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LT8335 vs. LT3580

In my design I need 2 supply rails +12V & -12V from a +5V. I will be using these supplies to power transceivers that are rated a max current drawn of 120mA\130mA. In order to give enough spare in the power ratings of the required solution I thought of looking for DC\DC that would provide 0.5A max (maybe this is over kill ???). We are concerned about the heat and limited space requirements of our design. I found the LT8335 (Typical application : 5V to 12V Input, –12V Inverting Converter) and the LT3580(Typical application : 2MHz Inverting Converter Generates –12V from a 5V to 12V Input) and now comparing them as well as potential options of other suppliers solution. 

Is there someone who could possibly give me some advice on these potential solutions ?

  • Hi shueld,

    Both, lt3580 and lt8335 will work for you, but neither will give you a 500mA guaranteed max load. 

    The lt3580 offers some flexibility with its programmable frequency, if you decide you want to reduce the switching frequency. Also, it might run a bit cooler, with all else the same, since it's physically larger. 

    You can reduce resistor package sizes, as long as they are rated properly, but input and output capacitors packages must remain as recommended in the datasheet, or larger. Capacitance droops with smaller packages when biased. Reducing package size can get you in trouble in terms of loop stability. 

    If you want to save space, you may use a single "coupled" inductor for the lt8335 -12V converter, instead of 2, 3.3uH. If you do that, please make sure you connect the polarity right, as indicated in the lt8335, -5V application below.

    That can also be done with the lt3580.

    Cheers!

  • Dear dulcevida,

    Thanks for your informative reply ! 

    1. I am not expecting that my design will draw more than 150mA, what do you think the temperature will be for both solutions ?

    2. "....but input and output capacitors packages must remain as recommended in the datasheet, or larger. Capacitance droops with smaller packages when biased. Reducing package size can get you in trouble in terms of loop stability. " if I have a smaller package but with the same voltage rating - will this still cause potential trouble e.g. For the LT8335 (Typical application : 5V to 12V Input, –12V Inverting Converter) : C3: MURATA GRM32ER71E226K ==> 22uF, 1210, 25, is not a good idea to reduce this down to 22uF, 0805, 25V ?

    3. "If you want to save space, you may use a single "coupled" inductor for the lt8335 -12V converter, instead of 2, 3.3uH. If you do that, please make sure you connect the polarity right, as indicated in the lt8335, -5V application below."

    a) Would this single inductor go in the place of L1 or L2 ?

    b) Could this be used for the LT3580 solution as well ?

    4. In what situation would I want to reduce the frequency and how would this impact on the performance of the solution ?

    Thank you so much fro all your help 

    Shmuel  

  • Hi, 

    1. The worst case total power loss will be for the -12V supply, using the lt8335. There will be about 300mW. If we assume half of that will be inside the chip, the junction to ambient temperature rise will be around 15 degrees C. Of course, this is with  your 150mA load. It would be more with higher loads.

    2. If you do an actual bench test of those two capacitors, you will find the smaller package has less capacitance than the larger one, regardless of the claims.

    A better question would be:

    How much can I reduce the "actual" capacitance for my design and still be fine?

    That will require testing the application to find out. Since the load is less than half, it might be fine, but you will have to try it to find out. 

    3.a. The single inductor is in reality 2 inductors wound in the same core, as you can see in the page 16 schematic, located in the middle of the page, which I added for reference in my previous reply. So, it will go in place of both, L1 and L2. 

    3.b. Yes, the dual inductor approach can be used with either part.

    4. If you had a problem switching at any other frequency, besides 2MHz, you could not do it with lt8335.

    Slower frequencies have lower switching losses, at the cost of larger components. 

    Some customers want to stay away from certain frequency ranges for EMI qualification reasons.

    There are many others.

    In short, if you are not going to be operating in some high ambient temperature, and are OK with 2MHz, the lt8335 is a better option. That is my humble opinion.

    Cheers!

  • Dear dulcevida,

    Thank you again. 

    Following your advice I think I will leave the input and output caps as stated in the data sheet as we would prefer to reduce all risks where ever possible. Also I think I will stay with the 2 inductors for the same reason unless we really get stuck for PCB space.  

    Re: Heat dissipation. Besides the fact that the LT3580 has a slightly bigger package, the fact that its  θjA (for the MSOP package) is much lower(35-40 deg C) than the LT8335(80.5 deg C) doesn't this also indicate that it will be cooler ?

    Our product is industrial rated thus we will be doing higher ambient temperatures testing thus it appears that LT3580 will be a better choice even though it appears that this solution will take more area than the LT8335. Also it appears to have a better Iout rating, Vin range and optional switching frequency.

    However what about the age of the chips ? I noticed that the data sheet of the LT3580 is 2007 c.f. the data sheet of the LT8335 is 2016. Is this an indication of how new the chips are ? When did both these chips start their production ? I would much prefer to take a newer chip than an old one !? 

    Best regards

    Shmuel 

  • Dear Shmuel,

    Sticking to what is proven to work seems wise to me, so I'm with you on that.

    The lower θjA in the 3580 is lower precisely because of its larger size. It is cooler because it is easier for it to get its heat out. It has better heat transfer.

    Regarding age:

    The lt3580 is older, yes, and the lt8335 newer. They are both great quality parts, with different features. Which one you should use is up to you, but being familiar with both parts, I would not recommend making a decision based on age.

    I hope my son can prove to be as reliable as me, or better.

    Time will tell! Pensive

  • Dear dulcevida,

    Well it looks like we are going with the LT3580 and implementing 2 of the Typical Applications :-

    a) 5V to 12V Boost Converter Switches at 2.5MHz and Uses a Tiny 4mm × 4mm × 1.7mm Inductor

    b) 2MHz Inverting Converter Generates –12V from a 5V to 12V Input

    For these applications - can I use the same Diode ?

    Thanks

    Shmuel 

  • Hi, 

    Yes you can, as long as it is properly rated for the converter that has the higher requirement, and that is the inverting application. The negative converter must have a diode rated for at least 30V. If you wish to use the same diode for both converters, please use one that is rated for at least 30V.

    Regards!

  • Dear dulcevida,

    Thank you fro your reply.

    Just to double check - I can use the diode CMMSH1-40 for both applications right ?

    Best regards

    Shmuel 

  • Yes, Shmuel, you can.

    That comes up as a 1A, 40V diode, so yes, it will work fine for both applications.

    Page 17 of the datasheet shows layout suggestions for both applications. I'd recommend you use that guidance to avoid any surprises during test.

    Good luck with your design!

    Dulcevida

  • Dear dulcevida,

    Thanks again. 

    Yes I noticed that layout and I have already implemented the associated placement accordingly for both applications ! 

    Thanks for all your help once again !

    Best regards

    Shmuel