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LM5149: LM5149 Vout to Vin shottky and reserve protect function question.

Chao Fu Chen
Intellectual 1030 points
Part Number: LM5149

Hi Expert,

I would like to ask about the reverse protection description of LM5149. When VOUT is greater than VIN 2V, there is an internal ESD diode to help VOUT discharge, but I looked at the Block diagram and did not see it. Is it actually there?

In addition, the instructions suggest adding an external schottky diode from the Vout pin to VIN. How should I select the schottky specifications required for this method? Since this one is not on the demo board, do I have to add it?
We currently apply it to 38V to 25V 300W output and 38V to 12V 180W output. Please provide suggestions, thank you.

Eddie

  • 0
    TI__Mastermind 30020 points

    Hello Eddie,

    We do not display it in the block diagram but it exists. if VOUT>VIN DC current will flow through the said diode and cause damage to the device.  place a Diode to VIN pin to prevent DC current flow from VOUT to VIN.

    Hope this helps,

    David.

  • 0 in reply to David Baba
    TI__Intellectual 1030 points

    Hi David,

    Got it . thank your comment for question1.

    How about the adding an external schottky diode from the Vout pin to VIN ?

    Is that the necessary ? any recommend spec of schottky ?

    Eddie 

  • 0 in reply to Chao Fu Chen
    TI__Mastermind 30020 points

    Hello Eddie,

    Theoretically the bypass diode approach, will provide protection, however there is nothing limiting the current should by chance the V forward drop gets close to the breakdown voltage of the ESD diodes.  I would suggest following datasheet recommendations and use the blocking diode method.  if you don't, you will need to ensure that the bypass diode method is providing adequate protection under all conditions.  If it were me, I would use the diode blocking method as detailed in the datasheet.

    Hope this helps.

    David.

  • 0 in reply to David Baba
    TI__Intellectual 1030 points

    Hi BaBa,

    But the datasheet only said this ;TI recommends adding a Schottky diode in series with the VIN pin of the LM5149 to prevent reverse current flow from VOUT to VIN.

    No any spec recommend , could you please share any guide line how to select the Schottky diode of spec ? Or we can miss this diode just use internal ESD diode to defend the external spike of stress ?

    Eddie

  • 0 in reply to Chao Fu Chen
    TI__Mastermind 30020 points

    Hello Eddie, 

    Please add a small signal Schottky capable of 1A IF and VRRM >VINmax-VCC.

    Place diode as shown below, hope this helps.

    David.