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I'm trying to add power control to a USB hub based on the Terminus Tech FE1.1s hub chip. From the datasheet, I know the chip supports ganged power control, so all ports share a single switch. That's fine for me, I'm actually planning to use the switch to control only one of the four downstream ports the hub has.

As far as I understand the datasheet, the PWRJ pin (pin 25) is a digital output pin that outputs low (<0.4V) for when power should be enabled, and high (>2.4V) when it's disabled. These levels are at 4mA.

I'm now trying to figure out what kind of transistor (and accompanying resistor) I need to use for switching the power of one of the hub's USB ports with the output of the PWRJ pin. The consumer at that port (soldered in) could theoretically draw 500mA, but in reality, I believe even 100mA could be enough.

I imagine the circuit to look like this: enter image description here

I'm a beginner when it comes to electronics, and I still find the range of transistors rather complex. I believe I need an NPN transistor (since low should enable power), and in order to get minimal voltage loss (USB needs its ~5 Volts), I probably need a MOSFET?

What transistor could fit my requirements here? What characteristics do I have to look for?

Any help is highly appreciated!

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    \$\begingroup\$ search for MOSFET high-side switch \$\endgroup\$
    – Marcus Müller
    Oct 18, 2019 at 9:45

2 Answers 2

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You want to use a P-channel MOSFET for the high side power switching. Connect the source terminal to the +5V and the drain terminal to the USB load. There are a multitude of devices to choose from but you could look at something like a NTR3A052PZT1G from OnSemiconductor. It is nice part rated at a couple of amps in a small SOT23 package.

You will have to do some looking at the hub chip output. If its power switch output control does not swing up to near 5V (data sheet only promises 2.4V) then you will have to provide some extra circuitry to translate that output to a full 5V swing. This is needed because you want to pull the gate terminal of the P-FET all the way up to the 5V rail to ensure that it turns off.

(The extra circuitry involved to take care of the P-FET gate drive may very well drive you toward using a regular USB power control chip as everything would be taken care of in a nice small SO-8 package including a compatible logic input for the power control. If you go this route make sure select a chip with active low enable because there are also chips with active high enables).

If you have a pullup resistor on the PWR switch control output make sure to select it so that when the output is low and the pullup is dropping 5V that the current does not exceed the current rating of the HUB output.

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  • \$\begingroup\$ Thank so much for your detailed answer! I'm measuring about 3.35V for the PWRJ pin when power should be disconnected. Would that be enough for the transistor solution? \$\endgroup\$
    – FD_
    Oct 19, 2019 at 16:54
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    \$\begingroup\$ @FD_ - No it would most likely not be enough. (5V - 3.35V) = 1.65V would be the Vgate-to-source on the P-FET. Even for FETs to not be considered as "logic level" FETs this still places the FET at the threshold of being in the turning on state. \$\endgroup\$
    – Michael Karas
    Oct 19, 2019 at 18:21
  • \$\begingroup\$ Alright then. Will look for these power switch chips. Thanks for all your helpful tips! \$\endgroup\$
    – FD_
    Oct 19, 2019 at 19:59
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I would advise against using a simple transistor here in the first place.

USB hubs usually use dedicated USB port power switch devices that have overcurrent limiting and can tolerate large surges.

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  • \$\begingroup\$ Well, there are a few reasons why I'd rather go for a transistor here: 1) The port I want to toggle has a consumer soldered into place, so the solution can be specific to that downstream device (ethernet adapter). 2) This is for retrofitting a device that only cost a few dollars, and the solution should be cheap and fit into the case. \$\endgroup\$
    – FD_
    Oct 18, 2019 at 10:24
  • \$\begingroup\$ Power switch chips cost around fifty cents to a dollar. \$\endgroup\$
    – Chris Stratton
    Oct 18, 2019 at 13:11
  • \$\begingroup\$ @ChrisStratton Where can I find these and what terms should I search for? \$\endgroup\$
    – FD_
    Oct 19, 2019 at 16:59
  • \$\begingroup\$ @FD_ - Search for "Power Switch" ICs or "USB Power Control", \$\endgroup\$
    – Michael Karas
    Oct 19, 2019 at 18:38

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