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TPS7B4256-Q1: Can TPS7B4256 sink current?

Tomasz Jastrzebski
Intellectual 252 points
Part Number: TPS7B4256-Q1
Other Parts Discussed in Thread: OPA206, TPS7B84-Q1, TPS7B86-Q1, TPS7B91, REF2030

Tool/software:

Hi TI, can TPS7B4256 sink current? Probably not, but I just want to double check.

  • 0
    TI__Expert 6640 points

    Hi Tomasz,

    Your intuition is correct, TPS7B4256-Q1 cannot sink current.

    Under what operating conditions would this be necessary or desired? None of our LDOs have this functionality, so it would be good to understand if there's a common use case where sinking current might be desired.

    Regards,
    Kelsey

  • 0 in reply to Kelsey Farr
    Intellectual 252 points

    Hi Kelsey, I need LDO to power high voltage frontend unity-gain opamp buffer. In case of overvoltage some extra current needs to be sunk. Ofc, Zener diode is a standard solution, but in this particular case it is problematic since opamp (OPA206) works at max Vc 36V so very precise current sink is needed and it must be able to dissipate considerable power (5mA x 36V = 180mW).
    It is not directly related but are you aware of any other LDOs with both FB and ADJ/EN (Vref) pin, preferably max Iout 250 mA? TPS7B4256 max current is only 70 mA.

  • 0 in reply to Tomasz Jastrzebski
    TI__Expert 6640 points

    Hi Tomasz,

    Thanks for the details! Is the 36V expected in case of overvoltage on the input to the op amp? Just want to make sure I'm understanding this correctly.

    TPS7B4253-Q1 is a tracking LDO with FB, rated to 300mA.

    I'm curious though, if this is for powering the op amp, wouldn't a traditional LDO (with internal Vref, like TPS7B84-Q1 or TPS7B86-Q1) be a better fit? Or there might be there some nuance in the "buffer" aspect of the use case that I'm missing.

    Regards,

    Kelsey

  • 0 in reply to Kelsey Farr
    Intellectual 252 points

    Hi Kelsey,
    36 V is power supply voltage. Circuit must survive as high input voltage as possible. In this case, according to specs, up to -10 mA input current can be expected and this current must be sunk so if this opamp has to die, it dies alone because of exceeded inrush current, not because of power supply overvoltage possibly taking other circuits down with it.
    Why tracking LDO? Because circuit operates at the opamp limits and I was thinking that power supply voltage accuracy better than 1% offered by the typical internal LDO Vref may be required but I am not certain about it yet, I admit, it may be overkill but I am just contemplating available options.
    Kind regards,
    Tomasz

  • +1 in reply to Tomasz Jastrzebski
    TI__Expert 6640 points

    Hi Tomasz,

    Thanks for the clarification and taking the time to explain!

    About the tracking LDO -- While the tracking LDO does offer excellent accuracy, this isn't Vout accuracy in the same sense you'd think of a traditional LDO. The accuracy is tracking accuracy, which measures how well Vout will adhere to the VADJ/EN (Vref) voltage. The difference here is that because Vref is provided externally, if Vref moves, Vout will track it accordingly. So your LDO output accuracy for a tracking LDO is limited by whatever the accuracy is of the external Vref you provide.

    If you're looking for tight accuracy, TPS7B84-Q1 (150mA) or TPS7B86-Q1 (HSOIC, 500mA) have maximum accuracy <1% across line/load/temp, but their max Vout is 18V. TPS7B91 (150mA) can provide 36V Vout, but it's not nearly as accurate.

    Since the input range for OPA206 is 4.5V to 36V, could you possibly lower the supply rail? Or are there some auxiliary loads that require 36V?

    Regards,
    Kelsey

  • 0 in reply to Kelsey Farr
    Intellectual 252 points

    Hi Kelsey,

    I do understand and since I use REF2030 anyway I can expect pretty good LDO accuracy.
    At all cost I do NOT want to lower the power supply voltage. Quite opposite, I want to keep it as high as possible since it defines circuit dynamics - input voltage range.
    In any case, new TI TPS7B and TPS7A LDOs are excellent products and there is plenty to choose from. The only LDO I am missing from TI portfolio is low power, small package high voltage (60/80V transient) chip like AP7387Q-50FDCW-7 (Diodes Inc).

    Kind regards,

    Tomasz

  • 0 in reply to Tomasz Jastrzebski
    TI__Expert 6640 points

    Hi Tomasz,

    Oh good, yes if you're using REF2030 as well, then a tracking LDO here would work (aside from the ability to sink current, of course).

    For the 60V use case, TPS7A1650AQDGNRQ1 is the closest match for the Diodes device. We also have TPS7A4350DGQR rated to 85V input if you do need >62V abs max.

    Regards,
    Kelsey

  • 0 in reply to Kelsey Farr
    Intellectual 252 points

    Hi Kelsey,
    TPS7A4350DGQR requires too many external components while TPS7A1650AQDGNRQ1 can survive only 62V.
    I like AP7387Q-50FDCW-7 because it is small, simple and dies hard.
    Regards,
    Tomasz

  • 0 in reply to Tomasz Jastrzebski
    TI__Expert 6640 points

    Hi Tomasz,

    Noted, I appreciate your feedback -- it will be taken into account for future developments. The last option I can think of in our portfolio for this type of higher voltage use case would be TPS7A4001.

    Feel free to reach out if you have further questions; I'm closing this thread for now.

    Regards,

    Kelsey

  • 0 in reply to Kelsey Farr
    TI__Expert 6640 points

    Hi Tomasz,

    Sorry to reopen this thread again, but I'm curious for the 60V/80V input LDO -- what maximum output current is required of the LDO? I see that AP7387Q-50FDCW-7 is rated to 150mA.

    Regards,

    Kelsey

  • 0 in reply to Kelsey Farr
    Intellectual 252 points

    Hi Kelsey, maximum current I still need to determine but probably 50mA will be sufficient.

  • 0 in reply to Tomasz Jastrzebski
    TI__Expert 6640 points

    Great, thanks for the response, Tomasz! Really appreciate it.

    Regards,

    Kelsey