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LM5025A: LM5025A Evaluation Board - Feedback Circuit

Rolf Lerch
Prodigy 240 points
Part Number: LM5025A

Hi

In Figure 9 of SNVA097B you showed the Application Circuit. We understood most of it, but not the Function of D7 in the feedback path.

  1. Please explain exact the function of the two parallel diodes.
  2. We replaced D7 on the eval board with a Schottky an suffering now  from an unstable output. After few minutes the output voltage starts increasing. Any idea why?

 Best Regards Rolf

  • 0
    TI__Expert 7900 points

    Hello Rolf,

    I need to know the part number of the schottky diode you used to replace D7.

    Also, what is the typical voltage at +SB / TP4 when operating normally?

    Thanks,

    Eric

  • 0
    Prodigy 240 points

    Hi Eric

    We are using dfls1100q-7. Can't tell /measure the voltage on T4, the board is in a case.

    What's the purpose of D7?

    Best Regards

    Rolf

  • +1 in reply to Rolf Lerch
    TI__Expert 7900 points

    Hi Rolf,

    I was double-checking with my colleague before describing the functionality of D7.  We believe this diode clamps the reference to U3B-5 during startup (i.e. +SB is very low) and provides a bit of a soft start function before U4 (1.2V reference) is fully on.

    Can you remove D7 and test/compare startup waveform?

    A schottky diode has a relatively high leakage, especially when it is hot.  If D7 has significant leakage it will raise the reference to U3B-5 via R33 (20k).  I think this explains why your output voltage starts increasing after a few minutes.

    Regards,

    Eric

  • +1 in reply to Eric Reicher
    TI__Expert 7900 points

    Hello again Rolf,

    There is another possibility for D7.  When the system shuts down and SB+ becomes 0V, D7 will discharge capacitor C30 very quickly.  C30 is located at U3B-5, the reference for the control amplifier.  Discharging C30 quickly sets the system for a quick restart.  Without D7, C30 must discharge through R33 and R34 (30k), forming a time constant of 3ms.

    Regards,

    Eric

  • 0 in reply to Eric Reicher
    Prodigy 240 points

    Hi Eric

    Thanks for supporting me. I'll try your suggestins.

    Regards Rolf