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LM25116: buck boost regulator, problem output voltage

Ivan Ortega
Prodigy 20 points
Part Number: LM25116

Tool/software:

Hello there

Sorry if i write something wrong, english is not my native language.

I am having troubles designing a buck boost with the LM25116, my requirements are:

Vin: 7-42v

Vout:14v

 max Iout: 20A 

I am planning in use it as a regulator to feed a solar battery charger to charge a lead acid battery, as show in diagram.

I decided to use the LM25116 based on this product  https://www.amazon.com/-/es/Convertidor-Dc-DC-Regulable-Cargador-M%C3%B3dulo/dp/B07RBKVDHW but made of my own.

According to datasheet and other examples found on internet, I calculated my values for my components and schematichs are as follows:

When applying range of voltage from 5v to 24v, the output voltage stays at 2.9v

What could be wrong? Thanks in advance.

  • 0
    TI__Genius 17245 points

    Hi Ivan,

    Thank you for sending us your schematic. There appears to be a design calculator tool for this product that can help with component selection. Please utilize the design calculator to more easily design your application. You can also go through section 7 of the datasheet if you want more insight onto the components required to support your application.

    Please feel free to reach out if you have any more questions.

    Thank you,

    Joshua Austria

  • 0 in reply to Joshua Austria
    Prodigy 20 points

    Thank you for your advice, I am trying to calculate the materials, but I am stuck in the switching frecuency.

    What should I use as Fsw for my inputs and outputs voltages?

  • 0 in reply to Ivan Ortega
    TI__Genius 17245 points

    Hi Ian, 

    Thank you for your question. I noticed that the minimum input voltage you put was 6 V whereas output voltage was 15 V. VIN must remain higher than VOUT for the device to operate properly. The switching frequency will be limited by the VIN/VOUT ratio and that minimum 6 V is causing the calculator to calculate the switching frequency lower than it needs to be. If you put your nominal VIN into the VIN (min) box, that should allow you a wider range of useable switching frequencies.

    I also noted that your ripple current is 80%. Generally we like to keep the ripple current between 20% to 40% which will give an optimal tradeoff between size and efficiency. Please see section 7.2.2.2 of the datasheet for more details about selecting your output inductor to accommodate this.

    Thank you,

    Joshua Austria

  • 0 in reply to Joshua Austria
    Prodigy 20 points

    Ohh, thank you, now is all correct, but was wondering, the LM25116 is used on the product I linked as a buck boost, but with that setting is only working as a buck converter, how can I make it work as a buck boost?

  • 0 in reply to Ivan Ortega
    TI__Genius 17245 points

    Hi Ivan,

    To my knowledge, I believe the only way TI recommends our buck converters to be used as a buck-boost is with an inverting buck-boost configuration. This application can help with the design of this topology: Working with Inverting Buck-Boost Converters. 

    Thank you,

    Joshua Austria