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I'm building a keyboard which will use SK6803 mini-e LEDs (ws2812 driver). I'm using a Pi Pico and as such the USB limit for current draw is ~500mA. I'm looking to limit the current draw from the LEDs to around 300-400mA and I found out that the TPS2553DBVR IC can do this, however I'm a bit confused about the NFAULT and EN pins.

In my current design I don't have enough GPIO pins on my MCU for the EN and NFAULT signals so I was wondering if I can ignore the NFAULT, either leaving it floating or pulling it high (or low, I'm not sure tbh), and for the EN pin either just having 5V+ going in or using the same data signal which turns on the LEDs (also a 5V signal, but driven) so that the current limiter is always on? I just want it to consistently limit the current in the given range.

My schematic so far is looking like this: enter image description here

Any help would be greatly appreciated!

Also if I use 5V+ for the EN pin, should I use VBUS or VSYS from my Pico, and would a pull up or down resistor make sense?

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  • \$\begingroup\$ What do you want to happen when the current gets limited? You can't expect the WS2812s to keep working normally when their supply voltage is reduced by the TPS2553. \$\endgroup\$
    – Finbarr
    Commented Nov 21, 2023 at 16:32
  • \$\begingroup\$ @Finbarr I just want the current to be limited to the LEDs in general so I don't surpass the 500mA from the USB. Otherwise the LEDs will draw too much current (currently using 110 LEDs, each draw around 9mA at full brightness). Their voltage will still be 5V. \$\endgroup\$
    – printsmith3d
    Commented Nov 21, 2023 at 16:38
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    \$\begingroup\$ You can't limit the current while keeping the voltage the same. \$\endgroup\$
    – Finbarr
    Commented Nov 21, 2023 at 20:54

2 Answers 2

Reset to default
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You can tie the ENABLE pin to USB+5. The only reason to connect it to a GPIO would be if you wanted to shut off power for some reason, which seems pointless if you have addressable LEDs that you can already turn off digitally.

Do you have a reason to check the FAULT status? If not, you can also ignore it. It is open drain so you can leave it floating.

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  • \$\begingroup\$ Thanks so much! I wasn't sure if this was the case but had a hunch, needed someone with more knowledge to confirm :) Would it be better to use VBUS or VSYS from the Pi Pico? Right now I'm using VSYS for my 5V power from the Pi PICO. EDIT: Also should I put a pull down or pull up resistor on the EN and the IN pins of the TPS? \$\endgroup\$
    – printsmith3d
    Commented Nov 21, 2023 at 15:25
  • \$\begingroup\$ I would connect the enable pin to whatever power supply is powering the chip. A pull up is a resistor to the power supply. If you connect something to the power supply directly then it cannot have a pull up resistor (since both ends of the resistor would be connected to the same thing). \$\endgroup\$
    – user1850479
    Commented Nov 21, 2023 at 15:39
  • \$\begingroup\$ Everything is being powered with the USB from the Pi Pico, so I guess VSYS works fine. No resistor needed then. So it would be 5V+ to IN and EN and that's it? \$\endgroup\$
    – printsmith3d
    Commented Nov 21, 2023 at 15:42
  • \$\begingroup\$ I think that is all you need. Just the "Typical Application" circuit without the GPIO pins connected. \$\endgroup\$
    – user1850479
    Commented Nov 21, 2023 at 15:51
  • \$\begingroup\$ Just to confirm (sorry Im paranoid) this is my new circuit: imgur.com/RkRxaPF \$\endgroup\$
    – printsmith3d
    Commented Nov 21, 2023 at 16:36
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FAULT is open drain, so it shouldn't matter which choice you make. Datasheet doesn't mention any alternative way to connect FAULT than a pullup (another way to implement the signal is described in 10.2.2 Auto-Retry Functionality). If this is a PCB you're creating, add a pullup for the PCB but don't populate it, and do the same for GND but just a bridge, this leaves you with options to both correct and to use the signal if you find the need.

or using the same data signal which turns on the LEDs

Sounds good.

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  • \$\begingroup\$ Since it's open drain I can just leave the pin floating though correct? No need for a pullup on nfault? Also I'm thinking maybe using the 5V from the VSYS of the Pi Pico rather than the signal from the LED data, it's 5V either way I just figure using 5V from VSYS means its always on limiting the current to the LEDS. Also not sure but should I have a pulldown on the EN pin if I'm just grabbing 5V? \$\endgroup\$
    – printsmith3d
    Commented Nov 21, 2023 at 20:34
  • \$\begingroup\$ Yes that's what I'm saying, you can add the tracing for pullup as well as bridge to GND, but don't use them. Just in case there will be an issue, or that you have second thoughts and wanna use the FAULT signal, then just solder things in place. \$\endgroup\$
    – MiNiMe
    Commented Nov 21, 2023 at 23:23
  • \$\begingroup\$ Ah okay, and you mean to put those footprints both on the FAULT signal right? \$\endgroup\$
    – printsmith3d
    Commented Nov 22, 2023 at 13:39
  • \$\begingroup\$ That is right.. \$\endgroup\$
    – MiNiMe
    Commented Nov 22, 2023 at 13:49

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