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LM5168: Boundary condition for PFM and PWM operation LM38010S

Part Number: LM5168
Other Parts Discussed in Thread: LMR38010

Dear Experts,

--- Sorry I could not insert the proper part number LM38010S to the Part Number field because it gave me Part number "LM38010S " not recognized so I inserted another power product in order to stay under Power Management forum. ---

I would like to use LM38010S which has PFM capability for light load and normal for higher loads, so basically the auto mode version.

My question is how the IC decides when to switch from PFM to PWM is it based on the sensing of discontinuous conduction mode

or this there something else?

Actually checking the efficiency curve I do not see difference for higher current so it could stay also in PFM till the max current, right?

Please the the plot:

The purpose of my question is because I want to choose the inductor for my application and since the typical current consumption won't be higher than 0.3A

I am wondering how big the inductor should be. I checked the equation in the in datasheet and for Ioutmax=0.5A, k=0,5,fsw=400k,vin=52V,vout=11V

it yielded 86uH which I would like to reduce it since most of the time the buck will work in PFM anyway. The webench and datasheet table 9.1 gives me 68uH

but I am wondering if I can still go under this value and how much to reduce the size of the inductor or it's not worth?

Thanks for you answer in advance,

Andras Magyar

  • Hi Andras,

    Basically as load decreases, inductor current begins to exhibit DCM behavior. This means that as the load decreases, the duty cycle also decreases accordingly. Eventually, the peak current of the inductor will be below minimum peak current limit threshold. Therefore, IC knows it has entered PFM. 

    For the LMR38010, you can either buy parts that come in either light-load AUTO mode or FPWM mode.

    In your calculations, it may assume for CCM behavior for most of the input conditions, so you may go lower. If anything, I would try to size the inductor such that you are about to enter DCM at 0.1A, as shown by the above chart. 



  • Hi Richard!

    Thanks for the answer. I understood clearly!

    One more question left. I am playing now with the PFM/DCM/CCM boundary changing the inductor.

    I have two other questions:

    1. If I choose a lower inductor let's say 56 uH it means only that the DCM zone is wider and it does not mean that the PFM threshold is higher

    since it is fixed, right?

    2. This connects to the first one the is it possible to make the auto mode IC work always in PFM for all currents? I suspect no and the answer is within the first question that it has a fixed threshold and only the DCM zone gets wider if I lower the inductance. I am asking because the efficiency map shows both PFM and PWM together so it is hard to see at what current they melt together and when CCM takes over and I also asking because I know that other IC vendors have similar buck where I can select PFM or FPWM with a single resistor for the whole range of output current. For example this: MAX17760ATC+

    Thanks for you answer,

    Andras Magyar

  • Hi Andras,

    1) Yes you are correct. A smaller inductor will make the range that you enter DCM larger, but PFM threshold is still the same. 

    2) It is possible depending on your components and load current. However, I suspect that you will need quite a small inductor, and a limited range of load currents. If your expected load current is always small, then you will be in PFM. You can approximate when PFM/PWM crossover by the efficiency. For example at 64Vin, the efficiency curve for PFM and FPWM overlap at around 0.2A. Therefore, if your load current is less than 0.2A, it is better to operate in PFM.