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LTC3225, LTC4412, Supercapacitors Can Replace a Backup Battery for Power Ride-Through Applications − Design Note 450

Category: Software
Product Number: LTC3225, LTC4412

Hi

I tried the circuit with LTC3225 and LTC4412 from the Application ("Supercapacitors Can Replace a Backup Battery for Power Ride-Through Applications − Design Note 450").

My Goal is to have 4.5V or more for approx. 1s, the current I need is about 330-450mA. For the moment I don't use a DC/DC converter at the output.

So the first issue I had was, that the LTC3225 didn't seem to charge (PGOOD stays low), when setting RPROG=4k7 (according to datasheet RPROG must be >2k).

So I set RPROG = 12k.

This is what my schematic looks like (the orange highlighted parts are slightly different from the application note):

I use the P-MOSFETS NTLUD3A50:

Unfortunately the Voltage drops way to fast.

Also when I measured the signals it seems as if the FET's are not switched digitally (with opposite levels) but rather are in a kind of amplifier mode.

For a short time the PGOOD oscillates when only one FET is fully conductive.

Zoom of previous oscillation:

Can you give me any advice to improve my schematic.

Or is there any mistake I am missing?

Thank you.

  • Hi Irina,

    Does the LTC3225 charger work when you set the Rprog to 4.7k ohm? If yes, can you try isolating the LTC4412 first and test it on its own? Try verifying the design using LTspice first.

  • Info:
    With the first approach I used, I had only 220mF and they had a very worse ESR of 40 Ohm! (2x 220mF in Series, with CX in middle, schematic^^)
    (Sorry, I forgot to mention that I also used an other value than in the Application Note, because I didn't had this capacitor then.)
    So I guess the oscillation at PGOOD is because of this high ESR and the fast voltage drop because of the too small capacitance.

    Update: 
    When changing the supercaps to 2F/3V and only 100mOhm ESR the result was much better.
    Now the voltage drop is less and the discharging curve looks as expected.
    (I have still around 4 Volt after 2.5 s)

    However, I can still see that the FET's have loss in the path when switching to the backup voltage of the supercaps (so they are not fully conductive respectively blocking).

    Update2: 

    I changed the FET's, now it's looking much more digitally Slight smile

    Instead of the P-FETs NTLUD3A50PZTAG I use now the FETs Si9953DY (which I had available, but are obsolete).

    It works now properly :)

    But now my Question is, why does it work with the one FET's but not with the other (similar specs)?

    Which parameter for the FETs are important?

  • Hi Ron,

    Thank you for your answer.

    It seemed that the LTC3225 had problems when i set the Rprog to low, but when I did the tests the other components were already there, so isolating it first is a good idea.

    The design itself should work as my newest results show, however there is still room to optimize, especially the path losses in the FET's...