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LTC3108 C1 capacitor and Auxilliary bypass capacitor design


I am in the process of designing a power supply using the LTC3108. In the application note, there are certain recommended values for the capacitors at pin C1, C2 and the capacitor at the auxilliary pin. I would like to know how to calculate the values of those capacitors specifically for my design so that I can motivate every design choice and have the circuit working optimally.

Any help would be greatly appreciated.



  • Hi,

    The values of capacitors C1/C2 were determined empirically to optimize the resonant circuit with transformers of different turns ratio. Page 14 and 15 gives the recommended set of capacitor values for different transformer ratio.

    There are examples for application circuits with different turns ratio and optimized C1/C2/Caux values.


    Lower value of capacitor at auxiliary pin is recommended for transformers with 1:100 turns ratio as it will start up faster for lower input voltage range of 20-25mV. Larger cap value reduces the voltage ripple of Vaux and is recommended for lower transformer ratio. So there is a tradeoff between faster response and lower voltage ripple for selecting Caux depending on the minimum startup voltage.


    There are various parameters affecting the design. Two key factor in designing the converter is the minimum startup voltage (Transformer turns ratio is selected based on the value) and the source impedance(should be between 2 to 10 ohms).


    If you share us the design specification, we would be able to suggest you further in component selection and design.