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LM3485: LM3485 WEBENCH Load Transient Ananlysis with different Cout

Wenbin Zhang
Prodigy 10 points
Part Number: LM3485

I am using the LM3485 to convert 24V DC to 8.15V DC. Once I change the Cout to different brand (same Capacitance and same ESR resistance), the load transient analysis shows the output voltage falling to a very low level. I know the Capacitance and ESR value affects the DC-DC performance, what else parameter will affect this for this Cout selection?

  • 0
    TI__Expert 3400 points

    Hi Wenbin,

    The capacitor's DC voltage rating may also cause performance issues, which does not appear in your schematic. Can you please share the part number of the WEBENCH recommended capacitor and your chosen capacitor so we might return to you with a more complete understanding? Please share the BOM for each build.

    Thank you,

    Ryan

  • 0 in reply to Ryan
    Prodigy 10 points

    Dear Ryan,

    Thank you so much for kindly response my question. 

    Here is the example. This is the default recommendation from WEBENCH. a Vishay 100uf, 125mohm capacitor is recommended. The load transient analysis is fine under this condition. Once I switch the capacitor to very similar capacitor (the only difference is the RMS current), the output voltage is down to a very small level. I do not know what are causing this behavior. 

    BR,

    -Wenbin

  • 0 in reply to Wenbin Zhang
    Prodigy 10 points

    Sorry, forgot to attach the BOM. the only different between two builds is the capacitor. 

  • 0 in reply to Wenbin Zhang
    TI__Guru 50120 points

    Hello Wenbin,

    This is a hysteretic control device so the switching frequency depends on the external components and operating conditions. 

    If in your simulation you picked different output capacitor brands but made sure to keep the same capacitance and same ESR, I can't imagine why there would be significant difference in the load transient analysis.

    The output capacitances ESR since this value impacts the operating switching frequency. 

    Also note that at light load the device will operate in discontinuous conduction mode so the operating frequency will lower and switching losses reduce.

    Have you tried picking an MLCC capacitor with a resistor in series to see if this results in similar load transient analysis? 

    Regards,

    Jimmy