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Electronics => Projects, Designs, and Technical Stuff => Topic started by: tapasxplore on March 20, 2018, 04:58:07 am

Title: Connecting positive supply to power plane
Post by: tapasxplore on March 20, 2018, 04:58:07 am

I am making a four layer PCB. The top and bottom layer are signal and the internal layer has power and ground layer.

L1 --- Signal,
L2 --- Power,
L3 --- Ground,
L4 --- Signal

I am using Li-ion battery and then using LDO to convert it to 3V. 3V is then powering the whole circuit. Should L2 should be connected to 3V LDO output or Li-ion battery positive output. Both are positive supply source, I am unable to decide should I connect LDO 3V output or Li-ion output to the power plane.

Regards,

Title: Re: Connecting positive supply to power plane
Post by: killcode on March 20, 2018, 05:20:23 am

Consider the purpose of a power plane....

If Vbat is only provided to the LDO and the output of the LDO is distributed to all other components, then it doesn't make sense to connect the plane to Vbat does it?

Title: Re: Connecting positive supply to power plane
Post by: bson on March 21, 2018, 02:05:20 am

Connect it to the rail, i.e. the LDO output.

Consider swapping your power and ground planes so they're L3 and L2 respectively.  Most signals end up on the top layer, because that's where most parts are, and the ground plane typically has lower impedance than the power.  With ground in L2 it makes it super easy to put ground right under an XO, oscillator, reference clock network, or elsewhere where you think it might be beneficial - in fact, this is what just happens unless you go out of your way.  Parts on the bottom often end up being decoupling and pull resistors, that connect to the power plane.  Or card/battery holders, connectors, that kind of thing - maybe the occasional part that just didn't fit on top.  Then, in the future, if you use blind vias you'll save substantial board real estate with this stackup.  You'll develop habits and practices based on your stackup, and it's nice to immediately realize a gain if you do decide to pony up for blind vias.  (Not much use for buried vias with either of these 4L stackups. :) )

Title: Re: Connecting positive supply to power plane
Post by: soubitos on March 21, 2018, 03:05:10 am

You might want to take into account the low but present voltage drop of the LDO you have chosen.
Your Li-Ion might be powering your pcb only down to 3.6-3.3V at best. This means you loose maybe up to 30-40% of its capacity. You might want to search for a buck-boost switching or something similar available