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LM2679: BEWARE: Startup problem!

Gregg Nardozza
Prodigy 30 points
Part Number: LM2679

The LM2679-5V part does not start properly if the input voltage is brought up very slowly.  The LM2679 should not try to start until the input voltage reaches the minimum required voltage for regulation, else the device gets locked into a non-regulating condition if the input voltage current limit is less than the 5V output supply current.  I solved this by increasing the soft start capacitor to 1uF to delay the start of the LM2679 so that the input converter would be through soft start and regulating at full voltage, but it is not a robust solution.  The LM2679 is poorly designed in my opinion.

  • 0
    TI__Mastermind 25590 points

    Gregg,

    I would like to try to reproduce this in our lab.  Please share the following information:

    • Schematic
    • Layout
    • Input Voltage, Input Current
    • Output Voltage, Output Current
    • Waveforms of the issue (showing VIN, VOUT, SW, Soft Start)

    Thanks!

    -Sam

  • 0 in reply to Samuel Jaffe
    Prodigy 30 points
    Sam,

    The schematic is identical to your app note. Here is what to do to see this: set the 5V load current to 3A resistive (1.7 ohms). Set the current limit on the 24V input supply to less than 3 A (perhaps 1A). If you turn on the 24V input quickly, then everything should start properly. BUT, if you bring the input voltage up very slowly, the converter will being to switch when the input voltage is too low, thus drawing enough output current to keep the input source in current limit and the input voltage will never get high enough to start the converter properly.

    It is an easy test. Try it.

    Gregg
  • 0 in reply to Gregg Nardozza
    TI__Mastermind 25590 points
    Gregg,

    Which app note are you referring to? I've ordered a few EVMs to try to recreate this issue.

    At what voltage does the LM2679 start switching?

    -Sam
  • 0 in reply to Samuel Jaffe
    Prodigy 30 points
    Sam,

    See Figure 13 of the data sheet. The LM2679 seems to begin switching immediately... that is the problem.

    There should be a divider off the input voltage which feeds the chip to know when the input voltage is high enough to begin to switch such that there is not a current limit tug of war.

    Gregg
  • 0 in reply to Gregg Nardozza
    TI__Mastermind 25590 points
    Gregg,

    I see what you're saying. We do spec the minimum supply voltage as 8V, so one could assume that the part shouldn't start switching until the supply voltage reaches at least 8V.

    But the recommended minimum supply voltage pertains more to steady-state and, well, recommended. If the part starts switching below 8V, the part can start regulating as the input ramps so the output can get to regulation faster. Now this can obviously cause issues, as it has with you. But this is why we have that soft start option.

    The thoughts that went into this decision were considering the tradeoffs. Your specs, unfortunately, fell into the wrong bucket. Fortunately, though, we have that soft start pin to make this work : )

    -Sam