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Using an LMR16006 to generate -30.0V from +5V?

Douglas Stierman
Prodigy 90 points
Other Parts Discussed in Thread: TPS5430, LMR16006

Hello, I need to generate -30V from a +5V supply. I'm familiar with the various articles involving Inverting Buck-Boost converters as well as the article on using a plain buck converter as an inverting regulator on this site at: /lit/an/slyt286/slyt286.pdf

I initially had two candidates for this topology: A TPS5430 or an MC3x063A, but these both have drawbacks and far exceed the amount of current supply I need(I only need 20-30mA, but I'd like the supply to have reasonable margin, so I'm shooting for 100mA). The TPS5430 is oversized at 3.5A Max and has a SO Power Pad, which will not be pleasant to hand solder. Since my load is very small, I could possibly get by with just providing a SOIC footprint and leaving the power pad float, I suppose. The MC3x063A series are very small and cheap. Reportedly slow, but I don't expect any large current draw transients(I'm just powering/programming a small older EAROM chip), but calls for a HUGE output capacitance of 1000uF, and on top of that it would have to be >30V rated, so I'm imagining either a very large and/or expensive part. Again, perhaps my small load and lack of large transients would tolerate a smaller output capacitor and still keep ripple to a minimum, but I'd hate to find out that I don't have pads for a big cap after fabricating my first proto PCB.

What I'd really like to use is the LMR16006 simple switcher. It has a nice footprint, is rated for 600mA which, even after derating for it's (mis)use as an inverting boost, should still supply the 20-ish mA I need, and should not require any large or exorbitantly expensive capacitors. The question is, will it work in this application and is the documentation sufficient to provide me with guidance on Cap and Inductor sizing when used in this topology?

Any help would be greatly appreciated!

Cheers.

  • 0
    TI__Guru 73585 points

     I think that the LMR16006 should work in your application.  I can send you an EVM, free of charge, if you would like to experiment with it before committing to a final design.

    If so please send you shipping address and phone to the following email: simpleswitcherapps@ti.com.

    The following app notes may also be helpful, in addition to the one you have looked at already:  SLVA317, SNVA722. 

    You mentioned leaving the power pad float; this is a very bad idea for any regulator.  This pad provides an electrical as well as a thermal connection to the regulator and must always be soldered to a ground plane.

    FD

  • 0 in reply to Frank De Stasi
    Prodigy 90 points
    Hi Frank,

    Thanks for getting back so quickly, and for your generous offer of an eval board. However, It looks like the eval board is configured for a buck configuration and might not be easily configured for the inverting boost I need. My biggest fear in using this part is that there is no clear guidance on how to choose the inductor and capacitor values as there is in the data sheets for the parts that are advertised as inverting buck-boost capable. I'm pretty sure I'm going to need a large inductor due to the large boost, but I want to avoid discontinuous mode as well, and with my application drawing very little comparative current, I may need to even load the output down with a resistor to guarantee staying in continuous mode. I'll be sure to download and read the two App notes you referenced, hopefully they will provide some guidance on component selection.

    Cheers,

    Doug.
  • 0 in reply to Douglas Stierman
    TI__Genius 10115 points
    Hi Doug,

    For a first pass on component values. Go to the product page
    www.ti.com/.../LMR16006
    Then enter 35Vin and 30Vout at 0.6A into Webench. This will size the components close to what you would want for your design. Your 5V input looks like 35 across the part once it has generated the -30V.
    Avoid the mistake of making the inductance value too large as this will increase the amount of minimum load current that you need to keep the boot capacitor charged.
    First pass you would probably want to pull 10mA of static load through the FB resistors. So your FB resistors will be on the order of 3K total. If this is working then you can start to scale up the resistance if necessary.

    Marc
  • 0 in reply to Marc Davis-Marsh
    Prodigy 90 points
    Hi Marc, Thanks for the input! I actually never noticed the Webench tool off on the side of the page. It appears that the inductance requirement goes up as the current decreases(15uH at 500mA, 100uH @ 100mA). I'm also a little leery of the small input and output(2.2uF and 1.0uF) cap's in the generated design. They seem to small to be true. Do these values look right to you? I'm sort of new to switcher design. Thanks again for the help!
  • 0 in reply to Douglas Stierman
    TI__Genius 10115 points
    The inductance is often calculated to provide a ripple current proportional to the load current entered. Although this works well near the full load current of the device, it tends to calculate a higher inductance than necessary when you are using only a small portion of the available output current.

    The 2.2uF and 1uF values would be minimums. The 2.2uF would be based upon input voltage ripple of a certain percentage. The 1uF is probably based upon stability for 30V. You could safely add some more capacitance, but avoid adding too much because the amount of current needed to charge your output capacitors during start-up can add up quick and you could easily hit current limit with this high output voltage.
  • 0 in reply to Marc Davis-Marsh
    TI__Guru 73585 points

    As I mentioned, we are happy to send you an EVM for the LMR16006 that can easily be converted to inverting as detailed in app note SNVA722.  By adjusting the feedback resistors to give a 30V output, it will probably work on the first pass.  Then you can adjust the components to get the performance you desire. 

    FD

  • 0 in reply to Frank De Stasi
    Prodigy 90 points

    Hello Frank,

    Does the EVM have provisions for swapping the diode and the inductor in order to facilitate testing the inverting buck-boost topology? If so, it could be very helpful. Thanks!

    Doug.

  • 0 in reply to Marc Davis-Marsh
    Prodigy 90 points
    Hello Marc,

    After going through the design exercises in slva257a.pdf, but substituting in the specifications for a LMR16006Y, I realized that my design will require quite a large inductor due to my expected small actual use current. The component is actually expected to draw less than 8mA, but I want the regulator to be stable when the no load is connected, so I used just the feedback resistor load current of 4mA as my Io,min. With those numbers, the equations call for a 300uH inductor. At this low current, the WebBench designer doesn't generate a circuit as it states that the design is not stable. The lowest load current I can enter into WebBench is 100mA, which IIRC generates a circuit with a 100uH inductor. Not sure where the 100uH is coming from as the parameters show it is designed for 60mA ripple current. My spreadsheet calls for a 20uH inductor at 60mA if you assume min current to match the desired ripple current. That is the assumption the app note makes if we want to stay in continuous mode. It's pretty clear I may need to do a bit of experimentation. I could always crank up the static current through the feedback resistors as long as I use a package that can handle the power and get away with a smaller inductor that way, but I'm not too concerned with that. I'm going to use a Bournes SRR1240 series part which comes in values from 1uH to 1000uH. I'm also not too worried about large static current draw(other than needing higher wattage resistors), as this is for a low duty cycle vintage EAROM chip programmer, but I'd still like to figure out how to make this work by design as opposed to by experiment. There may be aspects of the Inverting Buck-Boost topology that attempting to fool Webench can't resolve. I tend to trust the equation based app notes more, however, since I don't have the pole and zero info for the LMR16006Y, the output capacitor will still be a bit of an experiment. I'll probably just start with a 10uF and I suspect it i will be perfectly fine as long as the rest of the circuit is correct. It would be great if Webench could also handle the inverting B-B topology explicitly an option.

    Thanks for all your help thus far, though!
  • 0 in reply to Douglas Stierman
    TI__Guru 73585 points

    You do not need to exchange any components.  Only the input and output connections are swapped as shown in SLVA478; attached.

    If you would like an EVM for the LMR16006, send you shipping address and phone number to simpleswitcherapps@ti.com.

    FD

     slva478b.pdf

  • 0 in reply to Douglas Stierman
    TI__Genius 10115 points
    There is a description of how to connect the EVMs of buck converters into buck boost by swapping the ground and vout wires in the following ap-note
    www.ti.com/lit/an/snva425a/snva425a.pdf
  • 0 in reply to Frank De Stasi
    Prodigy 90 points

    eMail sent. Thanks again! If you have them, it's fine to send just a bare board since I'll have to replace the Inductor and output cap anyway, not to mention the Schottky diode, which may not be rated for the -30V+25% margin recommended.

    Cheers,

    Doug.

  • 0 in reply to Marc Davis-Marsh
    Prodigy 90 points
    Ah, Of course! The generic depictions show the diode and inductor replaced, but when you actually go to implement on an actual part, it becomes more clear that the GND and Vout on the chip are just changing roles. Thanks for the heads up!