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PMP9595: input voltage range -min value with 3V

Part Number: PMP9595
Other Parts Discussed in Thread: LM5122

Hi! I need to design a boost convert the specs with 

Application Field                               : Cold Crank (start stop)

Required  Specs for the application : In case of cold crank, the input voltage can drop to 3 volts.So  3 volt and higher input voltage value is required in accordance with this situation.

                                                           Vout: 9V output and up to 20A.

If I use the pmp9595 circuit as a reference here, I know that I can adjust the output to 9V volts by changing the values of the reference resistors, that's fine.
The point is, what exactly are the requires the  change I need to make when the input is 3v?

  • Hi Electronx,

    Thank you for your question.

    Next to the change of feedback resistors for 9V Vout that you already mentioned, I would recommend to check the feedback compensation.
    In the reference design, compensation is optimized for the given voltage ranges, therefore it might need to get adapted to run stable during all worst case scenarios with the new conditions. The datasheet of LM5122 provides a section on loop compensation in chapter 8.1.1 (p. 24 ff).

    Additionally, I would recommend to check all external components, to see if voltage and current ratings still line up or if parts can be optimized. E.g. check if no inductor falls into saturation.

    TI offers a general tool for this called power stage designer:
    This tool can give a quick overview on voltage and current flows and suitable values for capacitors, inductors and resistors.

    Regarding LM5122 itself, please keep in mind that this device needs an initial input voltage of 4.5V to start up before being able to support Vin of 3V.

    Please let me know if there are additional questions.
    Best regards,

  • Hi Electronx,

    I got some additional feedback from a colleague with additional notes for your consideration:

    At 9Vout/20Aout we got 90W per phase (it’s two phase interleaved, 2x LM5122) by boost factor of 3:

    We got an input current of 65A..67A at 3Vinput, please be aware of this.
    Design corrections necessary:
    - Current sense resistors: min. trip level 65mV, peak current 35A, so less than 2milliOhms, around 1.5milliOhm
    - UVLO: for start/stop would set to 6V ON / 3V OFF, at upper current level use large hysteresis
    - Slope compensation: this is a function of Vin, Vout, Rcs
    - RHPZ at load resistance 900milliOhm (single phase), duty 70% and magnetizing inductance is less than 6kHz
    - Loop gain to be reduced for loop bandwidth 1kHz, 1.5kHz is maximum
    - Compensation zero to be corrected for new load pole
    - Compensation pole to be set at ESR zero 55kHz
    - Would add additional pole 30kHz at fb divider to improve S/N at error amplifier, small cap across LS resistor

     Furthermore power stage needs to be checked:
    - LS FET sees 28Arms
    - SR sees 18Arms
    - Inductor sees 33Arms/35Apk, XAL1010 is at it’s limit 32Arms/34Asat
    - Output capacitor sees 15Arms, each capacitor rated for 4.65Arms, at the limit as well

     For further redesign, there are the following questions:
    - How long is the design working at that cold cranking level?
    - What is the max. ambient temperature range?

    Additionally, start/stop sounds automotive – this design is NON-automotive.

    I hope these comments are an additional help.
    Best regards,

  • Hi Electronx,

    As there has been no further comments, I will close this thread for now.
    If there are no additional questions, feel free to click on the resolved button.
    You can re-open the thread anytime by posting a reply below.

    Thank you and best regards,