LT3757 Discontinuous Operating Conditions

Hi,

I am interested in using the LT3757 to build a boost converter (that'll be powered by batteries later) in discontinuous mode. However it seems that most of the data sheet related to the LT3757 is about continuous functionality. If someone could specify the requirements to get the LT3757 to work in discontinuous mode, that would be greatly appreciated. Thanks.

  • 0
    •  Analog Employees 
    on Oct 11, 2018 8:11 PM

    Foody,

    Please provide your input voltage range, desired switching frequency, output voltage level and load current requirement.

    Cheers!

  • Input voltage: 12V

    Switching frequency: 1MHz

    Output voltage: 80V

    Max load current: 2A

  • 0
    •  Analog Employees 
    on Oct 11, 2018 10:36 PM in reply to Foody

    From page 14 find the duty cycle, then calculate IL(max), from the equation on the next page, as below.

    Duty cycle should be about 85%, which corresponds to about 0.85uS, based on your switching frequency.

    Now find what inductor value you need for IL(max). So, (Vin*Ton)/IL(max))=L. This is the inductance you need to be at the boundary of CCM, and DCM, and should be around 0.7uH. Any value below that will have your converter operating in DCM.

  • Hi thank you for your response. 

    I believe I can follow this, but I am curious how you derived the inductor L=(Vin*Ton)/IL(max) relation. Could you also shed light on why the inductance I'll need is below the 0.7uH threshold? Thanks again.

  • +1
    •  Analog Employees 
    on Oct 12, 2018 7:46 PM in reply to Foody

    The IL(max) is the ripple current you need in the inductor, to be at the boundary of CCM/DCM. I guess I should call it IL, or peak-peak inductor current.

    So, for you, find the average input current first and then, multiply by 2, and that is the inductor current you need to be at the boundary. The inductor ripple rides on the average input current, and when it hits 0A, DCM starts. 

    So, your output power, Pout=160 watts, assume 90% efficiency and the input power is 177 watts. That gives you about 15A for average input current.

    The inductor current you need to start going into DCM is 2 times the average current, so about 30A. So, that is actually about 1/2 what I calculated yesterday, or about 0.34uH.

    I am not sure you want to be operating in DCM for this application, but, that's how you get there. Let me know if this helps.