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LM5118: Power dissipation at LM5118

Saurabh Kapoor
Intellectual 1220 points
Part Number: LM5118
Other Parts Discussed in Thread: CSD19538Q3A, CSD19538Q2

Please  let me know how to calculate the Power  dissipation at LM5118 when operated for an output of 22V when a 32V input is given

  • 0
    TI__Mastermind 21720 points

    Hello Saurabh,

    Thanks for reaching out with your question and for using the LM5118.

    Are your trying to calculate the power dissipation in the LM5118? If so, I need to know the MOSFETs that are being used and the switching frequency. Most of the power dissipation comes from the internal LDO which is primarily used to drive the external MOSFETs.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 20 points

    We are using 400kHz and CSD19538Q3A as our Mosfet . Please let me know the power dissipation calculation for the device .

    Our input voltage is 18-38VDC and generating 22V and 1.2A using the LM5118. We are also having a 5V output from other buck converter running through same input voltage . We have connected that 5V to VCCX pin of the LM5118 through 1 k resistance . Idea is , initially internal LDO will up 22VDC and meanwhile 5V will up also and this will lead to shutdown of internal LDO. Will this lead to reduction of power dissipation of LM5118 ? If yes , how to calculate in this case ? We want to compare both the case .

  • 0 in reply to Dipesh Chuahan
    TI__Mastermind 21720 points

    Dipesh,

    The power dissipated by the internal LDO is calculated as below. This is from normal operation when the VIN pin is used to power the LDO.

    PVCC = (VIN-VCC)*{(Qg*fsw)+IBAIS)

    Qg is the total gate charge of the selected MOSFET. If both the low-side MOSFET and the high-side MOSFET are switching this is the sum of the total gate charge for both devices.

    VIN is the input voltage

    VCC is the VCC voltage

    fsw is the switching frequency

    IBIAS is the operating current of the LM5118 = 5.5mA worst case. This values is from the datasheet.

    For the case where the VCCX pin is biased with 5V the LDO will not be operating and the equation is just based on the RDSON on the VCCX switch

    PVCC = (RDSON)*[{(Qg*fsw)+IBAIS)^2]

    RDSON of the VCCX switch is typically 5Ohm. This value is from the datasheet.

    Please let me know if you have any questions.

    Thanks

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 20 points

    Just wanted to get confirmation on my calculation with csd19538q2 MOSFET and 400kHz switching frequency . Does it mean once 5V power is up and connected to VCCX , the power dissipation will drop to 0.7 mW ? 

     

  • 0 in reply to Dipesh Chuahan
    TI__Mastermind 21720 points

    Dipesh,

    Yes that is correct.

    -Garrett