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LM25085: Vout Rising With Increasing Load & Rising Vin

Eric Page
Prodigy 110 points
Other Parts Discussed in Thread: LM25085

I'm working on a DC/DC buck regulator using the LM25085.  Design parameters are as follows:

  • Vin(min): 12.8V
  • Vin(max): 32.0V
  • Vout: 5.2V
  • Iout(max): 4.8A

I used this schematic, which is a copy of a design provided by WEBENCH with a few part substitutions due to availability:

My prototype board works fine except that Vout rises between 0A and 0.5A load, then holds fairly constant up to 4.8A.  The following table was derived at Vin=14.2V:

  • Iout  Vout
  • 0.0   5.216
  • 0.1   5.235
  • 0.2   5.255
  • 0.3   5.274
  • 0.4   5.287
  • 0.5   5.298
  • 1.0   5.298
  • 2.0   5.295
  • 3.0   5.294
  • 4.0   5.292
  • 4.8   5.292

Also, Vout varies with Vin, particularly under load.  For example, at no load...

  • Vin   Vout
  • 13.0  5.210
  • 32.0  5.222  [12mV change; no problem]

...but at a constant 1A load...

  • Vin   Vout
  • 13.0  5.290
  • 14.0  5.296
  • 15.0  5.302
  • 16.0  5.307
  • 17.0  5.311
  • 18.0  5.315
  • 19.0  5.319
  • 20.0  5.322
  • 21.0  5.326
  • 22.0  5.329
  • 23.0  5.332
  • 24.0  5.334
  • 25.0  5.337
  • 26.0  5.339
  • 27.0  5.342
  • 28.0  5.344
  • 29.0  5.346
  • 30.0  5.348
  • 31.0  5.350
  • 32.0  5.352  [142mV change from Vin=13V, Iout=0A / 62mV change with Vin rise]

I can live with Vout rising with increasing load, as it actually helps offset voltage drop in the output cable, but the Vout rise with increasing Vin will cause an over-voltage under light load conditions.

Do you have any suggestions as to what's causing this, and how to resolve it?

Please let me know if additional information is needed.

Thanks!

  • 0
    TI__Genius 11490 points

    Hi Eric,
    The VOUT rising with VIN is common in constant-on-time converters. This is because the ripple voltage injected at the feedback node due to to R4-C6-C5 circuit increases with VIN. I know of two ways to alleviate this issue.
    1. To tune the ripple injection (R4-C6) network to reduce the ripple injection. This reduces line variation but increases the possibility of jittery behavior at low VIN.
    2. Injected VIN feedforward current at FB node using a resistor to VIN(large resistor). This has to be tuned.

    Thanks and regards,

  • 0 in reply to Vijay86929
    Prodigy 110 points

    Option 2 sounds like it might be a little easier and faster to fine-tune.  I'll give it a try.  Thank you, Vijay, I appreciate the quick and helpful reply!

    Eric

    *UPDATE*

    Vijay, you're a genius!  I started with 10M ohm from Vin to FB, and it brought everything right into spec.  At no load, Vout now varies from 5.18V at Vin=12V to 5.16V at Vin=32V.  I still get a small Vout rise with increasing load, which is fine, but with the same tiny inverse function between Vout and Vin.  With the 10M in place, Vout never exceeds 5.25V.  It's fixed!

    Thanks again,

    Eric