Using main board power supply rails in add-on boards

Question

Many boards come with board-board connectors that can be used to connect another PCB to add new functionality. There are many examples of this. Some that come to mind are Arduino shield, Click boards™, mikroBUS™, PMOD, HSMC, FMC among others. Some are designed for high-speed applications like HSMC and FMC. Then we also have the concept of a system on a module.

ICs on the add-on board will also require power and the schemes mentioned above contain several pins for different voltage rails and GND. This brings me to my question:

1. What is the correct way to supply power to add-on board ICs from the main board?

   i. Connect the 5V, 3.3V e.t.c rails directly to the ICs and just use some decoupling capacitors?

   ii. Take the high voltage rail from the main board and connect it to a linear or switching regulator on the add-on board to generate all the voltage rails required on the add-on board?

Or maybe there is another option? The main issue is that the power supply network of the add-on board must have power integrity. Else it shall not work reliably.

Answer 1

It really depends on what the ICs on the board need, but it really depends on the voltage drop across the connector and you need to be able to handle multiple cable lengths.

Another thing that you need to know is the max current pull from any of the rails, because that will also affect the voltage drop.

Let's say the connector is 100mohm and the current is 50 mA, that means you'd have 5mV which is fine for almost any application. However if it were 1ohm, you would have 50mV, and some digital systems may not be happy with that, so in that case the application May need to have a switching regulator

Another thing is inductance if you have short-term spikes you need to have enough local storage to be able to cover the drops so really power intensive modules like Wi-Fi or cellular modules that need amps of current could run into issues and may need a converter

Answer 2

As commenters above have already noted, it really depends on your specific set of requirements. Equally there are probably several approaches which would all work. That being said, here are some factors to consider:

Providing a high voltage rail is useful if the add-on card draws a lot of power. For the same amount of power, higher voltages have less current. It is this current draw that can create voltage drops, which may become out of spec of the ICs that are being powered on the add-on board.

Local voltage regulation from a higher voltage is best for sensitive analogue circuitry. The closer regulation is to the load, the less chance of noise being added onto that rail.

A higher system voltage makes it easier to adapt your add-on card at a later date. If a new technology allows improvements to the add-on card then you are not tied to a limited set of voltage rails; simply change the regulators on the add-on card to something else. Be careful with the digital logic levels, which are likely fixed on the main board.

When creating a pinout of a board to board connector, be sure to check the current limit on each pin and include a reasonable safety margin.

Place a large decoupling capacitor as close as possible to where power enters the board. This prevents larger current loops for the board which may cause EMI issues. You may also want to consider more complex filtering such as LC or Pi filters depending on the application.