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So I was looking at SMPS IC's the other day. I needed a couple of low current bipolar power supplies. I found these switch mode, three terminal voltage regulators. Like the old linear 78xx series. Here's the spec sheet of one, (CUI P7805-S) but there are a number of suppliers. You can even hook 'em up backwards for a negative output voltage. (though at reduced current and efficiency.)

I've order a few from digikey, but as I await their arrival I wonder if anyone has used any of these? Are there any "gotcha's"? (Besides the ripple and noise.)

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  • \$\begingroup\$ I've used the 3.3V version of those to run a PIC32 circuit. Didn't give me any trouble at all. \$\endgroup\$
    – brhans
    Commented Jan 19, 2016 at 15:21
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    \$\begingroup\$ 30 mV peak-peak output ripple. No big deal for some applications. Show-stopper for others. \$\endgroup\$
    – The Photon
    Commented Jan 19, 2016 at 18:40
  • \$\begingroup\$ @GeorgeHerold: Your question is unlikely to be of help to anyone else (contrary to the objective of SE) because you don't reference a part number in your question title or body. Anyone searching for advice on that part number would never find your question or the wonderful answers that are sure to arrive. Can you edit the question? \$\endgroup\$
    – Transistor
    Commented Jan 19, 2016 at 18:51
  • \$\begingroup\$ @transistor, Well I thought part of the idea was not to include part numbers.. but to be more general. I'll add the Cui part number in the body. \$\endgroup\$
    – George Herold
    Commented Jan 19, 2016 at 19:48
  • \$\begingroup\$ @ThePhoton, Well they should arrive later in the week. I thought I'd try an LC low pass on the output. And maybe a cap. multiplier if that's not enough. (I know more parts.) \$\endgroup\$
    – George Herold
    Commented Jan 19, 2016 at 19:50

2 Answers 2

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  1. Price. These parts typically sell for $1 or more. 7805's can be well under $1. Your CUI part is about $4 at US distributors. A name-brand LM7805 is about $0.25 (100 pieces).

  2. Output ripple. Your part specifies 30 mV peak-to-peak output ripple. This might not affect some applications. But where it does it means adding additional filtering, which adds cost and degrades the load regulation spec.

Of course these are just considerations that should be taken into account and balanced against the higher efficiency of the switching design.

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  • \$\begingroup\$ Excellent, (+1) I was comparing them to doing a switcher with some IC (LM2594, or perhaps a more expensive LT part) and external L and C's. This was a case where a linear regulator would have to dissipate too much power. \$\endgroup\$
    – George Herold
    Commented Jan 19, 2016 at 21:22
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Aside from the output ripple and noise at a poorly defined frequency (280 to 450kHz), the input voltage tolerance (against damage) may not be quite as good as for a 78M05 (35V).

One sort-of gotcha is that the 12V version is marked "P7805-Q24-S12-S", which might result in entertaining lighting, aroma and sound effects if you put it in a 7805 'socket'.

If you're planning on using them in negative voltage mode, note the minimum current that sometimes applies and check that the start-up surge (if any) is not objectionable. I've found that with some other switching inverting regulators the surge can interact with a current limited supply to cause problems.

78x05 chips are also pretty difficult to destroy from the load end- these may not behave with such decency and civility if you short or overload them.

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  • \$\begingroup\$ Nice (+1) I'll note there is what looks to be a very similar part from CUI with a different part number V78XX-500 where the XX is the voltage. cui.com/product/resource/v78-500.pdf The spec sheet's look similar.. but you pay a dollar or two more for the part (at DK). By start up surge you mean the IC demands a lot of current from the supply? I'll add that to the list of tests I want to do. (Noise, load and thermal shutdown.) \$\endgroup\$
    – George Herold
    Commented Jan 20, 2016 at 13:40
  • \$\begingroup\$ Yes (I don't know that this particular chip does it, but it was another +ve SMPS regulator used in inverting mode) and it drew something like 0.5A momentarily- or it would fail to start. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jan 20, 2016 at 14:00

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