data/text/DLP1/899559

LM5175: Inductor selection

Part Number: LM5175 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.

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

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] (company 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.

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