nRF9151 SMA DK power input questions

[m4rkw]

Hi

I'm working on a new project using the nRF9151 SMA DK and I have some questions that I haven't been able to answer by searching this forum or reading through the documentation.

1. The hardware guide says the board should be powered from either USB or the VIN pin (pin 9 on P20). However it would be advantageous for my project if I could instead create a shield during prototyping that supplies 5v via pin 5 of P1. SB35 (closed by default) connects the P1/P3/P7 5V pins to VDD_5V, so electrically they appear to be the same net as VIN_5V on P20. Is this a supported way to power the DK, or are there caveats I should be aware of (e.g. the power switch SW6, power-input selection FETs, etc.)?

2. The guide says only one source should be applied at a time but doesn't say why. My assumption is that it's to prevent reverse current flow between the two sources. For my application I'd like the DK to remain powered from the vehicle's 12 V (via a buck) while also being able to connect USB for programming/debugging, without having to disconnect either side. I'm planning to put a Schottky in series with my buck output (cathode toward the DK's 5V net) to prevent USB current from flowing back into my buck. Looking at the power input section of the schematic, it appears that Q9 (MCM2301-TP) provides similar reverse-blocking on the VIN_5V -> VBUS path via its body diode. Is that the intended behaviour, and if so, would my Schottky + the existing Q9 be sufficient to safely connect both sources simultaneously? Are there any other failure modes (transient behaviour during USB plug-in, SW6 interaction, USB_DETECT logic, etc.) I should be aware of?

Thanks,
Mark

[Simon D-M]

Hi Mark,

First, be sure to keep in mind that I'm not really a hardware person so, my knowledge on the subject are somewhat limited.

1. The hardware guide says the board should be powered from either USB or the VIN pin (pin 9 on P20). However it would be advantageous for my project if I could instead create a shield during prototyping that supplies 5v via pin 5 of P1. SB35 (closed by default) connects the P1/P3/P7 5V pins to VDD_5V, so electrically they appear to be the same net as VIN_5V on P20. Is this a supported way to power the DK, or are there caveats I should be aware of (e.g. the power switch SW6, power-input selection FETs, etc.)?

It is definitely not recommended as you are bypassing some protections (and the "POWER ON/OFF" switch) and supplying voltage to a pin that should be used as an output.

However, I think it should theoretically work, but we cannot give any guarantee that you won't break your board after some time if you are supplying your board that way.

2. The guide says only one source should be applied at a time but doesn't say why. My assumption is that it's to prevent reverse current flow between the two sources. For my application I'd like the DK to remain powered from the vehicle's 12 V (via a buck) while also being able to connect USB for programming/debugging, without having to disconnect either side. I'm planning to put a Schottky in series with my buck output (cathode toward the DK's 5V net) to prevent USB current from flowing back into my buck. Looking at the power input section of the schematic, it appears that Q9 (MCM2301-TP) provides similar reverse-blocking on the VIN_5V -> VBUS path via its body diode. Is that the intended behaviour, and if so, would my Schottky + the existing Q9 be sufficient to safely connect both sources simultaneously? Are there any other failure modes (transient behaviour during USB plug-in, SW6 interaction, USB_DETECT logic, etc.) I should be aware of?

Yes, this is to prevent reverse current flow. Even though we don't recommend doing it, the DK is capable of handling USB and VIN:5V at the same time. It will switch to the available source with a preference with the USB source. So when both way are powered, the VIN:5V is not used. This mechanism does not allow to power from VIN:5V and use USB, it will switch between fully USB powered + USB data or fully VIN:5V (USB not connected).

However, if you are supplying power directly from the 5V pin, this mechanism doesn't work, and you may break something.

Basically if you want to use USB with another power supply, you will need to somehow "replace" the USB V_BUS before it enters the board.  

Please take what I said with a grain of salt, as I am not a hardware expert.

If you have more questions, feel free to ask.

Best regards,

Simon D-M

[m4rkw]

Hi Simon

Thanks for getting back to me and I appreciate you're not an expert, is it possible to get a definitive response from someone who is?

I feel like these are fairly basic things that should be covered by the technical documentation but right at the start of my project when I'm trying to design the power circuitry it seems like things are a bit vague. I would like to know:

> It is definitely not recommended as you are bypassing some protections

Which protections specifically?

> and supplying voltage to a pin that should be used as an output

What are the specific implications of doing this that I should be aware of?

> we cannot give any guarantee that you won't break your board after some time if you are supplying your board that way

Obviously I don't want to break the board but I'm confused as to what you mean here. Do you mean that since I'm bypassing protection circuits then I might make a mistake and cause over-voltage or over-current? Or is this inherently going to cause damage for some reason even if I'm careful and put sensible protections in on the source side?

> However, if you are supplying power directly from the 5V pin, this mechanism doesn't work, and you may break something.

Ok so if I've understood this correctly I think what you're saying here is that if I do what I wanted to do in question 1 (supply via the 5V pin of P1) then the mechanism that defers power input to the usb won't work and something might break as a result.

If I instead supply 5v through VIN the way the documentation says to, is it the case that the device will switch from 5v to USB power when it's connected and switch back when unplugged? Are there any specific implications or risks of operating it like this? I can add protection to the things I'm connecting to it but without knowing what you're referring to it's a bit difficult to design them.

> Basically if you want to use USB with another power supply, you will need to somehow "replace" the USB V_BUS before it enters the board.

I'm really not sure what you mean by this. I think powerless USB cables exist, is that what you're referring to? Or do you mean I'd need to put something in the path of the USB connection and turns its power off when VIN is present?

How is this supposed to work in products built using the nRF9151 platform? It surely can't be the case that connecting them to both a DC input and USB at the same time would cause problems so this must be a unique quirk of the dev board.

I mean no disrespect here and I do appreciate you trying to help but I'd rather not design power circuits based on vague notions about how things might work. Would be very grateful if you could get a definitive answer from someone.

Cheers,
Mark