lpcware

LPC1769 and voltage divider for USB VBUS

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by robert hulsebos on Fri Dec 11 09:04:49 MST 2015
Hi all,

We have a PCB that uses the LCP1769 and configures its USB port as an USB device. It is a self-powered design, the LPC1769 does not derive its power from the USB bus.
In more recent versions of the LPC176x datasheet / AN11392 it is recommended that two resistors are added to the VBUS signal of the USB port. These resistors are to function as a voltage-divider for bringing down the 5V of the VBUS signal to the maximum allowed 3.6V. (see paragraph 5.1 of most recent datasheet)
As our PCB design predates these recommendations, it does not include the voltage-divider. However, it does include a series resistor of 1K in the line to the VBUS pin.
My questions now are:

1. Is VBUS voltage-divider really required ? What is the real risk of not having the voltage-divider ?
2. Is the 1K series resistor a sufficient means of mitigating this risk ?
3. What are the recommended values for the resistors of the voltage-divider ?

I noticed in other forums on this site that for other LPC families NXP recommendations are [u]not[/u] to use a voltage-divider, but other circuity instead.
( e.g. see https://www.lpcware.com/content/forum/usb0-vbus-voltage-divider )

3. Does this now also apply for the LPC1769 - has NXP insight on this matter changed?

Regards,

Robert

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