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I have a USB 3 Type A (i.e. 9pin connector) connector on my PC and connected to a USB 3 Type A connector on my PCB with a USB 3.0 Cable. This USB3 A Type connector on my PCB is connected to FT232H chip. It means i am having USB 2 Communication between that connector and FT232H chip. My question is : is it possible to get 900mA Current from the Pin1(VBUS) of that USB Connector. Currently VBUS(5V) from that USB Connector goes to FT232H chip and 2 DC-DC Regulators. These regulators are used to make 3V3dc and +/-2.5Vdc power supplies for use among the various ICs on the PCB.

In case it is not possible with the above setup, what can be done to get maximum current out of VBUS Pin.

Thanks

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  • \$\begingroup\$ usb 3.0 ports provide 900ma no questions asked (there is no communication pin or anything to set any limits or anything as far as I know). I think you must be ok. But don't forget to protect it against short/overcurrent, you don't want to cause any trouble on the mb side (or better power it from some other supply until all testing is done and you're 100% sure about it) \$\endgroup\$
    – Ilya
    Commented Jul 10, 2020 at 14:49

2 Answers 2

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USB 3 Type A (i.e. 9pin connector) port must provide AT LEAST 900 mA if the host system is powered from AC or with a sizeable battery. So draining 900 mA should be fine.

However, what if your device gets plugged into a regular USB-A 4-pin port? These ports are mandated to supply AT LEAST 500 mA. Is this sufficent for your needs? Of course, 95% of USB2 ports would provide quite a bit more than 500 mA, but there are some (rare) cases when USB ports have overcurrent protection in hardware tuned to 600-700 mA cut-off.

Therefore, to comply with USB standard, you should include some detector of whether the port is USB2 or USB3.

One way to detect the presense of USB3 port is to have 45 Ohms termination on Rx+ and Rx-pins, and check if there is electrical activity. The USB3 host, when it's Tx sees termination on the opposite side, will periodically issue Rx_detect pulse, and then send LFPS signals (10 kHz bursts with 500-600 mV amplitude (1V p-p differential), for 360 ms. This activity should be relatively easy to detect with any MCU, and make an appropiate decision to engage or not to engage.

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Connecting two devices by a cable with USB-A on each end is allowable under the USB 3.0 spec but there should not be any power transfer. A cable that complies with the USB 3.0 would not have the 5VDC pins connected. (The D+/- pins for USB 2.0 data would also not be connected on such a cable but that's an aside.) If there is concern about compliance with the spec then using the correct connectors would be a part of that.

Any USB device should not pull more than 150 mA until it has been "enumerated", that is the host has recognized the device is connected and communicating. Once enumerated then power negotiation can begin. The PCB may be using only USB 2.0 for communication but that doesn't mean it cannot pull USB 3.0 kind of power. I've seen USB 2.0 devices that pull 1.5 amps, but again they pull this amount of power only after getting permission from the host to do so. Well, it might technically be the USB-BC spec (not the USB 2.0 spec) that allows 1.5 amps, the USB specifications can be so confusing.

USB 3.0 allows for up to 3.0 amps from USB-A ports, and even more than that if using the option of USB-PD power negotiation but that's perhaps an aside as well. The two points I am making on not burning something up is to use the correct connector, then not pull more than the minimum allowed current until a request to do so has been given the OK by the host.

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