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LM25011: supercapacitor charger

Alen Barisic
Intellectual 1300 points
Part Number: LM25011

Hello,

we are using the LM25011 as a supercapacitor charger and have some unexpected behavior where we would appreciate some help.

The issue is that after first charge, the output regulation start to decrease very slowly and it is not stable, and seems that the regulator does not perform regulation any more.

I made some investigation:

  • FB pin is stable to 2.51V, SW and BST are switching, CS has 0 current
  • Without supercap and with 1k resistive load, the output is stable to 4,84V as expected

I have also re-tested the demo board in the same configuration and this behaviour does not exist.

Here is what we have changed from demo board:

Then, I made some investigation:

  • I changed R6 to 100mohm, but nothing changes
  • Changed R2 and R3 to 4,7k (near to the demo), but nothing changes

Please also find waveforms of VOUT, SS, SW, BST, and IL

YELLOW: VOUT (supercap voltage)

RED: SS

BLUE: SW

GREEN: BST

As we can see, SS is not dropping down, BST and SW are switching so ti seems that the regulator is on. But on yellow we can see on “widetime” that the voltage regulated drop down.

  • 0
    TI__Genius 10665 points

    Hello,

    It appears that your switching waveform is not switching properly. Instead of looking like the expected square waveform, the SW and BOOT look more sinusoidal. 

    From your description it sounds like the LM25011 was able to properly regulate the output voltage and charge up the super capacitor, but after the first charge something happened that caused the regulation to stop. Was there a situation after the first charge where the input voltage to the LM25011 was less than the voltage at the output? 

    Regards,

    Harrison Overturf