RP2040 Flight Computer Power Supply Advice

[ThomasRocketMan]

Hey !

I am trying to design a flight computer for a model rocket using the RP2040 MCU. I want to be able to power the board with either a USB-B micro, or a 2s LiPo. I need 3.3v to power the MCU and onboard sensors, and 5v to operate my servos that I will be connecting.


I have attached schematic of my power supply, and MCU design. Please let me know if you see any glaring errors, have advice on where to research more information on power supply circuity, or just have advice on this to change/adjust.


Thank you all!

 

[swfa]

How much voltage and current can VBUS supply? Servos can require a lot of power.

 

[heada]

Bleed resistor for the bypass caps?

 

[cerving]

I assume that those 100 nF caps are all for the RP2040's power pins. Good practice, as is using a separate regulator for any device that uses a lot of transient power (i.e. an RF device over about 100 mW output).

Do NOT power your servos from the same regulator as your logic. Use a separate regulator, preferably a separate battery, with a decent sized capacitor across the servo's power leads (220 uF or so). All you need to connect the servos to the logic is GND and the signal; good practice would be to put a moderate value resistor (330 ohms is fine) between the logic outputs and your servo's input.

 

[Mason T]

I want to echo Chris with use a separate supply for the Servo's. I would recommend that you use a buck converter rather than a linear regulator to generate your 5V for your servos. My quick calculations show only being able to pull 376mA from a 8.4V battery in a 100°F environment before the NCP1117 will thermal regulate. TI has some super simple ones.

 

[cerving]

Actually, you probably don't even need a regulator for the servo, if you're using a micro servo most of them will run off a 1S Lipo. If you're using a larger servo, yeah, get a buck regulator, rated at least 2x the current requirement of the servo. Stall currents are going to be a lot higher... so stalls should be avoided (i.e. do not move the servos all the way to the stops).

 

[tfischer]

Check your RUN pin connection, it is connected to the 1.1V VREG output rail (some kind of sequencing?), but appears to be a 3.3V level input pin so VREG powerup may not take it out of reset.

 

[ThomasRocketMan]

Actually, you probably don't even need a regulator for the servo, if you're using a micro servo most of them will run off a 1S Lipo. If you're using a larger servo, yeah, get a buck regulator, rated at least 2x the current requirement of the servo. Stall currents are going to be a lot higher... so stalls should be avoided (i.e. do not move the servos all the way to the stops).

Thank you for the replies! I took everyones information into consideration and spend last day or two redesigning some areas of concern. I just posted my full schematic , I am wondering if you would be able to take a look at it and leave some comments on anything you deem needing to be changed? link is here, thank you! https://www.rocketryforum.com/threads/flight-computer-schematic-review-request.186506/

 

[ThomasRocketMan]

Check your RUN pin connection, it is connected to the 1.1V VREG output rail (some kind of sequencing?), but appears to be a 3.3V level input pin so VREG powerup may not take it out of reset.

Yeah im having a hard time grasping that one. In the RP2040 data sheet the board can be powered with 3.3V, but there is internal voltage regulator that outputs 1.1V from VREG_VOUT. Datasheet goes on to say DVDD is powered from 1.1V and can be directly connected to VREG_VOUT. Ive been advised against an external reset so the data sheet states that RUN can be directly tied to IOVDD *note now looking at my final schematic I just posted in a separate post, I tied RUN to DVDD, must've mistaken it, it is now changed*.

Anyways, I can't think of any reason I would need 1.1V to power anything on my board, so Im now thinking just connect the 1.1V output to a test point to ensure everything is working properly.