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LMZ14201: Rated up to 5V but recommended by WEBENCH for -3.3V Output

Marius Raducanu
Intellectual 680 points
Part Number: LMZ14201
Other Parts Discussed in Thread: LMR33640

WBDesign_OUT_NEG3p3V_500mA.pdf

Hello,

I have used WEBENCH to generate a -3.3V power supply.

The datasheet of LMZ14201 specifies the minimum voltage 5V.

The topology used is Inverting Buck Boost and might be the reason the generated schematic is correct but I need confirmation for this.

Question: Can LMZ14201 generate -3.3v although in the specs the minimum voltage is 5V (5-0=5 > 0-(-3.3v) = 3.3V)?

Regards,

Marius Raducanu

  • 0
    Intellectual 680 points

    Hello,

    I changed my design and use a LMR33640 converter to generate the negative -3.3V.

    I found TI documentation that explains how to use buck-boost converters to generate the negative voltage.

    I have tried unsuccessfully to use WEBENCH for generating the schematic of the negative power supply (-3.3V).

    How can I force WEBENCH to generate schematics using LMR33640?. It does not accept negative (voltages minimum 1.24V).

    Regards,

    Marius

  • 0 in reply to Marius Raducanu
    TI__Mastermind 36740 points

    Hello Marius, 

    A crude way to do it, is Vin + (Mod VOUT) so if you have 5V in -3.3V out, this is 8.3V in 3.3V out.  This will not give you the accurate input stage of the Cap that goes to Ground and a recommended Cap going from Vin to minus Vout.  Also, the output current will need to be Iin+Iout., not just Iout.  where Iin is Pin/Vin, where Pin is Pout/Efficiency - estimate 0.85, where Pout is Vout*Iout.

    for a more accurate approach, suggest reading application note, link below and check the results WEBENCH provides!

    Working with Inverting Buck-Boost Converters (Rev. B)

    Hope this helps.

    David.