I'm trying to design a simple 48V to 5V power circuit using the MAX17640. The basic circuit shown on page 19 is simple enough for me to follow but doesn't provide all the resistor values and directs me to the evaluation kit for more details. The relevant evaluation kit circuit (page 12) has the following pieces which I'm having trouble understanding.

First, here's the whole circuit:

enter image description here

Here are the parts I don't understand. enter image description here

  1. what's the significance of the open inductor (L201) and open capacitors (C202, C204)? If they are simply disconnected, why label it as an inductor/capacitor at all?
  2. What do the vertical arrows for VIN2 and GND (separate from the VIN2 and PGND2 power inputs) indicate?
  3. If the L201 inductor is installed, does the electrical path above it run in parallel?
  4. What benefit do all of these additional capacitors add to the circuit? i.e., why not just run VIN directly to VIN on the MAX17640?
  • 1
    \$\begingroup\$ (1) Those "open" don't need to be stuffed. If you do stuff them, then you do NOT bridge over L201 with a jumper and the wire becomes an "open," instead and L201 is used when stuffed. The "open" capacitors themselves aren't stuffed, unless they are. (2) Elsewhere there will be similar arrows and when you see them they will be connected to the same node. (3) No, the jumper is removed in that case, or not soldered. (4) Longer, but less certain, story and why I'm not bothering making this an answer. Too many questions and the last one leads to writing a book. \$\endgroup\$
    – jonk
    Commented Mar 5, 2022 at 23:17

2 Answers 2

  1. They are extra components that are drawn in but not present in the circuit. It is far more easier to draw in the extra components and then not place them on the PCB, than to leave them out and find out later they were needed but there's no place for them.

  2. The VIN2 is the name of the circuit node and the arrow indicates it is a power supply node called VIN2. The fact that the power input connector reads also VIN2 is different as it is likely just a comment tag for the connector.

  3. The intention is that when the inductor L201 is needed and placed in the circuit, the path that bypasses inductor L201 is cut off.

  4. Because there are rules how much electromagnetic interference (EMI) a device is allowed to emit, and these parts reduce the EMI. They are not needed for functionality, they are needed to be able to pass official tests to be able to sell the device. It is explained in the chapter "Electromagnetic Interference (EMI)" in the page 3 of the evaluation kit data sheet that the components are optional.

They are not placed in the official evaluation boards because the components cost money, and they are not necessary for evaluating the performance of the circuit, the engineer who is evaluating the circuit may not be even interested in evaluating the EMI performance at the first phase of evaluation, and each jurisdiction has different legal limits for radiated and conducted EMI.

So there would be a requirement to manufacture boards with different components for different markets, or if a single board is manufactured to all markets, the components would have to be such that the device will pass tests for all markets, and for some markets the components would be too expensive and they would be changed to cheaper components and it would require re-evaluating the system with the cheaper components again.


It would seem that all of the "open" components have footprints on the PCB but are not populated. The footprints allow you to easily add those components if you so wish.

The C202-L201-C203 footprints are for an LC pi filter to filter the incoming power. If you decide to add the filter you should also cut the trace that shunts L201.

You could also just populate L201 and C203 for an LC filter. Again, you would want to cut the trace that runs across L201.

C204 is for an additional filtering capacitor if you decide you need it.

The arrows on VIN2 and GND are just standard symbols used to identify power supply nodes in a schematic.


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