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I have seen several products ( such as the pitft for the raspberry pi, and most other small tft screens) which use a 3.3V regulator on the 5V power rail to provide power to the 3.3V system. In some of these products, given that they are breakout boards for systems which have a 3.3V power rail which can supply the needed current, it seems that the extra 3.3V regulator could be omitted. Is there any special reason why it might be beneficial to include this external regulator?

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    \$\begingroup\$ The key phrase is "which can supply the needed current". For example, the RaspberryPi does not have a lot of spare current on its 3.3V rail, so it's generally a good idea to use a separate regulator. \$\endgroup\$ – Dave Tweed Oct 9 '16 at 22:38
  • \$\begingroup\$ "... it seems that the extra 3.3V regulator could be omitted" - What lead you to that impression? \$\endgroup\$ – try-catch-finally Oct 10 '16 at 6:49
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    \$\begingroup\$ The rpi also needs a clean 3v3, it may not be suitable after going through a header or connector wire - sure, you can add some capacitors, but how many? Its much easier to predict a noisy 5v + then go through a regulator \$\endgroup\$ – user2813274 Nov 9 '16 at 13:55
  • \$\begingroup\$ As an aside, I've used cheap chinese 2A supplies for rpi, which caused it to shutdown frequently. \$\endgroup\$ – user400344 Dec 9 '16 at 15:22
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Apart from too much current and heating issues when you connect to already existing regulator which supplies power to another board, there's a responsibility reason: if your board will fry existing regulator (in your case raspberry's one) whole configuration will stop working, and you may be held liable for failure of whole device. If your device will fry its own regulator, (hopefully) only your device will stop working. Thus while addition of part adds some cost, it is wise to have it on the board thus having predictable current consumption of its output, and protect other device from issues with your board.

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