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TPS62125: Charging a super-capacitor from a buck SMP without seriously draining the capacitor with no power input

Andrew Lohmann13
Prodigy 30 points
Part Number: TPS62125
Other Parts Discussed in Thread: LM5165

My application

Motor as a generator ---- bridge rectifier  --- single op-amp V and I monitor to control maximum power transfer ---- Buck converter ---- supper-capacitor----- Boost converter ---- 3.3V out

I am considering using TPS62125 because of its efficiency and low voltage operation but I am concerned about what the current drawn from the super-capacitor when there is no input power.

If it were 30uA say? that would be fine but if the regulator goes into some output discharge mode that would be a problem. I want to avoid adding diodes as these would limit the minimum operating voltage.

  • 0
    TI__Guru**** 150690 points
    Your question is not entirely clear. But I think it is around what happens when there is no input power to the TPS62125 and the output voltage is still held up by the super cap? Here is a paper I wrote recently, which discusses this topic: www.ti.com/.../slyt689.pdf

    The TPS62125 does not have output discharge. You can use the PG pin as a discharge function, but there is no other output discharge built-in.
  • 0 in reply to Chris Glaser
    Prodigy 30 points

    Thanks Chris,

    My application is similar to the note's figure 3. In any case what I am doing happens with every type of buck regulator as the note shows in figure 2 and 3. Your final comment about there being no other discharge is re-assuring - Having started to review a ST.COM part I found that there is a built over voltage detection and output discharge function - so I thought will that trigger as the input voltage and the reference voltage begin to fall?  

    I shall avoid the PG pin function.

  • 0 in reply to Andrew Lohmann13
    TI__Guru**** 150690 points
    Yes, every regulator is different, so just because one has a discharge which works one way doesn't mean another even has the same feature.

    Using the PG pin is fine--connecting it to the output voltage with a small pull-up resistor will create a discharge path. I think it is this second item which you wish to avoid.

    I assume there is an MCU in your system which can turn off the TPS62125, when needed? Or you can use its precise EN pin to do the same.
  • 0 in reply to Chris Glaser
    Prodigy 30 points

    Thanks Chris,

    The MCU does not switch off anything, when the super-capacitor voltage is low but will restrict what it is doing in order to keep the RTC running at least. Then there are watchdog and other things planned in order to ensure the MCU runs effectively without freezing. Power  will run down completely perhaps daily when there has been no movement of the generator for many hours.

    I give you this additional information in case there is some other issue that you have in mind in mentioning the enable pin?

    Lastly the other buck regulator I am considering is LM5165 as this has soft-start, used as a compensation input, which could give better MPPT control stability.

    Any way once again thanks.