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LMR23630-Q1: 220k pull down resistor on the EN pin

Part Number: LMR23630-Q1

Hello TI,

We enable our LMR23630-Q1 using the voltage from 4 other signals on the board. These signals are 3V or 1.65V (drop from 3.3V and 1.8V due to small signal diodes). In the absence of these signals there is a 220k Ohm resistor to pull down the enable pin. Is this acceptable? The intention is not to set the UVLO thresholds, instead there's the 220kOhm pull down resistor simply to prevent the EN pin from floating. Could you kindly advise if in your opinion there are any issues with this implementation? 

  • Pradeep,

    That looks fine to me. Keep in mind the EN rising threshold is 1.7V worst case so the 1.8V through the diode may not be able to enable the IC.

    -Sam
  • Hello Sam,

    thanks a ton for the prompt response! The datasheet mentions the typical EN rising threshold to be 1.55V, which is why I chose the 220k Ohm. In my tests over the last few hours, it seems to be working fine. I could increase the resistor value higher if there that would be a safer bet. Lesser current flows through the diodes and the voltage drop due to the diodes is lesser.

    Should I try and go for a higher resistor value?
  • Pradeep,

    The datasheet does say EN rising threshold is typically 1.55V but the max spec (worst case in your situation) is 1.7V. So we need to design for 1.7V if we want to ensure proper operation over the margins of this part.

    You're right, a higher resistor value will drop less over the diode but much higher and it might not be able to pull down from any leakage on that node. The datasheet says the EN pin leaks current into the pin so that is probably not an issue but I'd still not go over 1Mohm for the pull-down.

    If you have 3.3V DC available you could probably do something like this:

    If not you'd probably need a more complex level shifter circuit.

    -Sam

  • Hello Sam,

    thanks for the quick schematic! I'll have to evaluate our available options and see what would be the safest bet! Thanks a lot again.