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New to this and designing my first non-trivial circuit. I'll be using an ATMEGA168 and plan to run it at 3.3v/10MHz to maximize compatibility with other components.

I'll be placing this into some existing hardware and will not have any control over the input voltage. It could be 24v or 36v depending on the specific model. After filtering through regulators on DigiKey it looks like MIC4576-3.3WT is one of few that are rated for the input/output voltages I need.

Looking at the datasheet for the regulator it mentions "Guaranteed 3A Switch Current." Should I interpret this as:

  • It will provide as much current as my circuit demands, up to a maximum of 3A, or...
  • It will provide 3A and it is my responsibility to limit the current before it goes into the ATMEGA which has an absolute max of 200mA?

If the latter, how do I address it? Or would choosing something other than the 4576 have avoided this altogether?

Thanks in advance!

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  • \$\begingroup\$ The former, but 3 A (peak) switch current isn’t 3 A output current unless you are running at 100 % duty cycle. With deep CCM, it will be close though. \$\endgroup\$
    – winny
    Commented Oct 30, 2022 at 19:37
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    \$\begingroup\$ In circuit design terms, switching regulators are definitely toward the deep end of the pool. You're probably going to get some learning opportunities, in a "why are parts turning into smoke in front of my eyes" sort of way. TI and others make 3-terminal switching regulator modules that are exactly the pinout, and roughly the size, of a 78Mxx regulator + heat sink. If there's one of those that will take your input voltage range then it'll definitely be easier to use than designing your own switching regulator, even if you're starting from an easy to use chip. \$\endgroup\$
    – TimWescott
    Commented Oct 30, 2022 at 20:35

5 Answers 5

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It's a voltage regulator and won't need to provide any current if nothing is connected however, it will still produce a regulated voltage output despite no current being taken. If you have a load of up to 3 amps it will provide the current needed by the load and maintain the output voltage. If your load takes more than 3 amps, then it's likely that the output voltage will start to sag.

As with any voltage regulator, the power taken by the load is supplied by the unregulated power source connected to the input of the regulator so, bear that in mind and, also bear in mind that a little extra power is needed to keep the switching regulator working.

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  • \$\begingroup\$ Thanks for the reply, Andy, but can you clarify where the 12v comes in? If I'm reading the datasheet correctly (and it's entirely possible I'm not) this regulator produces a fixed 3.3v output. \$\endgroup\$
    – dazedandconfused
    Commented Oct 30, 2022 at 19:46
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    \$\begingroup\$ @dazedandconfused oops a misread - fixing ..... \$\endgroup\$
    – Andy aka
    Commented Oct 30, 2022 at 20:21
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3A switch current means that the peak current through that switch is guaranteed to be at least 3A. Note that this matches the peak inductor current (because of KCL). It does not match the average inductor current -- that's determined by the inductance you choose, as well as the input and output voltages. At best you put a high-value inductor in there and the average current is close to the peak. At worst, you put a teeny inductor in there and the average current is way lower than peak.

It's best to both fully understand how switchers work, do the math, and make sure the inductor is theoretically the right size and also simulate the circuit to make sure it's behaving nicely through your anticipated operating range (all combinations of current draw and input voltages). You should at least have at least a notion of how the thing works and also simulate -- and hope that your simulation is accurate.

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The regulator produces 3.3V (assuming that's the one you buy) and consumes maybe 10mA for its own operation (10mA at the input voltage so that's more than 1/3 of a watt) . Any additional current is completely under your control depending on what your circuit demands. The ATmega itself will not draw much of the ~3A* capability of this regulator. You should be able to find the current draw at 10MHz 3.3V in the datasheet, and it won't be many mA. Other things you may have (LEDs, motors, other chips etc.) will, of course, add to the consumption.

* note that the maximum current may be limited by thermal considerations- efficiency is only 72% at 3A 3.3V out so a fairly significant heat sink would be necessary to get full current capability on a continuous basis. For no heat sink you would have to limit the current to much less than 3A (and it would depend on the maximum ambient temperature and how reliable you want the device to be).

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Correct answer is the first one. It will provide a maximum of 3 A. The microcontroller maximum power consumption only applies when you are drawing current from the microcontroller pins, by for example putting a low resistance resistor on the pin. Without external circuitry, the microcontroller does not draw anything near 200 mA.

Check out adjustable ouput voltage regulators as well. And Digi-Key for example has another category for on-board DC-DC converters, those might be the best option for you.

Recom power R-78HB3.3-0.5L might fit your needs.

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It will provide up to 3A, depending on how much your circuit demands.

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