Author Topic: Best way to feed power into secondary boards?  (Read 721 times)

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Offline ANTALIFETopic starter

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Best way to feed power into secondary boards?
« on: May 29, 2021, 11:23:42 am »
Hi All

I am working on a multi-board design (hobby project) and am wanting to know, what is the best way to feed power into secondary boards? Like is it better to generate 3.3V locally or can I just feed it in via a filter?

For example, I have two secondary boards (RECEIVER_SENSOR-RIGHT & RECEIVER_SENSOR-LEFT) that are connected to a primary board (RECEIVER_MAIN-BOARD). With the current design I want to generate 3.3V on the primary board and then feed it into the secondary boards via a pi-filter. Reason for this is that board space is tight so it’s easier to just feed in 3.3V rather than generating it locally

Offline SeanB

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Re: Best way to feed power into secondary boards?
« Reply #1 on: May 29, 2021, 02:08:15 pm »
Short distances, so just feed 3V3 power in to the board. Remember those ceramic capacitors are very low inductance, so will ring with the inductance of the leads, so best to put a 4R7 SMD resistor in series with the power lead to provide damping. Will drop voltage slightly, but likely depending on current draw from the board you will be fine with 100mV of voltage drop. You do not even need the first capacitor, or the inductor, easy to replace it with the resistor, in place of L1 and delete C1. Will do a better job of decoupling, and also will provide hot plugging protection.
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Offline Siwastaja

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Re: Best way to feed power into secondary boards?
« Reply #2 on: May 29, 2021, 02:41:54 pm »
Filtering is hard, worth of PHDs and $100000 simulation tools, don't do it randomly in hobby projects for no reason.

Proper local bypassing is important, though. In addition to the chip-side 100nF ceramics, a larger high-ESR bulk capacitor at each card is always a good idea. Any cheap electrolytic cap like 100µF does the trick. This provides the damping that was talked about without the massive 4.7ohm series resistance suggested above.

Short runs low currents, just supply 3V3 as is. This is especially case with digital logic and IO voltages. Removes any power sequencing issues as well.

Whenever something requires larger currents and/or good voltage regulation, higher bus voltage regulated down with local regulators is a good idea. For example, a small DC motor with 100s of mA stall current would be something I would not try to power from the 3V3 bus shared with microcontrollers. In such cases, pay attention to power sequencing conditions of ICs with multiple separate power pins. Similarly, make sure ICs don't get voltages in their IO pins before their IO power supply has stabilized, unless the devices are explicitly specified to allow such operation without latch-up or other issues.
« Last Edit: May 29, 2021, 02:44:26 pm by Siwastaja »
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