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Why is my LTC3780 only giving 0.8V - 2.8V output?

tim687

Recently I bought two of these, I was needed to remove all of the pots since they where bad. I soldered a resistor (500K) like this to the board to make sure that the undervoltage protection won't go off, since that was the issue I had when I powered up the board for the first time. That issue caused the fault led to light up and the output to not work.

I got one board working, since I wanted to get one working before I changed the other one. I am not sure if the resistor fix is correct here.

image

 

Now, a few days (maybe weeks further), I continued with the project and tried a reverse polarity protection circuit. That didn't work very well, but it was fun to see something, that is not supposed to light up, light up (@28.5V). Everything continued to work after the short!


The next day I soldered the banana plugs that came in to the output wires and all of a sudden the device wouldn't power more than around 2.8V. The fault led is lighting up and the chip is getting very hot.

 

Did I blew up the chip by testing out the lighting effects on components?

 

The strange thing is that, the other board isn't working at all. Same problem as the first one.

I didn't test the shortcut capabilities of the second board though.

 

I have ordered two new LTC3780's and a few of the LM358N's to make sure that they are all good.

 

 

I am doing anything wrong?

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  • 0

    According to the datasheet of the LTC3780 (G package).

    Here are the voltages

     

    Pin:

    1: 8.89V

    2: 5.30V

    3: 0.18V (at lowest and highest voltage output setting)

    4: 0.18V

    5: 0.32V at lowest output and ~ 2 V at highest output

    6: 0.80 V at lowest and 0.08V at highest output

    7: 0 V

    8: ~ 9.2 V

    9: 0 V

    10: 1.53 V

    11: 0 V

    12: 1 V

    13: 6.42 V at lowest and 7.84 V at highest

    14: 1.12 V (lowest) ~ 3.45V (highest)

    15: 0.80 V (lowest) ~ 2.80V (highest)

    16: 5.84 V (lowest) ~ 1.18V (highest)

    17: 0V

    18: 0.06 V (lowest) ~0.12V (highest)

    19: 5.95 V

    20: 0 V

    21: 12 V (lowest = input voltage) ~ 3.80 V (highest)

    22: 0.77 V (lowest) ~ 2.00V (highest)

    23: 5.60 V (lowest) ~ 0.90 V (highest)

    24: 5.63 V (lowest) ~ 7.30 V (highest)

  • 0 in reply to tim687

    I don't quite understand what lines like this mean:

     

    21: 12 V (lowest = input voltage) ~ 3.80 V (highest)

     

    this is the power input - it should be constant

     

    The voltage on pin 8 looks like a serious fault - but it has that ~ again and I don't know what it means.

     

    What is connected to pin 8 - it should be a steady 0V or 5V  - I really need a full schematic to help properly.

     

    From the history it looks as if something has blown but I can't tell what.

     

    It is very ambitious to try to debug a switching power supply when you don't have access to a scope - can you borrow one ?

     

    MK

  • 0 in reply to michaelkellett

    21: 12 V (lowest = input voltage) ~ 3.80 V (highest)

     

    At the lowest output voltage selected, that line is 12 V.

    At the highest output voltage selected, it is around 3.80 V

     

    I do have access to a scope, but then I don't have access to my own gear.

     

    The chip get's very hot when I put the output voltage at max.

  • 0 in reply to tim687

    Well, if Vin is collapsing it suggests a huge current - what is the power supply ?

    The fact that the current is affected when the demand for output volts is altered suggests that the 3780 is trying to work.

    It could be that the 3780 is faulty or it could be one of the MOSFETs (or both).

    You could remove the 3780 and test the MOSFETs in place.

     

    MK

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