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LM25018: Doubts about the desired values ​​and placement

Paolo Gaggero
Prodigy 60 points
Part Number: LM25018
Other Parts Discussed in Thread: LM3150,

Hi everyone,
With a buck converter I need to obtain a stable output at 16.8V and a maximum charging current of 300mA.
Initially I started considering the LM3150, but after suggestions I was informed that the tolerance on the output values ​​of the limiting current could even reach 50% based on various factors such as RdsON, inductance value etc..
Since I wanted an Iout limited to max 300mA and needed a simple and fairly narrow layout with easy-to-solder components, it was suggested that I use the LM25018.
So I carried out some studies on the datasheet and generated a diagram with Webench.
I then created an Excel file with the calculations relating to the LM25018 which I carried out based on what was reported in the datasheet.

LM25018.xlsx

If you have a chance, check it for a moment because there are a few things that don't add up to me.
I am attaching the electrical diagram that Webench generates for me, which is partly correct and partly seems to me to generate values ​​that are not very correct.

the only difference compared to the webench scheme is the insertion of a potentiometer (after Rfbt1 which will not be 10k but 8.7K) so as to be able to more precisely adjust the output voltage to the value I want of 16.80V

My values ​​are as follows:

VALUES PROJECT
Vin min 20 V
Vin nominal 24 V
Vin max 28 V
I out 0,3 A
V out 16,803 V

1) From the datasheet the fSW should have the following formula:

and therefore from the calculations it should come out at around 604 kHz with a Ron of 309kΩ, however on Webench the fSW comes out at 544 kHz (which is very similar to the fSW(max) calculated with Dmin/Ton(min) )

2) Then on WeBench he inserts an inductance of 150uH, but WeBench calculates everything based on the fSW of 543 kHz, while I get L1 of 130uH but with an fSW of 604 kHz

3) The input capacitor calculated according to the formula:

choosing an input ΔV of 0.5V (imagining that I can vary for example between 23.5V and 24.5V) the Cin comes out at 24.82uF while Webench suggests the 1uF one which is lower, why?

4) Finally I have a question precisely about the peak current: the peak current calculated according to the formula :

I find it to be 337mA which is < 390mA (which from the datasheet is the minimum current limit threshold)
So what happens if the calculated peak current is smaller than the minimum current limit threshold?

At the moment these doubts have arisen about it, I hope I'm not too wrong in order to avoid mistakes. I have already placed the PCB a bit and it seems excellent also in terms of size and space occupied.
If we can resolve these doubts, I will show you the placement, whether it is correct and whether it can create problems, asking you for suggestions on the matter.
Thank you
Paolo

  • 0
    TI__Guru 73830 points

    Hi Paolo,

    Thank you for designing with the LM25018. Since your materials have a lot to review in detail, please allow me sometime and I should be able to get back to you in two days.

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Guru 73830 points

    Hi Paolo,

    Sorry for the delay. Here are answers to your questions in the same order.

    0). Using the potentiometer is okay for prototype I think.  Once you freeze your design, it's better to use fixed resistors to prevent accidental moving of the potentiometer. You may use few resistor combinations to get a close resistance value there IMHO.

    1). The frequency programming equation is an first order estimate. If you refer to Figure 6-9, you can see it is not dead on a fixed frequency. It is not a big problem for the COT circuit operation, and this equation does give you a good start point, and you can fine tune by test. I think the Webench is more closely to the actually measured frequency with that Ron.

    2). It is related to above.  You can design at your desired switching frequency and update Ron by test to operate at it, so your other component selections based on the frequency would remain valid.

    3). Webench model was largely based on the EVM for which the EVM designer did not assume much impedance between the voltage source and the dc-dc stage. In your design, it is wise to do it your way to make sure the input would not vary much.  Well done (thumb up :-) )

    4). Your design with Ipk = 337mA is good. The 390mA is guaranteed peak current threshold. The IC will not allow the peak current to exceed, which in turn limits maximum output load current. All designs should not exceed it at full load and max input voltage.

    By the way, I reviewed your excel calculations and they are good, and please just be reminded of my reply in 1).

    Good luck in your project.

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 60 points

    Hi David, thank you for the answers.

    Ok, so for the values ​​I try to stick to those of Webench, I adjust the Vout with the trimmer to obtain a fixed output voltage of 16.8V and as regards the current I only still have some doubts. I have not yet understood whether the current can therefore reach 390mA or whether it is limited by the calculations to a max of 337mA.

  • 0 in reply to Paolo Gaggero
    TI__Guru 73830 points

    Thank you Paolo.  Your calculated results of peak current may differ a little if your inductor value deviates from its nominal.  Anyway, please do the experiment and let us know if you encounter any problem.

    Thanks,

    Youhao