the laboratory is 0.1 to 1.0 mH) and,
  3. the frequency is in the range of 1 kHz to 100 kHz, the range of the LM3578.

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 the power.

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):

  1. Measure the output voltage and ripple under the full load conditions described above. Measure the switching frequency and duty cycle. The best place to see the switching and duty cycle is Vx (pin 5). Compare the observed and expected values of the duty cycle and switching frequency.
  2. Decrease the load current until discontinuous operation is reached. What happens when the operation becomes discontinuous? At what load current does the operation become discontinuous, and how does this compare with theory? Can you explain the waveform shapes?
  3. Vary the input voltage over some reasonable range. What happens? For proper operation how much larger than the output must the input be?
  4. Measure the inductor voltage and current waveforms. You can not do this directly. Use a scope probe at both ends of the inductor and the math mode on the scope. Repeat for R3.