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LP38501-ADJ: transient response from 5V down to 3.9V

steven heitbrink
Prodigy 230 points
Part Number: LP38501-ADJ
Other Parts Discussed in Thread: TPS54560

Hi,

I don't know if this is so much a question as a sanity check. I'm working will a celluar modem provider, and am trying to integrate one of their products.

We have are using a LP38501-ADJ providing 3.9V which is fed 5V by a TPS54560. I have 180uF of bulk capacitance on both of these power rails. The celluar modem provider is claiming that LDO's simply will not work because they aren't fast enough, and that switching power supplies are faster. This really contradicts what I know and what I've read in Application notes. They are claiming that they need to provide up to 2A of current. 

I've shown them bode plots from the datasheet of LP38501-ADJ , and a 4V power supply power supplies designed by WEBENCH. I've also put together a load tester with a power Mosfet and some load resistors. Every thing looks OK from where I'm sitting. 

I'm wondering am I missing something? Are there any conditions in which this LDO is slower than a typical switching power supply?  

 

  • 0
    TI__Mastermind 23361 points
    Hi Steve,

    I would like a little more clarifications on the comments about the LP38501-ADJ is not fast enough. The datasheet of this device indicate that the LDO can output a peak current of 6A and as you mentioned, the applications require up to 2A. It seems to me that the LDO is more than capable for the application. I wonder if you could get some feedback on what are their expectations and try to get quantitative requirements. If this LDO is not the best one, we may be able to recommend other ones.

    Switching power supply and linear power supply are very different in design and are targeting to different applications. Again, if you could point out your application criteria, we will be able to make more comments.

    Regards,
    Jason Song
  • 0 in reply to Jason Song
    Prodigy 230 points

    Hi Jason,

    I don't really know what "not fast enough" means either. I've thrown the below circuit together to do some testing. It seems that the device 3.9V rail dropped by no more than 50mV when the current jumped from 400mA to 1.6A. A lot of that looks like trace resistance. I can use R1 to see the current. The rise time was 1.2uS, so there is some inductance in my test rig, but it should be fast enough to test the LDO. 

    I suppose I need to get more clarification other than LDOs bad. I think a lot of their customers try to step from 5V down to 3.9 but their power rail that provides power to the LDO isn't fast enough to keep the LDO out of dropout. 

      

  • 0 in reply to steven heitbrink
    TI__Mastermind 23361 points

    Hi Steve,

    Your test rig looks okay, we also use FET to apply loads for some of our bench evaluations. You are right, once Vin has been lowered to 3.9V, the LDO is in dropout. Per datasheet, you will need to have the Vin higher than the dropout for your regulator to stay in regulation.

    The datasheet has data for 3A dropout for your reference. You should advise your customer that LDO needs to have Vin + Vdropout for it to stay in regulation. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    Prodigy 230 points
    Thanks for the help and the clarification.