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LMG708B0: DESIGN REVIEW OF PLMG708B0VBTR

Suraj Singh
Prodigy 20 points

Part Number: LMG708B0

switcher_28v_8v0 (1).pdf switcher_28v-8v0.xls LMG708B0_quickstart_A23-12V_20A_400kHz (1).xls 


Attaching the  PLMG708B0VBTR switcher schematic design pdf, BOM,Calc. sheet. Please review and share your feedback.
Vin---28V
Vout---8V
Iout(typical)--10A
Switching Freq.----400KHz
Please review and share your feedback. 

  • 0
    TI__Guru* 87517 points

    Thanks, Suraj. I'll review the files and get back to you tmrw.

    --

    Tim

  • 0
    Prodigy 20 points

    Waiting for the feedback!! ,Timothy

  • 0 in reply to Suraj Singh
    TI__Guru* 87517 points

    Hi Suraj,

    Note we have 36V/10A converters that could work here, as Vin-max is only 36V (no need for 80V GaN device).

    Here are some notes:

    Quickstart:

    1. The inductor ripple current is a bit high at 43% - check the inductor core loss.
    2. The ESR is entered as 50mΩ - this is very high. The ceramics in parallel will reduce the effective value. Note the ceramic value derates with voltage, so 188uF is also incorrect. This affects compensation calculations.
    3. The current limit is set at 12.8A with 2mΩ shunt. The CC loop setpoint is above this, so it won't engage. If CC loop isn't needed, just tie IMON low.

    Schematic:

    1. No need for the 10pF FF cap.
    2. Consider adding an input damping cap (see the EVM for example).

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 20 points

    Thanks for the prompt response ,Timothy

    I have gone through your feedback points and attaching the new Quickstart sheet:

    1-Inductor value changed from 3.3uH to 4.7uH(MPN: 744393605047 from wurth). Inductor ripple current 31%.

    2-ESR will be very low around 0.5-1mΩ.Also, the capacitor derated value can be expected around 110uF.

    3-We will be placing 0ohm at Imon res and demount cap.
    4-Could you please give more clarification on input damping cap? Are you referring to have RC type near to Vin pin ? If yes, 2.2ohm with 4.7uf combination b/w input cap and Vin pin will be good to use?

    LMG708B0_quickstart_28V_8V_20A_400kHz (1).xls

  • 0 in reply to Suraj Singh
    TI__Guru* 87517 points

    Thanks, Suraj.

    The quickstart looks good. The input damping cap is typically set at four times the derated ceramic value for Cin. Take a look at the EVM for reference. It's also mentioned in the datasheet.

    PS: you may be able to use the Wurth XHMI 1060P instead of the 1010P - this will help to reduce cost.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 20 points

    Thanks Timothy,
    I hope below point is also correct as per my understanding. Please acknowledge if anything need to change for below point.
    As CC mode disabled, Imon is low and Iset voltage is >1v (typ.1.2V using voltage divider ),Also, Vset >1v for internal Vref based output voltage regulation.

  • 0 in reply to Suraj Singh
    TI__Guru* 87517 points

    Hi Suraj,

    VSET and ISET have internal current source pullups to ~3.3V. Tie IMON low to disable CC mode operation.

    Regards,

    Tim