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LM2592HV: Low temperature issue

Johan Henriksson
Prodigy 10 points
Part Number: LM2592HV

Tool/software:

I am having problem with LM2592HV in low temperatures.

The board needs to be able to handle -40C but below -2C the LM2592HV drastically changes frequency. In the datasheet it says "The switching frequency is reduced when the second stage current limit is activated. " which is what I assume is happening. The problem is that there is nothing on the board that consumes more current when it gets colder. I currently have 220uF 340mohm 16V capacitor on the output. I have replaced it with a 220uF 160mohm 25V capacitor and a 10uF ceramic capacitor and now it can handle -21C.

It seems to be better when I add more capacitance with lower ESR even though the datasheet says 100mohm is the minimum.

What is the highest amount of low esr capacitance I can add?

  • 0
    TI__Guru 56345 points

    Hi Johan,

    Can you share the schematic or tell me what are the operating specifications of the application? If you remove the electrolytic capacitor, can the device operate near -40C? What type of electrolytic capacitor is being used? Is it an OS-CON type of capacitor? Try using OS-CON capacitors which offers low ESR values.

    Ben

  • 0 in reply to Ben C
    Prodigy 10 points




    C21 was originally a Nichicon UWD1C221MCL1GS

    What is not shown on the schematic is that there is another 10uF+1uF ceramic capacitor on the +5V rail.

    C21 was then replaced with Chemi-Con    EMZR250ARA221MF80G (ESR 160mohm @ 100khz).
    This is a capacitor that has low ESR and has low ESR effect due to low temperature but still the ESR will increase with low temperature.

    This solution lowered the temperature the board could handle but still not -40C.

    I also tried to remove C21 but it was worse. It looks like LM2592HVS-5.0 does not like when the ESR goes up to much which it does due to the low temperature.

    I could of course increase the ceramic capacitors total capacitance or find another electrolytic capacitor with lower ESR but what worries me is these sentences from the datasheet:

    "For the output capacitor, the ESR value is the most important parameter. The ESR should generally not be less than 100 mΩ or there will be loop instability."

    Is this not correct anymore? If you still have to comply to this then I do not understand how you are supposed to create a working solution that can handle -40C.

  • 0 in reply to Johan Henriksson
    TI__Guru 56345 points

    What is the output voltage as the temperature drops? What is the load current being applied to the output? Where able to try the test circuit in the datasheet?