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LM2679: Current limiting too soon

Danilo A.
Mastermind 34580 points

Part Number: LM2679
Other Parts Discussed in Thread: LM2678

Hi Team,

Posting on behalf of our customer.

I will share this E2E post to our customer so he can reply when needed.

I have an old design (has worked well for many years) based on the LM2676-ADJ that I'm attempting to migrate to the LM2679-ADJ in order to provide a bit more output power.

My problem is that my LM2679 seems to initiate current limiting much sooner than I'd expect.  To get started, I set the current limit resistor to 5.3 KOhm, to match the fixed 7A switch current limit of the LM2678, using the 37125/7 equation from the datasheet.  (The LM2679's datasheet doesn't really specify a max value for the switch current, at least I can't seem to find it, but I assumed it would be the same as the LM2678.)

Even under lighter loads, say 2-3 Amps, the LM2679 initiates current limit (as evidenced by the output voltage drop and by viewing the switch output waveform).  I wouldn't expect that output to require anywhere near 7A from the switch. (Vout is 12V, Vin is 16V for this test).  In addition, the current limit function seems so sensitive, that if I so much as put a scope probe on the current adjust pin, it immediately sends the device into current limit.  To further test my assumptions on the currents, I replaced the LM2679 with an LM2678 on the same board, and it works great, well past a 4A load.

I can "make it work" by arbitrarily lowering the current limit resistor (need to drop it down closer to 4KOhm), but I really don't want to go there, since in theory, that would allow switch currents beyond the safe operating area of the device.  I could use the LM2678, but I really want/need the current limit feature of the LM2679, so I'm hoping I can figure out what's happening.

Thanks for any help/guidance you can provide.

Schematic.pdf 

Regards,

Danilo

  • 0
    TI__Guru* 85755 points

    Hello

    It is possible that some noise is getting into the Ilimit pin.

    You might try a small capacitor across the Rset resistor.

    Also, it is difficult to make out from the PCB image the component placement.

    Can you include the silk screen for reference.

    It may be a good test to check your component values on a TI EVM for this device.

    Thanks

  • 0
    Prodigy 20 points

    Hi Frank,

    Thanks for taking a look at my issue.  My board doesn't have a silkscreen, but I've updated the Gerber plots in the attached file to serve as a poor-man's silkscreen, illustrating the placement for the pertinent components.  I had the same thought as you did regarding a cap across the current limit adjust , so I had already given that a try.  Unfortunately, any capacitor placed there sends the part into an all-out current limit.  I have the same issue even trying to probe the current limit adjust pin.  To illustrate, I've also added 3 scope traces to the attached file (see Schematic Rev A.pdf).  They show the switch output remaining perfectly stable throughout a 1 hour run (see Trace 1).  Then as soon as I probe the adjust pin, the device goes directly into what appears to be an all out current limit (See Trace 2).  If I then remove the scope probe on the current limit adjust pin, the LM2679 is unwilling/unable to return to a stable state.  It oscillates (see Trace 3).  But then, if I remove the scope probe on the switch output for just an instant, and then put it right back, the LM2679 has returned to the stable switch waveform.  So, I've obviously done something to cause the current limit function to be hyper-sensitive.  There is a reference to a behavior that sounds a bit like what I'm seeing in the datasheet regarding the value of Cout (see section 8.1.9 of the datasheet).  That might explain why the output goes unstable after a scope probe induced current limit, but it doesn't explain why probing the adjust pin resulted in a current limit condition in the first place.  I'll see if reducing Cout helps get me back to a stable state after the scope probe is removed.  Maybe you'll have some thoughts on how even the scope probe was able to get me into the current limit situation.  Hopefully, that will in turn provide some clues as to why I'm going into current limit too early in the general user case.

  • 0 in reply to Tim Oechsler
    Prodigy 20 points

    Sorry, accidentally replied before attaching: 1425.Schematic Rev A.pdf

  • 0 in reply to Tim Oechsler
    TI__Guru* 85755 points

    Hello

    A few more comments

    1. Try a little larger ceramic input cap; such as 1uF or so

    2. The feedback should be taken close to Cout

    3. The feedback path is somewhat long.

    4. Try using discrete resistors for the feedback and ground near pin 4; temporarily

    5. The current limit set resistor should be grouinded close to pin 4.

    6. The output capacitor ground should be near the diode ground.

    You might be able to try these hacks on your board, or get an EVM and hook up to your board and test.

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Guru* 85755 points

    Hello

    I will close this post due to inactivity.

    Thanks