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LM2651EVAL: Can this part output 5.1V?

Pol Francesch
Prodigy 40 points

Part Number: LM2651EVAL

Hello,

I am interested in the LM2651EVAL board and was wondering whether this part could provide 5.1V of power. I am looking to power a raspberry pi (which takes at most 1A at 5.1V) with a 12V power supply (all of this in DC). The LM2651 chip which is integrated into the board seems to be able to handle this type of load, however the board itself says that it has an adjustable power output but gives no range. So can I use this board for my application?

In addition, if I am able to do this, the user manual says that in the setup that the LM2651EVAL board is sold it outputs 2.5V and this can be changed by changing out the resistors. How easy is it actually to do this, and what kind of resistors are recommended for this application?

Thank you very much in advance, 

Pol.

  • 0
    TI__Prodigy 580 points

    Hi Pol,

    The LM2651 will work for your application. The simplest way to describe the output range of Vout is to say that it is mainly limited by Vin for a Buck converter. With a Vin max of 14V, Vout can be set at 5.1V. For more information on Vout limitations see this TI App Note http://www.ti.com/lit/an/slyt293/slyt293.pdf 

    To alter the board, you will need to be able to remove the current resistors and then solder 0805 surface mount resistors onto the board. A soldering iron and some experience soldering is needed to replace the resistors.

    To set Vout, you will need to follow equation 1 from the User Guide.

    For your case, plug in 5.1V and find the nearest standard resistor values that work. A combo of R1 = 66.5k and R2 = 21.5k would give you 5.067 Vout. You could always try other combos if you need to be closer to 5.1V.

    See the BOM in the User Guide for more specs on the resistors used on the EVM. 0805, 1% resistors are recommended. 

    Thanks,

    Alec Heierman

    TI Applications Engineer

  • 0
    TI__Mastermind 25590 points

    ****Oops, you've already got a response. I'll keep this here anyway****

    Pol,

    Yes this can output 5.1V. The LM2651 has a FB reference voltage of 1.238V so you just need to change R1 or R2 (see below) to get the resistor divider to give 1.238V when VOUT = 5.1V. For this specific case you can change R2 (red box below) to 6.65 ohms. You can use any resistor you'd like. Surface-mount 0805 is the original size but I think any reasonably small resistor would be fine.

    Another option would be to use a buck module. This is just like this converter but with the inductor (and other components) included in the part number. This will be a smaller solution.

    -Sam

  • 0 in reply to Alec_Heierman
    Prodigy 40 points
    Thank you very much for that Alec. I think that that pair of resistors should suffice in terms of the accuracy requirements.
    Thanks,
    Pol.