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Re: RPi 3B+ 5 volt lines

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In message <KLy*96JHz@news.chiark.greenend.org.uk>
          Theo <theom+news@chiark.greenend.org.uk> wrote:

> David Higton <dave@davehigton.me.uk> wrote:
> > Does anyone know where the RPi3B+ detects undervoltage?
> > 
> There's an APX803 chip which detects the voltage falling below 4.63 +/-
> 0.07V.  That chip then asserts PWR_LOW_N, a GPIO the GPU firmware can
> detect.  It's also used to drive the PWR LED (ie the LED going off is the
> same as PWR_LOW_N being asserted).
> 
> https://forums.raspberrypi.com/viewtopic.php?t=158777
> 
> > Does anyone have a schematic that includes the 5V lines?
> 
> https://datasheets.raspberrypi.com/rpi3/raspberry-pi-3-b-reduced-schematics.pdf
> 
> has the power supply and detection circuit.

Thanks, Theo.  That's the one that everyone has, and of course it's
incomplete.  But I looked up the data sheet for the RT9741C, so I have
to guess that "H5V" goes to all the USB-A sockets, which are not shown.

The upshot is that I'm going to have to reconsider power supply and
distribution for this one, which is used as NAS, so it needs to power
some spinning drives.  The incoming supply is rated at 8 amps, but its
output voltage is just 5V, and it appears that its transient response
may leave something to be desired.  One thing I can try is whacking
several 1000uF capacitors, via short leads, on the 5V line on the GPIO
connector.  I can also see where I would need to interfere with it so
as to increase the output voltage to about 5.1V.  I've cut the output
cabling as short as it can reasonably be made so that cable resistance
genuinely can be neglected.

It seems that mains-powered USB hubs are going out of fashion,
unfortunately.

David

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Thread

RPi 3B+ 5 volt lines David Higton <dave@davehigton.me.uk> - 2024-04-11 21:59 +0100
  Re: RPi 3B+ 5 volt lines Theo <theom+news@chiark.greenend.org.uk> - 2024-04-12 10:12 +0100
    Re: RPi 3B+ 5 volt lines David Higton <dave@davehigton.me.uk> - 2024-04-12 21:21 +0100
      Re: RPi 3B+ 5 volt lines Theo <theom+news@chiark.greenend.org.uk> - 2024-04-13 11:32 +0100
        Re: RPi 3B+ 5 volt lines David Higton <dave@davehigton.me.uk> - 2024-04-13 18:42 +0100
          Re: RPi 3B+ 5 volt lines David Higton <dave@davehigton.me.uk> - 2024-04-14 22:35 +0100

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