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TPS7A49: Current Limit Specification for TPS7A49

user4544767
Prodigy 240 points
Part Number: TPS7A49
Other Parts Discussed in Thread: TPS7A47, TPS7A33, TPS7A39

I need a 200mA capable LDO, with a Vref that is identical to the TPS7A3001 (complimentary negative LDO).

The TPS7A49 Datasheet (SBVS121E – AUGUST 2010 – REVISED MAY 2015), claims I_lim (min) is 220mA, but elsewhere all the documentation refers to 150mA.

Which is correct?

  • 0
    Guru 140200 points
    Hi User,

    this is no contradiction. 150mA is the recommended maximum output current the regulator can permanantly output, adequate cooling provided. And 220mA is the minimum output current which can be delivered in a brief overcurrent condition. Datasheet tells that the ouptut voltage has dropped to 90% of nominal output voltage in this situation. So, it's evident that the regulator is out of regulation in this overcurrent situation.

    Kai
  • 0 in reply to kai klaas69
    TI__Guru 55940 points

    Hi ,

    As  mentions both statements are correct.  TPS7A49 is designed, tested, and characterized for loads up to 150 mA; however, the current limit protection feature will engage at 220 to 500 mA.  The current limit is a protection feature and is not meant for normal operation.

    In order to help you select another LDO could you tell us more about your application requirements?  What is your Vin and Vout?  Are there any specific features that you are looking for?

    Hi ,

    Please note that the 90% is a test condition used to force current limit during the test sequence.  When Vout is bellow the regulated voltage, the error amplifier of the LDO will drive the pass element as hard as possible allowing as much current to flow through the LDO in an attempt to bring the output within regulation.  During our test sequence Vout is held outside of regulation; therefore, we are forcing current limit to engage so that we can measure the current limit. 

    In a real application, the LDO may or may not be in regulation still depending on application conditions even in current limit.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 240 points
    Thank you for the detailed information - with that background the datasheet makes more sense (although the complimentary TPS7A3001 is rated at 200mA and looks similar - though I guess the different polarity of the output stage may make a difference there).

    I'm trying to somthing similar to this:
    Use a tracking power supply to improve signal-chain performance, Scot Lester, TI
    www.eetimes.com/document.asp

    7V and -7V in, 5-6V (variable) and -5 to -6V output (variable), both at up to 200mA (rather than 150mA). I also need good Vin-Vout noise filtering at 500kHz.

    Therefore complimentary parts like the TPS7A49xx and TPS7A3001 would be useful (as I assume their BW and loop response are similar, as is their Vref) - with this style of servo control in the feedback loop, I'm worried about stability - should this be a consideration for LDO choice in such an application?
    Alternatives like the TPS7A4700 (1A) have a annoying packaging from a prototyping point of view.
  • 0 in reply to user4544767
    TI__Guru 55940 points
    Hi ,

    Unfortunately due to your voltage and current requirements TPS7A47 is likely your best option. We typically recommend pairing this with TPS7A33. It should be noted that as long as the resistor divider is properly selected for the specific LDOs used per their datasheets, it is not required for the LDOs to have the same reference voltage.

    Although your 200 mA requirement does eliminate TPS7A39 for this specific application, you may want to keep TPS7A39 in mind for future projects. TPS7A39 provides tracking on a positive and negative rail all within a single package.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 240 points

    Thank you for the concrete recommendations.

    As the TI article for the tracking supplies as the feedback as a "servo" voltage feedback - I guess that the bandwidth of most positive regulators would be sufficient (with regard to loop stability), so I consider various LDOs for the 200mA positive supply?

    PS could the TPS7A49 be paralleled for greater current output?

  • 0 in reply to user4544767
    Guru 140200 points
    Hi User,

    paralleling regulators, especially LDOs, is almost always criticial and should be avoided whenever possible. In any case you need additional measures to balance the even very little differencies in the output voltages. And this must work for the whole output current range and the whole temperature range. So, you will usually have to introduce balancing shunt resistors or similar to the outputs, which can heavily deteriorate the regulating performance:

    www.reuk.co.uk/.../

    Above you wrote that you need a high PSRR at 500kHz. But this can also be accomplished by the help of a simple Pi-Filter in front of the regulator.

    Kai
  • 0 in reply to kai klaas69
    TI__Guru 55940 points

    Hi ,

    While I agree with  that care must be taken to properly design a parallel LDO application, it is a possibility.  We typically do point out our higher current LDO solutions as they use fewer components and are simpler to design into an application; however, there can be some advantages to paralleling LDOs.  For instance you will split the heat dissipation and you can even reduce the output noise from the LDO itself.  Here are two TI Designs that you can use as references as you decide if you want to go down the paralleling path:

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 240 points

    Thank you for the links - I'll probably just bite the bullet and go for a 1A regulator :)

  • 0 in reply to user4544767
    Guru 140200 points
    Hi User,

    contact us if you need help with the dimensioning of PI-filter.

    Kai