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LM5123-Q1: Schematic Review

Phil Shen
Prodigy 20 points
Part Number: LM5123-Q1


Tool/software:

Dear Sir/ Madam：

I have designed a boost circuit that steps up 15V to 56V.

LM5123.pdf

The boost solution is LM5123-Q1

Please help to review the circuit..

Thanks

  • 0
    TI__Mastermind 19715 points

    Hello Phil,

    May I please ask you first, to properly fill in your personal information (The system only shows questionmarks instead of your name).


    Can you please provide me with the following information: Vin min and Iout max / maximum output power.

    Ideally, please download the QuickStart calculator from our website: LM5123-Q1-DESIGNCALC Calculation tool | TI.com

    Then fill in your information and upload the spreadsheet here.

    Thanks a lot,
    Harry

  • 0 in reply to Harry Hofner
    Prodigy 20 points

    Hello, Harry：

    Thank you for your response.

    What personal information should I provide?

    The circuit Vin is 15V and Vout is 56V for PoE PSE (802.3 af/at)function

    So the maximum output power is 30W

    Phil 

  • 0 in reply to Phil Shen
    TI__Mastermind 19715 points

    Hello Phil,

    I assume that the system can only handle latin characters for the name.
    Otherwise it only shows questionmarks.
    Can you change that, please?

    From your comment, I derive that the 15V are pre-regulated and will not really drop.

    I have entered the numbers you gave into the QuickStart calculator here:
    https://www.ti.com/tool/LM5123-Q1-DESIGNCALC 
    (Again, you can easily use this tool yourself.)

    I get the following result:  The required inductor is 20µH, 4A saturation current.

    So, the 6.8µH inductor would be too small and not allow for a proper regulation.
    When selecting an inductor, please also verify the frequency rating (that it is rated for more than 100kHz).

    A current sense resistor of 1.5 Milliohm would result in a peak current of 40A! This is way too high.

    For 30W output power, please set it to 15 Milliohm.
    Please note that the current sense resistor is not only used for an overcurrent protection.
    Instead it is very important (together with the right inductor value), that this resistor has exactly the correct value.
    Otherwise the cycle-by-cycle peak current control loop will not work.

    4x 2.2µF ceramic capacitance on the output side is much too small.

    For our PoE reference designs, we usually use 3x22µF ceramic capacitors plus some smaller ones (1µF, 100nF).
    Please make sure that you will place the ceramic capacitors very close to the high-side FET and close to the sense resistor.

    Please do not insert a resistor between the HB pin and the HB capacitor (R84).
    Also, we recommend adding a 100pF filter capacitor close to the VOUT pin.

    The parasitics of a PCB have a big influence on the compensation network.
    Therefore, the proper components need to be determined by tests and measurements on a physical board in the lab.


    All information in this correspondence and in any related correspondence is provided “AS IS” and “with all faults” and is subject to TI’s Important Notice: http://www.ti.com/corp/docs/legal/important-notice.shtml


    Best regards
    Harry