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LMZM23601: used in 12V output design

Yuta Nishie
Genius 3010 points
Part Number: LMZM23601

Hi,

In connection with the question at the following URL, for example, if you want to output 12V, is it correct to recognize that a resistor of 291kOhm x 6 = 1.746MOhm is connected in series with the FB terminal?

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1025871/lmzm23601-used-in-6v-output-design?tisearch=e2e-sitesearch&keymatch=LMZM23601#

Also, please tell me how to calculate the resistance value in the URL.

Please also tell me the resistance value of FIXED VOUT OPTION.

Best Regard

Nishie

  • 0
    TI__Genius 9980 points

    Hi Nishie,

    For the 5V fixed output version, if you add a ~106k resistor as Rfbb from FB to ground, then it should output 12V. Unfortunately I cannot disclose how I arrived at that value.

    Thanks,
    Andrew

  • 0 in reply to Andrew Kutzler
    Genius 3010 points

    Hi Andrew-san,

    Thank you for telling us about the resistance value.
    Is what you are saying correct in the circuit below?


    In addition, please tell me the FB resistance value when Vout = 6V and 9V.

    Best Regards,

    Nishie

  • +1 in reply to Yuta Nishie
    TI__Genius 9980 points

    Hi Nishie,

    I apologize, I did my calculations wrong. I redid them and got the old value for VOUT = 6V. Here's what I got:

    Vout = 6 -> RFBT = 292kOhm

    Vout = 9 -> RFBT = 1167kOhm

    Vout = 12 -> RFBT = 2042kOhm

    (leave RFBB unpopulated).

    The issue I see for the larger resistance values is that the small leakage current from the FB node may distort the voltage value to something else.

    However, I think I might have a workaround. If you make a resistor network connected to FB with sufficiently small resistors then the larger internal FB resistors shouldn't affect the output voltage. However, this method might require additional losses as the circuit will lose more power through the FB resistors.

    Use this equation where VREF = 1, and set either RFBT or RFBB to ~10k.

    Thanks,

    Andrew