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DC1589A/LTC4425 Behavior

Hi,

I have the input being driven by a DC power supply, 3A capable no current limiting. It is set to 5.0V.  I have a pair of Eaton 300F caps attached to the board, connected as required to Vout, Vmid, GND.  The board is set for NORMAL, with Vclamp at 4.9V. The board jumpers are set for 2A output limit.

I begin by discharging the caps through a load until voltage is around 4V. I then enable the power supply, and as expected, current on the supply is indicating around 210mA, about 1/10th.

Then:

4.4V, current rises to 600mA

4.5V, 1A

4.6V,1.7A

4.7V,2A

4.75V,500mA

4.78V, 200mA

When it reaches 4.85V, it shuts off as expected. It sits around that for about 30 seconds, the power supply indicates it's consuming a bit of current, about 30mA. Then the cap voltage starts to bleed down until it reaches 4.78V, then it switches back on, goes to 4.85, then shuts off...this repeats over and over.  It's like the 4425 sprung a leak and is bleeding off current.

Can someone explain why the part (board) is behaving in this bizarre manner?

Thanks, Gary

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  • Just a quick update, this seems to be significant.

    Once the cap reaches top charge, and just when I see the power supply current drop to 0, I remove the ENABLE jumper and put it to DISABLE. The capacitor voltage then remains stable. I can basically do the same thing by shutting off or unplugging the power supply. I do not see anything in the LTC425 data sheet about this behavior, other than a paragraph on page 9 where it says the following:

    "In the event VIN suddenly drops below VOUT, the controller will quickly turn the FET completely off to prevent any loss of charge due to the reverse flow of charge from the supercap back to the supply."

    I don't think the FET is completely off. Any ideas...suggestions? This part would be completely unusable in this state. I've used the LTC3225 and it does not do this at all.

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  • Just a quick update, this seems to be significant.

    Once the cap reaches top charge, and just when I see the power supply current drop to 0, I remove the ENABLE jumper and put it to DISABLE. The capacitor voltage then remains stable. I can basically do the same thing by shutting off or unplugging the power supply. I do not see anything in the LTC425 data sheet about this behavior, other than a paragraph on page 9 where it says the following:

    "In the event VIN suddenly drops below VOUT, the controller will quickly turn the FET completely off to prevent any loss of charge due to the reverse flow of charge from the supercap back to the supply."

    I don't think the FET is completely off. Any ideas...suggestions? This part would be completely unusable in this state. I've used the LTC3225 and it does not do this at all.

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