I'd like to start using switching voltage regulators such as the LM2575 in the Arduino-like boards I design, instead of the linear regulators I usually use, such as LM7805 and LM7812, mainly to avoid overheating.
However, I'm finding a little difficult to make the transition, as there are a few more things to worry about when using switching voltage regulators in such designs. The following are a few of the obstacles I've found while studying the LM2575 datasheet:
- Inductor selection based on maximum voltage and current;
- PCB layout issues, such as single-point grounding and ground plane construction;
- Continuous and discontinuous modes of operation (when to select each mode).
Could someone provide some guidelines for beginners like me who would like to transition from linear to switching regulators? Are there any traps or dangerous situations to avoid?
Edit:
To be more specific, I have three examples I would like help with:
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I have an input of 15 to 18V and want the regulator to output 0.8 to 1.0A @12V. But during operation the load may require less current (when less 7-segment displays will be lit). What inductor should I use for that situation? Should I select the inductor for continuous and discontinuous operation mode?
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What would be a good choice of inductor, input and output capacitors if I wanted to replace the LM7805 linear regulator for a LM2575@5V on an Arduino Single Sided Serial board (S3V3 – Severino).
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Will I have to worry about EMI to a RF receiver on the same board I use the LM2575?
Best Answer
Answers to 1 and 2 should come mostly from the datasheet of whatever switching power supply chip you plan to use. First you have to make sure the chip is intended for your input voltage, output voltage, and maximum current. After that you need to follow the directions, carefully.
These kinds of chips are designed assuming a certain range of parts external to them. The datasheet will tell you what is acceptable and it may even give you some guidance what values to use over a range of parameters. For example, the datasheet might recommend a larger inductor for the upper half of the input voltage range. However, you'll probably pick a chip because it's max characteristics exceed your requirements by some reasonable margin. In that case, you follow the recommendations for the max case, since that's effectively what you will be using. Some datasheets assume this and only give you one set of recommended values. Just follow them.
As for question 3, yes, switching power supplies can cause more EMI. The main trick is to keep all the local loop currents localized to the power supply sub-circuit, and to keep these loops physically small. This means starting with a good layout, then carefully routing the key nets. Make a separate local ground net for the switcher, then tie that to the main ground in only one place, probably right at the ground side of the output capacitor. This keeps the loop current thru the diode, inductor, and output cap localized and off your main ground plane. Only the net delivered current should flow thru the power and ground feed points from the power supply sub-section.