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LM2854MH max vout?

Karl Morant
Prodigy 30 points
Other Parts Discussed in Thread: LM2854

Hello Ti,

Looking at the datasheet for the LM2854 I am unable to find the maximum Vout for the part. Web bench only allows designs up to 3.3V for this part. However the website says up to 5V for this part. So what is the maximum Vout?

What I want to do is have a Vout of 3.36V at 4.75 to 5.25 Vin.

Is that possible?

Thanks

Karl

  • 0
    TI__Guru 61007 points

    Hi Karl,

    Yes, that is possible. 5Vin to 3.3Vout is referenced extensively in the datasheet and app notes, and your application fits that closely. In this buck regulator, max VOUT is only limited by the VIN level.

    Regards,

    Tim

  • 0 in reply to Timothy Hegarty
    Prodigy 30 points

    Thank You for the reply Timothy,

    But what is the minimum Vin/Vout for normal operation?

    Thanks

  • 0 in reply to Karl Morant
    TI__Guru 61007 points

    Theoretically you can get 100% duty cycle as the high side FET is a PMOS. However, I would say 90% of VIN is a good target for max practical VOUT and still get good load transient response.

  • 0 in reply to Timothy Hegarty
    Prodigy 30 points

    OK thanks Timothy,

    I would like to have a design as follows:

    VIn=4.75 - 5.23

    Vout=3.38V

    Iout=3A

    The inductor I we would like to use is Coiltronics HCM0703-1R5-R (L=1.5uH, DCR=14 mOhm)

    Can you help me come up with Rfb1, Rfb2, Rcomp and Ccomp values for this design?

    THanks

  • 0 in reply to Karl Morant
    TI__Mastermind 19891 points

    Hello Karl, 

    Set the top feedback resistor to 150kohm (recommended value table 1 of the datasheet, page 18).

    Set the bottom feedback resistor to 46.4kohm (this will get you 3.38V output).

    Select Rcomp and Ccomp values based on your output capacitor parameters (this is from table 1 of the datasheet):

    Alternatively, you can create a WEBENCH design for 3.3V output (close enough), modify the bottom feedback resistor in the BOM and run a simulation. If you want to do this, go to the product folder at http://www.ti.com/product/lm2854 and enter the design requirements in the WEBENCH panel on the right. 

    Please note that in the datasheet the top feedback resistor is designated with RFB1, but in WEBENCH the top feedback resistor is designated as RFB2. That's why I just called it TOP FB R in the table above.

    I hope this helps. 

    Regards, 
    Denislav