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LTM8065 how to select optional SBD and bulk cap


I'm DFAE in Japan. Our customer designs the power supply with LTM8025. He'd like to know how to select optional schottky diode and input bulk cap on figure below. 

Could you please help us to select them ?



  • Hi Hiroyuki,

    Sizing the input cap depends on many factors.  Your AC input current will depend on your input voltage, output voltage and output current.  Sizing the cap will also depend on your system requirements, including EMI and the capability of your input supply (current slew rate).  You can download LTspice and LTpowerCAD to assist in calculating the input current and ESR losses.  You can also contact an ADI FAE if you need more assistance.

    The schottky diode will need to handle the power from the turn-on transient described in the datasheet.  LTspice can also help demonstrate how much energy is transferred to the output during turn-on.  This will depend on the capacitor sizes, capacitor ESL and voltage slew rate of your input supply.

    As you can see, the first step is to simulate your design in LTspice.  Then using that information you can design or tweak the capacitor and schottky diode parts.

  • Dear tkozono,

    Thank you very much for your support.

    I've additional questions by the customer.  Could you please let me know ?

    Q1. If a sudden voltage change occurs during hot swapping, does the soft start function not work ?

    Q2. Should the soft start capacitor be connected between TR / SS and GND even when using the LTM8065 with negative voltage output ? I think it could be ok.



  • Hi Hiroyuki,

    1) The voltage change in negative output configurations is separate from the soft-start.  The soft-start will always work.  Voltage change on the input can be reflected onto the output due to the series capacitors connecting the input and output as described in the datasheet.

    2) The TR/SS capacitor should be connected to chip GND for negative output applications, not system GND.  Soft-start will not work if connected to system GND.