We have implemented the Vbus protection circuit for the LPC1837JET100 (Figure 46 in datasheet p.136). The goal is to avoid the 5 V to be applied on the USBx_VBUS pin when there is no VDDIO. The datasheet specifies the voltage on that pin to be within 3.6 V and 0.7*VDDIO.
We used the resistor values (names from the datasheet figure) R2 = 10k and R3 = 12k. Which leads to a voltage on the pin of 2.7 V when the Vbus is 5 V.
Unfortunately, when connecting the USB to a desktop, the device is not recognized.
We solved that problem by replacing the resistor values by R2 = 150 R and R3 = 270 R.
Is there a range for these resistor values ?
Could you please explain why the resistor should be lower ?
What are the values you would recommend ?
Many thanks in advance,