lpcware

USB0 VBUS voltage divider

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by apriori on Thu Aug 13 00:00:43 MST 2015
In my current, self-powered design, the USB0_VBUS pin of a LPC4337JDB144 is connected to the USB connector's VBUS via a voltage divider as recommended in AN11392. With resistor values of R1=1.74 kΩ and R2=3.24 kΩ, I measure 3.25 V at USB0_VBUS when VDD is 0 V and VBUS is 5 V. With the device turned on, P2_7=LOW and booting into USB0 boot mode, I get 2.08 V when VDD is 3.3 V, just below the 0.7 * VDD threshold that is needed for the DFU firmware to recognize the attached USB host. That leads me to believe there's something equivalent to a 1.5 kΩ pull-down resistor at the USB0_VBUS pin instead of the 64 kΩ (+/- 16 kΩ) mentioned in the manual. This also fits the reports on this forum where people have been measuring in excess of 3 mA going through this pin.

Can anyone comment?

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