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Buck Converter for IBB Configuration with high efficiency and low ripple at light load

AlexWalton42
Expert 4855 points
Other Parts Discussed in Thread: LM63615-Q1, LMR36503-Q1

I posted I need help replacing a Maxim power part for an automotive customer's radar system.  I am planning to use a standard buck in IBB configuration as discussed in:

–Create an Inverting Power Supply From a Step-Down

–Using a buck converter in an IBB topology

Their current design uses the MAX16945TGUT/V+.  This chip is an inverting charge pump that will invert 5.4V to -5.4V. Here is the datasheet for the Maxim part: Datasheet.  My goal is to pitch one of our parts to replace the Maxim part.  The efficiency of the MAXIM part is around 90% at nominal load 5V @10mA.  Ripple is also a very important consideration for this system.

 

I have been looking at a few buck converters: TPS54061-Q1, LM5165-Q1 and LMR36503-Q1.  The LMR36503-Q1 is the only one that is in stock.  Unfortunately, they are all out of stock.  Since this is for automotive, I have ruled out the buck converters with higher Iout(max) because the frequency will drop into the AM band for the Iout(nom) = 10mA of my system.  Is that correct? 

Do we have any automotive grade bucks that can:

  • Operate in IBB configuration
  • Forced Frequency to stay out of AM band 
  • Low current efficiency >= 90%

By the way, we have a few automotive charge pumps, but they don't meet the input voltage requirement of 5.4V.  

I'd appreciate any help.

  • 0
    TI__Guru 59025 points

    Hello

    I think that almost any buck can be used as IBB.

    But in your case I don't think that a buck is a good fit.

    As you mentioned, at light load the converter will fold back frequency to give

    high efficiency, but you nee constant frequency.  If you use FPWM then the 

    frequency will be constant but the efficiency will be poor.

    If the load is very light, such as the 10mA you mentioned, then the frequency

    will be low and may not interfere with the AM band.  You can see some curves

    for frequency vs load in some of our buck data sheets.

    The best way to evaluate would be to use the EVM for a given device.

    Otherwise you may need to go with a charge pump.

    I will forward this post to my colleagues to see if they have any other suggestions.

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Expert 4855 points

    Hello Frank, thank you for the help.  The input voltage is 5.4V and all of the automotive grade inverting charge pumps have Vin(max) = 5.25V.  I limited it down to a few bucks that have the best light load(10mA) efficiency. Those are the three devices I mentioned in my initial post.

    Maybe there is a different buck converter that I missed? Or, is there a different topology besides the inverting charge pump, buck-boost inverters (the one automotive grade buck-boost had poor efficiency @~10mA), and buck in IBB that could work for my application?  

    Best,

    Alex

  • 0 in reply to AlexWalton42
    TI__Expert 4855 points

    And frankly, they already have the inverting charge pump in their system, so I do not think they'd want to switch to our inverting charge pump unless it had way better efficiency and less ripple.  Besides us not having any automotive grade inverting charge pumps for their design, I figured that pitching a buck that allows for wider input/output range would be best in case they need to modify their system. I'm curious to see what your colleagues can think of. 

  • 0 in reply to AlexWalton42
    TI__Guru 59025 points

    Hello

    You might look at the LM63615-Q1.  It should have OK efficiency in an IBB at light load in PFM.

    But you would need to check on an EVM if it is OK.

    When my colleagues return I will ask them for more ideas.

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Expert 4855 points

    Frank,

    In general, when using a standard buck in IBB, does the efficiency or ripple decrease?

  • 0 in reply to AlexWalton42
    TI__Guru 59025 points

    Hello

    Yes, in general the efficiency will go down when converting a buck to IBB.

    And the ripple will go up.

    The hard question is how much they will change for a given case.

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Expert 4855 points

    Okay.  And, if the standard buck in IBB configuration is the best option, are there any that will work in my application that are still in stock?

  • 0 in reply to AlexWalton42
    TI__Guru 59025 points

    Hello

    I will need to check with our Marketing team to see what is in stock at this time.

    I will try to get back to you as soon as possible.

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Guru 59025 points

    Hello

    Base on some feedback from my colleagues it looks like the LMR36503-Q1 is your best bet since it is in stock.

    The best thing to do is to get an EVM and test it out.

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Guru 59025 points

    Hello

    We will take this discussion to email; i will close this post.

    Thanks