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What is are the resistor values permitted on Vbus for the LPC1837

Question asked by Nicolas Boucquey on Mar 29, 2019
Latest reply on Apr 1, 2019 by xiangjun.rong

Hello,

 

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,

 

Best regards,

 

Nicolas Boucquey

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