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Back biasing a LMR12015 DC/DC Buck Converter...

Brent LaPorte
Prodigy 10 points
Other Parts Discussed in Thread: LMR12015

I am using the LMR12015 to convert 9V from a 9V battery into 3.0V.  The LMR12015's AVIN (analog supply input), PVIN (power supply inputs) and EN (enable)  inputs are directly tied to the 9V battery voltage.  Here is my question.  When the battery is disconnected, so no 9V supply input, but the 3V output is held to 3V by an external supply source, does the LMR12015 care?  In this state, the PVIN, AVIN and EN lines are low, causing the LMR1215's outputs to be disabled and the analog circuitry in the chip is unpowered.  The potential issue I see is the external 3V supply will back drive the FB (feedback) input to 1V, with no voltage on the AVIN pin.  This condition could cause current to flow through the ESD protection diodes on the FB pin.  The specification doesn't state the DC current limit the ESD diodes would need to be kept under.

Note.  We are potentially going to build a very large number of these devices and I need to understand issues I may have with the LMR12015.

Thank you for any information you can supply about this issue.

  • 0
    TI__Guru 57555 points

    Your concern is valid.  With any buck you run the risk of damage when the input is shorted while the output capacitors are charged.  The current path is from the switch node to the input, since there is a parasitic diode from SW to VIN.  This can be a very damaging type of fault.  The FB path is less risky since the FB dividers will limit the current.  In your case if you are just floating the input, instead of shorting it, you may not have too much trouble.  However, since the output caps are discharging through the inductor to the input, you may see a resonant rise in the input voltage.  A zener on the input may be prudent.  The best plan is to build a prototype and test with the input opened, while monitoring the current throught the inductor.  We will be happy to look at your results.