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LM5116-HT: LM5116 vs LM5116-HT Start up

Ed Hildebrandt
Prodigy 30 points
Part Number: LM5116-HT
Other Parts Discussed in Thread: LM5116,

We have been working on a power supply design using the LM5116.  This is a high temperature design so we are going to use the HT part.  To save some cost (avoid destroying $300 parts),  we put a LM5116MH on a DIP16 carrier board and have been swapping the two parts in and out as required for testing.  We have a DIP16 socket on board to make this swap easier.  For the most part this has worked well.  We have two configuration of the board, one for 32V and one for 60V.  The 32V design worked well with either of the part.

The 60V design has a bit of an issue.  It works fine with the LM5116MH  part, but when we swap in the LM5116-HT part (same board with no other changes) the board does not want to start up without a significant load on it.  It seems to run fine with about a 300mA load, whereas if I use the LM5116MH part it will startup fine with just a very light load (~25mA).

Any suggestions?

Thanks
Ed H

  • 0
    TI__Guru 53011 points

    Hello Ed

    Since those two devices are functionally identical, I guess the issue is related with parasitic element (Parasitic capacitance/ Parasitic inductance). Also, I guess the issue is related with switching noise since the issue happens at higher input voltage. 

    The device turns on the LO driver by force if the SW node does not fall down enough within 150ns after the HO driver turns off. This hard switching can generate a large switching noise and may cause an unexpected behavior. 

    Please monitor V(SW-GND) together with V(LO-GND) at both 300mA and 25mA, and check if the converter does hard switching at 25mA but doesn't do the hard-switching at 300mA. 

    -EL 

  • 0 in reply to EricLee
    Prodigy 30 points

    I confirmed that the LO driver is turn of about 150nS after HS is turned off.  This happened with both the 300mA and the 25mA loads.  Also in both cases the LO stay on for a very short period of time, only about 25nS.  I presume this is because of the diode emulation mode although the inductor current is still positive when LO turn off.  On that note we are running in force diode emulation mode with DEMB tied to CSG through a 0R resistor.

    Some further information on the light load (~25mA) operation.  The device appears to be resetting continually every time going back into soft start.  The output voltage does start to rise but no where near the rated voltage of 60V. 

    Ed

  • +1 in reply to Ed Hildebrandt
    TI__Guru 53011 points

    Hello Ed 

    I think you are seeing the hiccup mode operation. If you can, please try to add a RC filter at between the CS and CSG pins and check if any improvement. 

    Also, please make it sure to locate the filter capacitor very close to the IC pins.

    -EL  

  • +1 in reply to EricLee
    Prodigy 30 points

    Eric,

    We did not get a chance to try your idea but we do seem to have resolved the issue.  We have gate resistors on both the high side and low side FETs.  20R on the high side and 10R on the low side.  We shorted the 10R low side gate resistor and that resolved the issue.

    In looking into this, we found one thing that we don't quite understand.  The data sheet says that the SS pin (soft start) should ramp up to 3V, but this is not what we see.  We do see the ramp on startup, but it ramps to 2V over about 1.3 seconds at this time it drops to 0.6V and stays there.  Just would like to understand why this is happening as it does not seem to match what the data sheet says.

    Thanks
    Ed

  • 0 in reply to Ed Hildebrandt
    TI__Guru 53011 points

    Hello Ed

    Please check the description about the SS-FB clamp on the page 18 of the datasheet.

    Once SS charges to 3 V the SS latch is set, increasing the DEMB bias current to 65 µA. An amplifier is enabled that regulates SS to 160 mV above the FB voltage. This feature can prevent overshoot of the output voltage in the event the output voltage momentarily dips out of regulation

    -EL 

  • 0 in reply to EricLee
    Prodigy 30 points

    Eric,

    Thanks for all the help and suggestions.  The issue appears to be resolved.

    Ed

  • 0 in reply to Ed Hildebrandt
    TI__Guru 53011 points

    Thanks for choosing TI. Please feel free to contact us if you have any question.
    -EL