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LTC3777 DRVCC Question

Hi,

I would like to use LTC3777 controller for a 5VDC to 150VDC input and output of 12VDC/10A power supply. According to the datasheet, DRVCC voltage can be set from 6V to 10V in 1V increments with DRVSET pin .

Q1: When EXTVCC is not used (tied to ground), DRVSET is tied to SGND (6V configuration), and VIN = 5V, the internal VIN LDO is not able to generate 6V because VIN is less than 6V. Does this mean that the voltage of DRVCC is going to be equal to VIN?

Q2: If the DRVCC voltage is less than 6V, do I need to select the MOSFET which can be driven by whatever voltage is on the DRVCC?

Thank you for your help.

Kind regards.

Liu

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  • Hi Liu,

    For Q1:

    This is correct. DRVCC will follow Vin if it is below its regulating voltage set by DRVSET.

    For Q2:

    That is correct.

    I suggest that you try to use LTSpice to simulate your design to have a general idea of what would be the behavior of your design.

    LTC3777 is also available in LTPowerCAD to design your power converter.

    Regards,

    Jhun

  • Hi Jhun,

    Another similar question:
    LTC3777 uses 2 LDOs to generate the drive voltage (V_DRVCC), which drives MOSFETs and powers the internal circuits as shown below:
    VIN_LDO / EXTVCC_LDO => V_DRVCC (6V at least) => V5_LDO (5.5 V internal circuit power)

    If LTC3777 Input Voltge (VIN) = 5V, and there is not other > 5V voltage can be connected to External VCC (EXTVCC) input, how to generate at least 6V V_DRVCC?

    I have used LTpowerCAD for my LTC3777 design. It's a great useful tool. However, LTpowerCAD doesn't show the detailed DRVCC circuit. It only lets the user to type a DRVCC voltage in the schematic. Please see picture below.

    Also, I will simulate my design in LTspice to make sure that the design works as we expected.

    Thank you for your help.

    Liu

  • Since you are designing a boost circuit, you can tie your EXTVCC to Vout. Another is you can use a charge pump to boost your input voltage and then supply it to your EXTVCC although it will increase your component count.

    Regards,

    Jhun

  • Hi Jhun,

    Our input voltage range is very wide (5V - 150V). It's not just 5V all the time.

    Liu

  • If this is the case, I would suggest tying your EXTVCC to Vout since it is fixed, correct? This will be a part of the load but it will be minimal.

    Regards,

    Jhun

  • Hi Jhun,

    This is what I thought so to connect EXTVCC to VOUT in the beginning. What I wants to figure out is how the LTC3777 gets started if only 5V VIN is available. V5 regulator provides the power to internal circuits of LTC3777 and it's output voltage needs to be 5.5V according to the datasheet. If VIN is 5V => DRVCC will be less than or equal to 5V => V5 regulator output voltage will be smaller than typical 5.5V. Are LTC3777's internal circuits able to operate correctly with less than 5.5V voltage?

    The datasheet doesn't clearly explain what is going to happen if V5 regulator is powered by the voltage that is smaller than 5.5V.

    Thanks.

    Liu

  • Hi Liu,

    sorry for the late reply as I have been in  contact with the applications engineer for this part. This is his comment below:

    "

    The ExtVcc, if connected to Vout, will be powered up once the part is active and switching. The top buck MOSFET A must be turned ON by the controller in order for Vin to feed the inductor and boost converter section of circuit.

     

    They will need to use logic level MOSFETs to go down to 5Vin but I haven’t seen a design going lower than 6Vin. As far as I know there are no MOSFETs  that go to 150V and have 4.5Vgs rating.

     

    I doubt that they will be able to do 5V-150V input design with this part. We have only done 6V-100V input design that is shown in the data sheet."

    I hope this helps answer your questions.

    Regards,

    Jhun

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