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LM5175: Inductor selection

d_zero
Genius 4865 points
Part Number: LM5175
Other Parts Discussed in Thread: LM5176

Dear *,

inductor selection for LM5175, different value from calculator and equation in datasheet

eg.
Vinmin =9V, Vinnom = 12V, Vinmax=27V, Vout=9V, Iout=2A, fsw=490kHz

also eg.

Vinmin =9V, Vinnom = 12V, Vinmax=27V, Vout=48V, Iout=0.5A, fsw=490kHz

Calculator output L recommended 25.2uH -> equation (14) in Ds L=14.02uH

why is this so, is there a error in calculator?

Best Regards.

  • 0
    TI__Guru* 78665 points

    Hi d_zero,

    Thank you for your interests in the LM5175.  These equations are used to help find the required inductance per ripple current limit (40% ripple current is used in the example), since the ripple current will change with duty cycle, this is why you get two different results.  Then, you need to do a trade-off to make your final decision. The selected inductance is usually between eq (13) and (14) results.  

    In your case, eq (13) does not apply.  Therefore, the calculator choose a value for you.  It calculates the inductance per eq (14) for min, typical and max input voltage, and choose the mean value among them.  It ends up to ~25uH.

    Hope this clarifies.

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Genius 4865 points

    Hi Youhao,

    1)

    if i understand correctly if booth equations 13 and 14 are apply, the output voltage can be buck, buck-boost or boost regarding to Vinmin, Vintyp and Vinmax then the read off is made regarding to eq 13 and eq 14  is that correct?

    2) 

    if only one of the eq13 or eq14 applies then the output voltage is either buck or boost regarding to Vinmin, Vintyp and Vinmax , and then i have to calculate the eq13 or eq14 with all Vinmin, Vintyp and Vinmax and do a mean value?

    3) can you tell why 40% ripple current is chosen in DS, on what does depend this? Ho does the ripple current change regarding to duty-cycle?

    How is the duty cycle calculated for LM5175, also how it is calculated in CCM and DCM mode?

    4) Regarding the inductor tread off, what is the tread-off if i choose bigger or smaller inductance value? Also does this depend on CCM and DCM mode of operation?

    5) i'm trying to calculate inductor for

    Vinmin = 9V

    Vinmax = 27V

    Vintyp = 12V or 24V

    But i want have adjustable Vout with DAC from 3V to 48V ( Pmax 24W, Ioutmax 2A)

    i cant do this with your calculator so i made an excel, please can you look at it and what do you think it is a good thing to go with 10uH inductor?

    Vin min 9 V diL 0.4
    Vin  12 V
    Vin 24 V
    Vin max 27 V
    Rt calc 49.751793 kOhm
    Rt 49.9 kOmh
    fsw calc 488.6869 kHz
    fsw 490 kHz
    Vout [V] 3 6 9 12 15 18 21 24 27 36 48
    Iout max [A] 2 2 2 2 1.6 1.333333 1.142857 1 0.888889 0.666667 0.5
                           
    Lrec [uH] ( TI calculator) 5.1 6.4 3.8 7 9.7 11.6 11.5 10.3 7.3 17.5 25.3
                           
                           
    L_buck_vin_min 5.11575 5.11575 0 -10.2315 -31.9734 -69.0626 -125.336 -204.63 -310.782 -828.752 -2128.15
    L_buck_vin_12V 5.7552188 7.673625 5.755219 0 -11.99 -34.5313 -70.5014 -122.778 -194.239 -552.501 -1473.34
    L_buck_vin_24V 6.7144219 11.51044 14.38805 15.34725 17.98506 17.26566 11.75024 0 -19.4239 -138.125 -491.112
    L_buck_vin_max (eq13) 6.821 11.93675 15.34725 17.0525 21.31563 23.02088 20.88931 13.642 0 -92.0835 -381.976
                           
    L_boost_vin_min (eq14) -138.1253 -17.2657 0 4.316414 6.906263 8.632828 9.866089 10.79104 11.51044 12.94924 14.02835
    L_boost_vin_12V -368.334 -61.389 -13.642 0 6.1389 10.2315 13.15479 15.34725 17.0525 20.463 23.02088
    L_boost_vin_24V -3437.784 -736.668 -272.84 -122.778 -73.6668 -40.926 -17.5397 0 13.642 40.926 61.389
    L_boost_vin_max -4972.509 -1087.74 -414.376 -194.239 -124.313 -77.6955 -44.3974 -19.4239 0 38.84773 67.98352
                           
    L_select 10 10 10 10 10 10 10 10 10 10 10
    eff 0.9                    
                           
    iLmax (eq15) 0.7407407 1.481481 2.222222 2.962963 2.962963 2.962963 2.962963 2.962963 2.962963 2.962963 2.962963
    iLpeak (eq16) -1.100929 1.021064 2.222222 3.193172 3.331297 3.42338 3.489154 3.538485 3.576853 3.653589 3.711141
    iLsat (eq17) -1.651394 1.531596 3.333333 4.789758 4.996945 5.135071 5.233732 5.307727 5.365279 5.480384 5.566712


    Best Regards.

  • 0 in reply to d_zero
    TI__Guru* 78665 points

    My answers:

    (1) Yes

    (2) Yes

    (3) It is a general good practice.  You can also choose 10% to 60%. The ideal inductor will have 0% ripple current but you cannot have it in reality.  If the ripple current is too high, you are more easily running into DCM at reduced load.  

    (4) The results under different conditions require different values. The trade off is not to go extreme.  You should choose one to be fair with all these conditions.

    (5) Our calculator is set for a single Vout level. What you calculated looks fine. You effort can be saved if you run our calculator for different Vout and Iout conditions and get a list of recommended inductor, then choose a mean value like said in (4).  You should conclude to the same.

    Thanks

    Youhao

  • 0 in reply to Youhao Xi
    Genius 4865 points

    Dear Youhao,

    I have ISNS set on the input to limit the power to 24W.

    1)

    when i using the calculator to calculate the inductor, up to 12V output voltage i set the Iout(max) field to 2A ==24W

    what for output voltages that are higher then 12V eg. 48V_Vout  do i set the Iout(max) field to 0.5A( 24W limit on input ) or do i also set also to 2A?

    2) Do i set the Iocp in calculator regarding to Iout(max) ? or can i always set to 125% of 2A

    3) regarding Rsense i get different recommended values from calculator , what is the proper way to select? ( used the 24W input power and Iocp always 2.5A)

    Vin_min 9 V          
    Vin_nom 18 V          
    Vin_max 27 V          
    f_sw 490 kHz          
                   
    Vout 3 6 12 24 48   V
    Iout 2 2 2 1 0.5   A
    L 4.7 6.8 8.2 10 22   uH
    Iocp 2.5 2.5 2.5 2.5 2.5   A
    R_sense_recommended 39 36.4 19 17.2 11.5   mOhm

    Best Regards.

  • 0 in reply to d_zero
    TI__Guru* 78665 points

    Hi d_zero,

    1) if you want to limit the power, and your ISNS on the input side, then you should use it to limit the input power which consequently limits the output power. 

    If you output voltage is fixed, you should place your ISNS to the output side for directly power limiting. Then, when you output voltage becomes higher, you need to adjust the limit for 24W, namely 0.5A at 48Vout.

    2) Depends where your ISNS are placed. Your goal is to use it either limits the input power or the output power, as mentioned above.  

    3) For one design to cover all these, you will need to select the min value from the least, otherwise you can high the pk current limit in some cases. 

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Genius 4865 points

    Hi Youhao,

    i decided to set the ISNS on the input to limit the input power, because my output voltage is variable.

    So i set one inductor for all cases = 22uH. And the Iocp is 1.25*Ioutmax expected for the power limit

    Vin_min 9 V          
    Vin_nom 18 V          
    Vin_max 27 V          
    f_sw 490 kHz          
                   
    Vout 3 6 12 24 48   V
    Iout 2 2 2 1 0.5   A
    L 22 22 22 22 22   uH
    Iocp 2.5 2.5 2.5 1.25 0.625   A
    R_sense_recommended 33.3 33.2 21.1 35.3 42.5   mOhm

    Is it ok to select 33mOhm or do i need to select 21.1mOhm ?

    What will happen on 12V if i select 33mOhm?

    Best Regards.

  • 0 in reply to d_zero
    TI__Guru* 78665 points

    If  you choose 33m, then you will hit peak current limit at 12Vin.  You should choose the min among the list to cover all conditions. 

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Genius 4865 points

    Hi Youhao,

    thank you.

    can you tell the in the calculator row 29 ( Peak inductor current iLpk) is this the value from DS equation (16) , (17) or (18) ?

    Best Regards.

  • 0 in reply to d_zero
    TI__Guru* 78665 points

    Hi d-zero,

    The peak current calculation is in the background, and it calculated with (16) under various input voltages (assuming Vo and Io are fixed in a regular design), and also with (17) under buck and boost conditions, and the max out of these calculation results is displayed in row 29.  I don't think (18) is relevant. 

    Thanks,

    Youhao 

  • 0 in reply to Youhao Xi
    Genius 4865 points

    Hi Youhao,

    i was referring to equations presented in LM5176, and now i see that they are not the same as in LM5175.

    Where you referring to LM5175 equations?

    Best Regards.

  • 0 in reply to d_zero
    TI__Guru* 78665 points

    Yes I referred to the LM5175 datasheet.

  • 0 in reply to Youhao Xi
    Genius 4865 points

    Dear Youhao,

    thank you for the replay.

    i send you a PM, please if it is no problem can you replay?

    Best Regards.

  • 0 in reply to d_zero
    TI__Guru* 78665 points

    Hi d_zero,

    I did not receive any PM.  Let me send an invitation from my side.

    Thanks,

    Youhao