Voltage of 5V pin when only connected to battery

Products & Technology XIAO
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1

Hello, I have a Xiao ESP32C3 and I am trying to understand what the voltage of the 5V pin represents when the board is powered only via battery.
Pinout available here

From my experiments, the 5V pin voltage is slightly above 5V when the board is powered via USB and there is no surprise there, especially given that the label on the pin is VUSB

If I instead disconnect the USB and only power the board from a battery (connected to the BAT + and BAT - pads) I have an unexpected voltage on the 5V pin. The voltage is not stable at all. I see it floating between 0.1V till 1.8V.
The voltage between the two battery pads is about 4V, which is sensible and tells me that the battery is fine. The 3V3 pin has the expected voltage as well, so again everything is fine on that front.

Is this expected? Is the 5V pin supposed to be used only when the device is powered via USB?

To add to my confusion, I also have an Adafruit QTPy ESP32C3 and on that board the voltage of its 5V pin is 3.75V, which sounds it could be the voltage of the battery minus the drop from a protection diode.

2

That is correct.
You can check the schematic on the Wiki.

3

Thanks for confirming!

Is there a recommended way to power a device from battery instead of the 3V3 pin?

I can think of 2 options:

  • keep the battery connected to its pads and connect the device to BAT+. I’m not sure if this is safe when using USB to charge the battery.
  • use a separate battery charger and power both the other device and the Xiao (using the 5V pin + protection diode). This should be safe but involves additional components and “wastes” the charger in the xiao
4

What is the operating voltage and current consumption of the device?

5

It’s an epaper display that works from 3.3V to 5V. It does work at 3.3V but it has an high deep sleep power consumption.

If instead I provide an higher voltage power consumption drops. I’m not sure why, I was advised to try this by the manufacturer.

The difference is substantial: with both Xiao and screen in deep sleep current goes from [email protected] to ~100uA@4V

Max current is 6mA during updates

6

HIGH on XIAO’s I/O is 3.3V. Is there a problem with the interface when 4V is supplied to the device?
What is the link to the device you are using? I think I can check the specs.

The battery pad is at 4.2V when charging.

7

The device I’m using is a WeAct Studio 4.2" B/W eink display. Documentation is available on GitHub. In particular, here are the schematics of the module and this is the datasheet of the display.

The schematics shows there are a couple things that can be configured. My board has the default configuration with 4-lines SPI and 3.3V-5V VCC.

8

There is a 3.3V LDO built into the display unit, so connecting it to BAT+ should not be a problem.

9

How do you apply the 3.3V and 4V voltages to the device and XIAO?
In my experience, the sleep current of C3 is about 50uA.

10

Thank you so much for your help. Really appreciated!

If I connect the battery to BAT+ do I need to add any protection diode or is a straight connection fine?

In my experiments I was using a Nordic PPK2 in source mode. In the initial experiment I connected it to the battery pads and then connected the device straight to the Xiao pins.
I then tried to do the same but connecting the device to the 5V pin, which is what led me to ask the initial question in this discussion.
That didn’t work so I got a breadboard out and used the PPK2 to power both the Xiao (using its 5V pin as the input) and the device.

11

I don’t think a diode is necessary.

As you can see in the link below, the sleep current of ESP32C3 is less than 50uA. if you have PPK2, you should check the current consumption of XIAO and display unit respectively.
The sleep current of the display unit chip is 3uA, so the rest might be the display panel and the boost circuit. Or maybe it is not sleeping properly.