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power from 3.3 V ok?
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power from 3.3 V ok?

by ralphhipps on Thu Jul 30, 2020 6:40 pm

I have purchased three different displays and need to connect them to a 3.3 V powered host:

https://www.adafruit.com/product/1431
https://www.adafruit.com/product/3787
https://www.adafruit.com/product/2088

I have looked at the schematics and on 3787 there seems to be one or two outputs that are connected to Vin rather than the LDO 3.3 V output, which concerns me. I'll blow up the host if it gets 5 V applied.

what is the recommendation here?

I tried applying 3.3 V to the Vout pin to provide power but so far that does not seem to work, at least on 1431.

I have been told I need to remove the LDO first by a colleague. I'd rather not if I don't have to, but if that's the only way then so be it.

What happens to the outputs that are supplied by Vin? I suppose I can try to connect them to the 3.3 V rail instead.

Any ideas here? are these supposed to work with 3.3 V systems? I think 1431 and 2088 look clean for that, other than the LDO question.

ralphhipps
 
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Re: power from 3.3 V ok?

by adafruit_support_carter on Thu Jul 30, 2020 7:57 pm


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Re: power from 3.3 V ok?

by ralphhipps on Thu Jul 30, 2020 8:54 pm

ok, it seems from all three guides that Vin can take either 3 or 5 V.

Not sure how the LDO works at 3 V Vin but I'll take that for granted at this point.

It looks to have a very low dropout voltage, so maybe that's the trick. I suppose it just passes current at lower Vin voltages with some small drop.

So removing the LDO is clearly not needed.

It seems on 3787 that the Vin should match the host, at least for the CARDCS, TFTRST, TFTCS signals.

Thanks for your help!

ralphhipps
 
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Re: power from 3.3 V ok?

by michaelmeissner on Sun Aug 02, 2020 1:04 pm

One thing to watch out for is how much power your 3.3v can put out. Some 3.3v devices can't put much power on the 3.3v bus, and some displays tend to use some amount of power. Now generally on the Teensys that I use, the Teensy can put out 250ma, which tends to be enough for 2 displays. But other processors that I've used in the path had smaller voltage regulators and might not put out as much power as you might need.

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Please be positive and constructive with your questions and comments.