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WEBENCH® Tools/LM5176-Q1: LM5176

user1292453
Prodigy 105 points
Part Number: LM5176-Q1
Other Parts Discussed in Thread: LM5176,

Tool/software: WEBENCH® Design Tools

Good morning,

Today, I would like replace an LT component  LT 8705    with the LM 5176-Q1  for the following  reasons:

- lower price

- VIn max requested form my design  = 40V ( VIn max LT8705 = 80V)

-  and also  Mr Ciolfi   LT Field application manager is no longer  available , if necessary , in the new structure AD

So now, I'm using WEBENCH , it is an impressive tool  with an excellent usability.

But if I check the WEBENCH  results  with the equations  for Lbuck  and Lboost ( pages 21, 22 LM5176 datasheet) for my design with the following inputs:

            VIn max= 32V   Vin min= 12V  Vout = 24V and Iout = 3.5A 

I obtain:   Lbuck = 12.3µH   ,  Lboost = 8,23µH      and Ilpeakboost = 11,4 A       but  WEBENCH propose 6.8 µH

It seems that , you choose an shielded inductor: SRP1270A in this case,   and   you select  the value of the inductor with  a current Irms  closed to ILpeakboost   --> Irms SRP1270-6R8M= 12A

I would like to now , why , you  do not change the switching frequency  (347 Khz -> RT = 23,2 Kohm  WEBENCH ),   with a switching frequency of  419 Khz  I obtain Lboost = 6.8µH.

Thanks in advance for  your answers

Michel Thiébaut

  • 0
    TI__Expert 8365 points

    Hi Michel ,

    Could you please share the WEBENCH design link using the 'Share Design' option?

    Thanks & Regards,

    Praveen

  • 0 in reply to Praveen Badiger
    TI__Expert 8365 points

    Hi Michel ,

    The LM5176 WEBENCH model is done as per LM5176 design calculator given on the product folder.

    https://www.ti.com/lit/zip/snvc208

    The WEBNENCH uses the below inductor selection equation for Buck mode:

    The difference in inductor values are observed because the peak-to-peak current ripple on WEBENCH is 0.8 whereas the datasheet uses 0.4.

    which results in an inductor of 6uH & 8uH for Buck and Boost mode respectively. A minimum of two values will be considered(6uH).

    The inductor limits are calculated and the best suitable inductor is chosen among the available list. The inductor selection is based on the inductance, DCR, current rating, footprint, technology, budget, package etc.

    The frequency range of LM5176 is 100KHz-600KHz.Since the 'Balanced' optimizer option is selected the mid-frequency(347KHz) is chosen. The frequency range is also dependent on minimum on/off time and FET selection.

    Thanks & Regards,
    Praveen

  • 0 in reply to Praveen Badiger
    Prodigy 105 points

    Hi Praveen,

    Thanks a lot for your celerity to answer.

    I found now the correct value with the ponderation of 80%  for the inductor   Lbuck= 6.174 µH

    But for the L boost , I found  with the equation in the datasheet ( ponderation of 30 %)    Lboost  = 8.232 µH  > 7.82µH

    If this ponderation is 31.5% , so  the value of Lboost is 7.84 µH  -> is it the correct explanation for this difference ?

    I noticed also with the LM5176 DC/CD controller design tool  (VIn max= 32V   Vin min= 12V  Vout = 24V and Iout = 3.5A ) ,  the recommended Inductance is 7.9 µH and not 6,8 µH .

    In this case, the choice is made on the highest value of the two inductors Lboost and Lbuck ?

    Thanks in advance for your answers 

     

  • 0 in reply to user1292453
    TI__Expert 8365 points

    Hi Michel,

    The Lboost equation on the WEBENCH model is taken from the LM5176 Design calculator and it is different from the datasheet equation. However, it considers the peak-to-peak current ripple as 0.3 and calculates the inductor value as Lboost=7.82 uH.

    The recommended inductor range for the above condition on WEBENCH shown below.

    The Lmin is taken as minimum of two values: 6.17uH & 7.82uH

    L target is taken as an average of two values :(6.17+7.82)/2= 6.99 uH

    Lmax is taken as maximum of two values: 7.82uH

    The suitable inductor is chosen within the calculated limits.

    Thanks & Regards,

    Praveen

  • 0 in reply to Praveen Badiger
    Prodigy 105 points

    Hi Praveen,

    Thanks a lot for answers.

    It is OK for the value of inductors.

    I have just one question about RSENSE.

    Input data:    VIn max= 32V   Vin min=12V     Vout =24V    Iout = 3.5A  RT=23.7 Kohm  -->  FSW=340,23 KHz  inductor = 6,8µH

    IL(max)= 24V *3.5A/(0.9 * 12V) = 7.78 A   ( page 22 LM5176-Q1)

    IL(PEAK)= 7.78A +  12V*(24V-12V)/( 2 * 6.8 µH* 0.3402 MHz* 24V) =9.074 A

     So, I found

    RSENSE(BUCK) =  80 mV/3.5A = 22.86 mohm

    RSENSE(BOOST)  = 120 mV /9.074A= 13.22 mohm

    But the   WEBENCH RSENSE value is 9 mohm

     Could you please explain to me this difference ?

    Thanks a lot in advance for your answsers.

    Michel Thiébaut   

    found now the correct value with the ponderation of 80%  for the inductor   Lbuck= 6.174 µH

    But for the L boost , I found  with the equation in the datasheet ( ponderation of 30 %)    Lboost  = 8.232 µH  > 7.82µH

    If this ponderation is 31.5% , so  the value of Lboost is 7.84 µH  -> is it the correct explanation for this difference ?

    I noticed also with the LM5176 DC/CD controller design tool  (VIn max= 32V   Vin min= 12V  Vout = 24V and Iout = 3.5A ) ,  the recommended Inductance is 7.9 µH and not 6,8 µH .

    In this case, the choice is made on the highest value of the two inductors Lboost and Lbuck ?

  • 0 in reply to user1292453
    TI__Expert 8365 points

    Hi Michel,,

    The difference in the Rsense calculation is observed because the WEBENCH has derated the values as shown below.

    RSENSE(BUCK) =  80 mV/3.5A = 22.86 mohm

    RSENSE(BOOST)  = 120 mV*0.8 /9.074A= 10.57 mohm

    minimum of two values is selected = 10.57 mohm

    Rsense target= 0.9*10.57mohm= 9.57 mohm

    Rsense max= 9.57 mohm

    Rsense minimum= 0.85*9.57 =8.13mohm

    Thanks & Regards,
    Praveen

  • 0 in reply to Praveen Badiger
    Prodigy 105 points

    Hi Praveen,

    Thanks a lot for answers.

    Ideally, it would be interesting to update the LM 5176  datasheet .

    Best Regards,

    Michel Thiébaut