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TPS63030 in parallel with LDO?

Mario Hernandez
Intellectual 695 points
Other Parts Discussed in Thread: TPS63030, CC2531, TPS2112A, CC2530, TPS62740, TPS63050

Hi Chris,

I am designing a system in which a TPS63030 will be in parallel with and LDO. I got some doubts regarding this:

-If they worked in parallel, should I protect the output of the TPS63030?

-Do I have to "duplicate" the number of caps for input and output?

-Any special concern about this practice? 

Thanks

  • 0
    TI__Guru**** 150650 points

    What power specs do you have that might benefit from this approach?

    Yes, you need caps for each device, located close to the device, as required by each device's D/S.

    You need to operate the TPS63030 in PFM mode so that it doesn't sink current.

    I recommend testing this with 2 EVMs first.

  • 0 in reply to Chris Glaser
    Intellectual 695 points

    Chris, thanks for replying. The specs are:

    Multiple power input will come from coin cell 3V + LiPo battery 3.7V + USB (previously multiplexed with a TPS2112A), so input range is [2.5 - 5.5V], although the USB can be regulated separately, so the input range would be [2.5 - 4.2V]. It needs to be uninterrupted, thus, if I unplugged one battery, the load (CC2531) should keep on. The load will vary from 1mA to 50mA aprox.

    Given some posts I read here, the ripple voltage when the TPS63030 is in PFM (power save mode) may be the biggest challenge from this design right?. Other things I think I should take care of are ballast resistors and input capacitor, to handle high internal resistor from coin cells. If you think I am skipping something, please let me know.

    Last question, when converters work in parallel (only one at a time), one has an active input and the other does not have any input. Even in those circumstances is necessary to protect the converter (for reverse currents)?  

    Thanks for your support

  • 0 in reply to Mario Hernandez
    TI__Expert 5425 points

    The TPS63030 should be ok with no input voltage even when there is output voltage from another converter. Power save mode needs to be selected to make sure that the TPS63030 does not sink current from its output since it is powered from another converter. In power save mode the converter will not have reverse currents. The converter powering the output will define the ripple of the output voltage.

    At a typical LDO at this condition current will flow from the output to the input if it is not powered. The backgate diode of the control FET is conducting this current. If this does not cause problems in the circuit it should be no problem either. The LDO itself most likely is fine.  

  • 0 in reply to Juergen
    Intellectual 695 points

    Looking at the efficiency in power save mode, I think I'll enable this permanently.

    My concern regarding the ripple and spikes is if those will affect the performance of my CC2530 device.

    Thanks 

  • 0 in reply to Mario Hernandez
    TI__Guru**** 150650 points

    I'm not sure that you need to parallel anything.  A battery charger would take the USB input and charge the battery.  It would also control those two sources and provide one voltage to the system.

    Then, your coin cell could be diode OR'd to this node, so that if the battery is absent or too low, the coin cell delivers power.

    To power the CC25xx, you just need a TPS62740 step-down type device, not a buck boost.

    If you have other, higher voltage rails in the system, then the newer TPS63050 buck-boost has better efficiency and lower ripple.

  • 0 in reply to Chris Glaser
    Intellectual 695 points

    Chris,

    Is it possible to diode OR the coin cell directly to the USB? As it's just 3V (and given that after little usage will become 2.7V or less), I do not want to reduce its nominal voltage with a diode, do you know any MOSFET, diode or IC that can do that with minor threshold?

    Yes, I need a converter valid for several power rails, the TPS63050 fits. Besides, it'll boost when the system is power from a semi-new (40% usage) coin cell.

    Thanks again

     

  • 0 in reply to Mario Hernandez
    TI__Guru**** 150650 points

    If you use a charger, then diode OR'ing it at the USB level would just add the charger in the path between the coin cell and system.  I see no advantage to this over ORing it on the charger output.

    The power interface forum would know how best to diode OR your coin cell.

  • 0 in reply to Chris Glaser
    Intellectual 695 points

    Agreed. The problem is that the charger will be placed at an external module, as well as the battery. However, the USB from the system will be able to power the charger in case the module is plugged. Otherwise, it will be a standard USB power. Sorry I omitted that.