lpcware

LPC43xx USB V_BUS 5V tolerance errata, removed on latest errata sheet

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by Zuofu on Sun Oct 27 15:26:32 MST 2013
On the LPC43xx series, USB_VBUS is only 5V tolerant if the 3.3V supply is connected to VDDIO. Therefore, connecting USB_VBUS directly to the 5V line on the USB bus is a problem if the 3.3V supply takes some time to power up (for example, if it is powered by a switch-mode power supply). This issue is mentioned in document ES_LPC43x0 revision Rev. 1.2 — 1 February 2012 under section 3.7 (page 8) as USB 0.1, which recommends a workaround using a voltage divider or load switch. However, the latest version of the same document ES_LPC4370/50/30/20/10 Rev. 6 — 21 October 2013 no longer mentions this issue, nor does this issue seem to be mentioned anywhere in the latest datasheet. I can confirm on a LPC4337-FT100 chip that connecting the 5V USB power directly to USB_VBUS when the 3.3 V line takes time to start up is a bad idea, it will cause a latch-up issue where the chip gets very hot.

Is there a reason why this issue was removed from the errata sheet? Is there a more in depth discussion of this in the proper documentation somewhere? Attached are 2 revisions of the errata sheet for the LPC43xx series. I think not mentioning this issue anywhere in the current official documentation will cause a lot of problems for people (it has for me). This is especially the case as the KEIL MCB4300 only has a protection diode on USB_VBUS (which works because when using USB power, the 3.3V rail is provided by a linear power supply with low start up delays).

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