LPC Microcontrollers中的主题 Re: LPC4370 - Voltage divider on USB0_VBUS really required?

LPC4370 - Voltage divider on USB0_VBUS really required?

mattiad — Fri, 24 Aug 2018 15:21:48 GMT

Dear All,

I’m using an LPC4370 in a device which is self powered and is used only as an USB device.

At the moment USB pins are directly connected to the USB external line, so USB0_VBUS has a 5V voltage when USB is PLUGGED.

I saw that in my case a voltage divider is recommended because there could be the possibility to have 5V on USB0_VBUS pin when the device is turned off.

In your opinion/experiences is it really necessary? At the moment I haven’t broken USB port yet.

Could it depend on device to device and I’m just lucky now?

Should I review the design?

Best Regards,

Mattia

Re: LPC4370 - Voltage divider on USB0_VBUS really required?

soledad — Mon, 27 Aug 2018 15:13:37 GMT

Hi,

For bus powered applications we are suggesting that you use a regulator.
For self-powered applications where OTG is not being used we suggest a voltage divider circuit using two resistors such that the pin receives 3.3v
For self-powered applications where OTG is being used we suggest you use an active circuit that connects VBUS based on 3.3v on VDD

Please note that the VBUS pads are only 5v tolerant when VDD is present, the condition where VBUS is receiving 5v when VDD is 0v will not cause immediate damage to the pad like blowing a fuse but that this condition will cause a very slight and gradual degradation of the pad over a very long amount of time. In other words, applying 5v to the pad during the time it takes for a regulator to power up VDD will cause no damage to the part at all.

Have a great day,
Sol

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Re: LPC4370 - Voltage divider on USB0_VBUS really required?

mattiad — Mon, 27 Aug 2018 15:34:55 GMT

Hi,

Thank you for your fast and clear answer!

In order to be sure VBUS is recognized high when its voltage is >0,7*Vdd, right?

I want to design a voltage divider which keeps in consideration the integrated pull-down resistor of the pin and the variability of the Vbus voltage.

Best Regards,

Mattia

Re: LPC4370 - Voltage divider on USB0_VBUS really required?

soledad — Tue, 28 Aug 2018 18:17:08 GMT

Hi,

Yes, I suggest to check the following application note:

https://www.nxp.com/docs/en/application-note/AN11392.pdf

Have a great day,
Sol

Re: LPC4370 - Voltage divider on USB0_VBUS really required?

mattiad — Wed, 29 Aug 2018 10:13:13 GMT

Thank you very much soledad.

Best Regards,

Mattia