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TPS82130: Concerns wIth 1F-2F output caps for long hold-up time

Shenhua Zhang
Prodigy 640 points
Part Number: TPS82130
Other Parts Discussed in Thread: TPS62130, TPS25940

Hi expert,

Customer needs to add 1~2F output cap for TPS82130 to achieve large hold up time, can you help check if this is acceptable?

What should we pay special attention to with such large capacitor?

Detailed spec below:

  1. Power :   2.5W
  2. Output Voltage : 3.3V +/-5%
  3. Input Voltage    : 12V
  4. Source 3.3V about 80ms after 12V is power off
  • 0
    Prodigy 90 points

    Part Number: TPS82130

    We need add 1~2F output cap for TPS82130 to achieve large hold up time, can you help check if this is acceptable?

    What should we pay special attention to with such large capacitor?

  • 0 in reply to user5951090
    TI__Genius 17905 points

    Hi Shenhua,

    This is an interesting design requirement.

    I do not think TPS82130 would be suitable to drive this load cap directly. The typical output cap range recommended for this device is 22uF to ~400uF per the datasheet and also as discussed in this app note (TPS82130 is similar to TPS62130 with 1uH inductor). You could potentially isolate the large cap from the TPS82130 output with an inductor/ferrite bead or series resistor -- for example, TPS82130 output with 22uF cap to ground, then series bead or resistor, then large cap. Feedback taken directly at TPS82130 output at the 22uF cap. A series resistor will also help limit current that can be instantaneously drawn from the source and TPS82130 but will lead to voltage drop and poor load regulation with the load current. But this may not be a problem if the 3.3V only needs to supply current during the 80ms when current is being supplied by the cap anyway.

    The other challenge is to prevent the 3.3V output from conducting back to the TPS82130 input through the HS FET body diode when the 12V rail turns off. You would need to add some kind of reverse current protection diode or load switch after the TPS82130 output. Without some kind of reverse current protection, any other circuits tied to the 12V node may be unintentionally or partially powered by the 3.3V minus a diode drop that will be at the TPS82130 input.

  • 0 in reply to Kris F
    Prodigy 90 points

    Hi, Kris, thanks for your reply.

    Can you suggest series inductor/resistor? Tps82130’s 3.3V output will supply current when 12V is ok, added 1F cap will supply current within 80ms, will there be load regulation problem?

    If there is any simulation model, that will be helpful.

  • 0 in reply to user5951090
    TI__Genius 17905 points

    Hi, apologies for the delay.

    I found this app note which uses TPS25940 to charge a supercap for backup/hold up applications. Putting this device after the TPS82130 would help provide reverse current blocking and give additional control on the charging current and current limit.

    I am looking for other recommendations for you to solve the challenges with this design.