DS90UB933-Q1: Pull down resistor value for PDB IO

Harry Shin1

Expert 4780 points

Part Number: DS90UB933-Q1

Hello Team,

My customer design wrong schematic. it input 3.3V for PDB even VDDIO=1.8

but they have serial resistor on PDB signal line as below.

So I think they can use this serial resistor to change PDB level to 1.8V if I know Pull down resistor in PDB IO.

I understand it may have weak pull down resistor.

Can I know pull down resistor value in PDB IO??

Thank you.

over 1 year ago

0 Nicholas Darash over 1 year ago

TI__Mastermind 18755 points

Hello,

I think what you are doing should be possible but the internal pull-down may very. Allow me a couple days to find what the value of this pull-down might be.

Regards,

Nick

0 Casey McCrea over 1 year ago in reply to Nicholas Darash

TI__Guru* 98485 points

Harry,

I would not recommend this method because the PDB pulldown would be very large value meaning that to divide the voltage down you would need a very high series resistor looking into the part. That means that your effective bias current through the resistor divider would be extremely low and leave the PDB pin quite susceptible to noise coupling. Also, the pulldown current is not going to be well controlled so I doubt it would be possible to find a single value for the resistor that would work well across PVT. So please instruct the customer to revise the design to use the correct voltage straight away or use an external resistor for the pulldown with reasonably low value rather than relying on the weak internal pulldown

Best Regards,

Casey

0 Harry Shin1 over 1 year ago in reply to Casey McCrea

TI__Expert 4780 points

Hello Casey,

This problem is also same as on 953 side. it has 1Mohm pull down in IO in datasheet.

With 953 , they also cannot use serial resistor to make 1.8V level for same reason?

Thank you.

+1 Casey McCrea over 1 year ago in reply to Harry Shin1

TI__Guru* 98485 points

Hello Harry,

Yes, same reason for 953 I would not recommend to do this

Best Regards,

Casey

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DS90UB933-Q1: Pull down resistor value for PDB IO