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I'm using a Raspberry Pi and have designed an extension board (Like the Pi-face) but I have a question about the power supply. I need 3V3 on my board, but the 3V3 from the Raspberry is not strong enough for my application. So I've used the 5V supply from the USB and I have the 3V3 from a LDO. My question is:

Can I supply an IC on my board with my 3V3 (from the LDO) and connect the IO pins from this IC to the Raspberry Pi GPIO port? Also can I use 2 different power supplies? The Raspberry Pi is supplied by its own LDO and my board has its own LDO and I would like to connect IO. Here is the block diagram:

enter image description here

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  • \$\begingroup\$ I don't see any problem here \$\endgroup\$
    – Andy aka
    Commented May 7, 2013 at 10:19
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    \$\begingroup\$ Looks fine to me as well. However, if you're powering both LDOs from the same +5V USB you should check the load on your power supply there. \$\endgroup\$
    – pjc50
    Commented May 7, 2013 at 10:30
  • \$\begingroup\$ You may want to add a capacitor on the LDO 3V3 output, check its datasheet for details. \$\endgroup\$
    – jippie
    Commented May 7, 2013 at 20:32

2 Answers 2

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Short answer: Yes.

Slightly longer answer:

You would need to make sure your 3.3V LDO has it's required passive parts (capacitors, resistors, etc).

You would also have to watch out how much current you are draining from the RPi's 5v/USB line. Older Rev 1 boards have 140mA Poly Fuses on the USB ports. Old and New model RPis have a 700mA Poly Fuse on the power in usb line. And they both have undersized capacitors, which can cause an issue with Voltage Sag resetting the RPi. Too much current draw, and you will have issues. Too much inrush draw, and you will have issues.

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Please see that both are powered at the same time. Driving an Unpowered Microcontroller's IO's are not recommended and bad practice.

Please see that nothing is driven, if any of the power supplies are not up. check the default states of the GPIO's at the startup.

If you are not targeting any battery based application, it's fine.

If cost is not that much problem, use a switch's like 3257

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