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I have a device with a microcontroller that is powered via a USB-C connector. It is a more or less a standalone device (a synth with a USB-C power input and a jack output) and probably user would want to power it from a power bank. The problem is that many power banks are way too "smart" and turn off due to the consumed power being too low - my device "sips" only about 30-40mA.

I found a solution that uses a short high-current pulse (through R19 and Q1) to keep the power bank on. However, some power banks seem to still turn off - even with R17 changed to 100k, essentially creating a 50% duty cycle high-current-flow. My questions are:

  1. Is there any solution short of just increasing the constant supply current by putting a ~50-70 Ω resistor in parallel to the power input?
  2. Are 5.1 kΩ USB resistors values valid in these circumstances?

Schematic

winny
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sx107
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  • it might be simpler to modify the power bank so that it doesn't quit. an you post a schematic of that? – Jasen Слава Україні Oct 10 '22 at 23:47
  • @Jasen the power bank is user's, not mine. So, theoretically, the device should work with any power bank. – sx107 Oct 11 '22 at 00:38
  • what you ask is basically impossible. perhaps make the device work with any li-ion cell instead. – Jasen Слава Україні Oct 11 '22 at 02:27
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    @sx107 You **can and should** enforce that the power bank is USB-C compliant. The power bank you describe is most definitely **not** compliant with the spec. In other words: the user is connecting junk to your system, and nothing works. The user should be instructed to buy a brand-name power bank with USB-C power outputs. Everything will "magically" work then (as the USB spec dictates it should!). – Kuba hasn't forgotten Monica Oct 11 '22 at 06:12
  • @Kubahasn'tforgottenMonica what usb c spec addresses power bank trickle charge disconnect features? – Passerby Oct 11 '22 at 06:14
  • Maybe one of the solutions is to "rethink" the device and add a Li-Ion cell to it? So that the powerbank only charges the device and even if the powerbank turns off, the device is still powered. Still, that's quite a major modification and does not actually answer the question "How to properly power low-power devices through USB". It does not matter if it is USB-C or micro-usb, powerbanks will still turn off (even some genuine Xiaomi powerbanks do in my experience) and that should be somehow avoided in my opinion. – sx107 Oct 11 '22 at 23:50
  • @Kubahasn'tforgottenMonica So essentially tell the user "You bought my device, so you need to update your powerbank as well". That's not how the real world works: User buys the device, it does not work with his (still very common non-compliant) powerbank, user is frustrated, reputation is lost, the device is not sold. – sx107 Oct 11 '22 at 23:54

2 Answers2

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These power banks are not compliant with the USB-C specifications, then. A power bank has no choice in "turning off" unless either the device disconnects itself, a fault condition such as overload is present, or the battery runs out.

The power bank needs to use CC lines to detect device presence. That's what the USB-C specs stipulate. It has a finite time from detecting the power sink on the CC lines to turning the VBUS on, and then it cannot turn the VBUS off based on load current. That's just not an option. The device can tell the power bank to turn off by opening up the CC lines. About 100ms later, the power bank would be turning the VBUS off.

My initial guess would be that the power banks you got are either a noname brand, or are fakes. Either way, they are not compliant, and basically don't work correctly. The whole point of USB-C is that this wholly arbitrary current-sensing nonsense that was needed with USB-A outputs is no longer necessary. What they did on those power banks was to just replace the A connector with a C connector, but they left the load detection logic in place. It can't be used with USB-C.

Kuba hasn't forgotten Monica
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  • Spec page or section number that addresses this? – Passerby Oct 11 '22 at 06:16
  • And then you go tell the user to never use a type A cable? – Anas Malas Oct 11 '22 at 07:14
  • Of course they are not compliant. Essentially, the question is in two parts: 1) Are 5.1k Ohm resistors valid for a low-current device 2) Non-compliant powerbanks still should be dealt with in some way to avoid user frustration, so, short of just increasing the current, what is the solution? – sx107 Oct 11 '22 at 23:46
  • @AnasMalas that would be very stupid. – sx107 Oct 11 '22 at 23:47
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Get fancy! Throw on some LEDs with a switch. Explain to the user that these LEDs can be used when the powerbank turns itself off and doesnt have a "stay on" function, or whenever they would like to look fancy.

Just add some red LEDs at 10 mA each and test it that way.

Another option is to straight up let the consumer know they need to plug another device in, or get a powerbank that has trickle charging modes. A cardboard insert can be included in the box.

Anas Malas
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