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TPS2121: Supercap backup power supply - 4A 5V

Part Number: TPS2121
Other Parts Discussed in Thread: LP2998, LMR33640, , TPS61022


I'm working on a backup power supply using supercaps.
The supply should be able to deliver 5V at 4A for about 10 seconds to be able to cleanly shut down an embedded pc.

Two supercaps in series seems to be the best approach for this power level when looking at the higher capacity number the 2.5V variants are sold in?
I came across this appnote which uses the LP2998 as a balancer:

SD of the LP2998 is tied to AVIN through a divider to make sure AVIN is higher then 2.2V before the device turns on.
Don't know if this is the best approach, a comparator might be better?

An LMR33640 is used to convert 12-24V to 5.1V, a TPS2121 power mux to switch to the supercap power rail when the LMR33640 drops out.
And a TPS61022 to boost the supercap rail to 5V during discharge.
In normal operation the TPS61022 should be getting 5.1V on VIN preventing it from having to regulate and just passing the 5.1V through.

Any remarks or suggestions for this kind of application?
ps: The TPS2121 and TPS61022 scare me a bit package wise, but couldn't seem to find any comparable alternatives with bigger (more reworkable) packages.


  • Hi Nick, 

    Do you have any particular concerns with TPS2121 here? TPS2121 can work for this application. We don't have a powermux in leaded package which can support 4A current, but TPS2121 is a good choice for this.

    My only comment is that this configuration will switch to supercap backup when VIN1 < VIN2, as the device is operating in VCOMP mode. If you want to tune the voltage where the switchover occurs, you can choose it arbitrarily by a divider on PR1.

    Please let me know if you have any further questions on this, otherwise could you please press the green button. 



  • Ok thanks for the feedback!
    Any remarks on the rest of the schematic?

  • Nick,

    I can't speak for the rest of the schematic , thats not my area of expertise. 

    TPS2121 configuration is OK, this is VCOMP mode and it will just switching to whichever VIN1/VIN2 is greater than the other. if this is the behavior that you want then this TPS2121 side of things looks oK