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RUN/SS Pin Problem on LTC3878 and LTC 3729

Hi,

I want to use LTC3878 and LTC3729 in my design.These component have RUN/SS pin. Before advance design, I tried to simulate macro model in LTspice. Of course, these models operated properly. When Css (soft-start capacitor) was changed SS time increased and decreased  according capacitor value. But I am going to use enable signal for these component in design. I connected enable signal (logic signal) to RUN/SS pin , soft start time did not change for any capacitor value. When enable signal was applied output of component rapidly increased and overshoot occurred. How can I solve this problem? Is there any probability on spice model have a fault?

Best regards.

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  • Thanks for your response. Your suggestion is second circuit,ı think. But I will use the PGOOD signal of another dc/dc converter. And this signal is active high. So when second circuit will be used my circuit will not work. When I simulated first circuit (except Vin and Rss) for RUN pin, Css started charging after active high signal. But ı do not have any information about selection D1 ( ı cant see any info in datasheet) diode and what is duty of D1 exactly?3729.asc