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LMR51610: Webench - inductor value

Andres Gomez
Intellectual 450 points
Part Number: LMR51610
Other Parts Discussed in Thread: LMR51606

Tool/software:

Hi,

We are going to use LMR51610 and we have these specifications:
Vin = 9-30V

Vout = 3.3V

Iout = 0.25A

Fsw = 400kHz

When we use the Webench tool, we are getting L = 18uH-22uH which means a input ripple 130%. It is common to select 20%-40%. Why did the tool select such value? Additionally, we know the boundary for CCM and DCM in a buck converter is:

2L/(R*Ts) = (1-D), Rcrit = 2L/(1-D)Ts. For L = 150uH, Rcrit = 134.8ohms and L = 18uH, Rcrit = 16.18ohms

so the higher the inductor, higher the range of R which the converter can work in CCM. However, when I simulated the circuit in PSPICE for TI, it showed me higher the inductor, it is more likely the system works at DCM. Furthermore, I was expecting a decline in the ripple between the two values, but it doesn't have any major change 

Full load L 18uH (R = 13.2):

Half-load 18uH(R = 33)

Full load 150uH(R = 13.2)

Half-load 150uH (R = 33)

  • 0
    TI__Mastermind 31315 points

    Hi Andres,

    Inductor value is based off of peak-peak ripple that is 20-40% of the output current, which is why the 22uH was recommended. 

    I'm not too familiar with the model, but perhaps there are some limitations in the model.

    Thanks,

    Richard 

  • 0 in reply to Richard Fu22
    Intellectual 450 points

    Hi Richard.

    I know the ripple current is peak to peak of the output current, which is the inductor current in steady state. Did you calculate the ripple current of the inductor with those values?

    K =1.335 so 133.5%. So how did you get that value? ( Fsw = 400kHz/ Iout = 0.25A/ Vin_max = 30V/ Vout = 3.3V and L = 22uH)

    Richard Fu22 said:
    20-40% of the output current, which is why the 22uH was recommended. 



    if I want K = 0.3, Lmin = 97.9uH

  • 0 in reply to Andres Gomez
    TI__Mastermind 31315 points

    Hi Andres,

    I believe the Webench calculator may be calculating with an assumed load current of 1A. I did rerun the Webench simulation and checked. It looks like even if you enter a small current like 250mA, the webench simulator will mention that the peak current ripple is ~160% of the average inductor current. It looks like this is a limitation of the model. Appreciate the heads up. 

    That being said, for your design- if that is a problem, you may try the FPWM option if you want CCM even at light load. The LMR51606XFDBVR is fixed 400kHz, FPWM operation, adjustable output. As long as your inductor current doesn't go below the LMR51606 negative current limit, it should be ok. 

    Thanks,

    Richard