lt8330

Hi!

LT8330, documentation p.17 has a reference circuit of 48V Boost Converter.

Duty cycle is ~94%. A max allowed duty cycle is ~84% even for 65 ns On-Time.

Is this circuit correct?

The table defines On-Time for Vin = 24V.

If On-Time has another value for Vin = 3V, where can I find this value?

  • 0
    •  Analog Employees 
    on Apr 5, 2018 1:08 AM

    I'm moving this to the "Power by Linear" forum for support.

  • 0
    •  Analog Employees 
    on Apr 5, 2018 3:46 AM

    Thank you for the post.

    The reference circuit mentioned above (3V - 6V input to 48VOUT) operates in discontinuous conduction mode (DCM), so the duty cycle calculation D = (VOUT - VIN)/VOUT for continuous conduction mode (CCM) is not applicable.  To determine the duty cycle for DCM, the first step is to determine the charge in the inductor during the switch off time: q ~ IOUT/(fsw*efficiency).  Once q is determined, I(PK) is determined and ton can be determined using I(PK).  I will leave this as an exercise but the exercise should show that the minimum on time of the switch is not exceeded.

    Minimum on time should not vary much with input voltage.

  • Thank you for your answer!

    Ok. I see on p.9:

    "If the above duty cycle calculations for a given application violate the minimum and/or maximum allowed duty cycles for the LT8330, operation in discontinuous conduction mode (DCM) might provide a solution."

    1) How can I understand that DC/DC converter can work with desired Vin.min/Vin.max and Vout in DCM mode?

    2) I don't see a design algorithm for DCM mode in the documentation.

    "first step is to determine the charge in the inductor during the switch off time: q ~ IOUT/(fsw*efficiency)"

    In design time I don't know IOUT. I have to calculate max reached Iout for the given Vin.min and Vout.

    3) How should I choose between the CCM or DCM modes?

  • 0
    •  Analog Employees 
    on Apr 6, 2018 4:01 AM

    I made a typo in my post above - I meant minimum off time instead of minimum on time.

    I would go to a power electronics text or just search on-line to learn more about DCM operation.

    Use IOUT max. Currents that are less than IOUT max. will just put the circuit deeper into DCM and that is o.k.

    DCM mode allows higher step up ratios generally and it is a good way to avoid duty cycle restrictions as you noticed.

  • "I would go to a power electronics text or just search on-line to learn more about DCM operation."

    It is very sad when you have to spend time for googling instead of using the IC specific design procedures from the IC documentation.