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LM3478: Switch Freq issue and startup problems

Caleb Munsill
Prodigy 40 points
Part Number: LM3478

I am having trouble using the following schematic in actual implementation 

When I try and run the device the system wont come up at 24V in I have to "ease" it in by first bringing it in at 10v and then turing the voltage up to 24v. additinally the switch frequency is way below expected (below 100kHz) and very unstable.

Also my target is to run the device at 100kHz but when I set the frequency setting resistor to 260k to achieve this,  the device again has a very hard time coming up and have to ease it. The the frequency will slowly get to 100kHz as the device sits on, almost like its a thermal relation. once "warmed up" the device come on and off nominally. 

Any help you can offer is greatly appreciated. 

  • 0
    TI__Guru* 76145 points
    I am afraid you hit the current limit during startup because your output capacitor is very huge. Could you reduce your output capacitor? It seems your output is set to about 32V, and you have a very small load (100 Ohm). Why use >1000uF?

    Please refer to the datasheet about current limit.
  • 0 in reply to Youhao Xi
    Prodigy 40 points

    This is one of the things I was afraid was happening, but doesn't the device have a soft-start feature that prevents this?

    Unfortunately the 100Ohm load is just one of the tests I was doing in the Webbench. The load can will be switched up to 2A. The load capacitance (output capacitance) is part of another engineers design and something I can't change.

    Additionally, can you explain the fact the the switch frequency is so unstable and doesn't appear to reach the desired value until much later (on the order of minutes) after the device comes up? 

  • 0 in reply to Caleb Munsill
    TI__Mastermind 21720 points
    Hi Caleb,

    What is the input voltage range for the application?

    The soft-start of this device is internal and might not be long enough to eliminate current limiting since the output capacitance is large.

    As for the switching frequency issue, can you tell me what input voltage and load current you are testing at? My guess would be that you are operating in pulse skipping mode so the frequency appears incorrect. Also please provide any waveforms of the switch node and the output voltage.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 40 points

    Hey Garrett,

    The input for the application is 24v nominal.

    As far as the soft-start do you happen to have any characterization information on it? The datasheet is pretty vague.

    Here is test example of the freq. drift I have been observing:

    Device has been sitting off for hours, with the freq. setting resistor at a nominal 86.6kohms. I brought the device up no problem by limiting the input to 1A and setting the load to 500mA the scope capture of the gate drive signal (freq. measured and noted in upper right corner):

    Here is ~5 min later:



    and here is after ~30min: