• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 64 subscribers
  • Views 950 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM25085 Not Working

simone righetti
Prodigy 30 points
Other Parts Discussed in Thread: LM25085

Hi,
yesterday I’ve designed and mounted a LM25085 buck converter projected with an input of 10 to 15V and 6V @ 7A output. Once plugged in, the output was quite the same of the input (about 500mV if drop at 3-4A). I’ve measured the sensor’s pin voltage and i get 2.5V instead of 1.25V. The command of the gate is allways on. What’s wrong with the cirucit? Thanks in advice.
p.s. I must admit that i’m not very good in pcb route so this might be the problem. 

  • 0
    TI__Genius 13700 points
    Hello Simone,

    There are a few things that I would like to cover that will should fix your issue and help improve your circuit.

    -Diode: Use both of the diodes in the package by connecting pin 1 to GND. These diodes should be well matched since they are on the same die and they will share current well. The benefit of this is that as the current through the diode increases so does the forward voltage drop which negatively impacts for efficiency. By paralleling these diodes the part will operate better thermally by spread the heat better through the die and it will improve your efficiency. You might also need a higher current diode much like you need a higher current MOSFET. Since there is no part number that is not something I can comment on.

    -Output Capacitor: According to your schematic the output capacitor is placed in reverse polarity which can be dangerous. An electrolytic capacitor that is given reverse voltage can explode so be careful when you build this and I would recommend fixing the schematic by flipping that capacitor over.

    -MOSFET: If you want to supply 7A on the output you are going to need a bigger MOSFET. I would recommend selecting a part that can support a bit more than your anticipated output current. Try finding a part that can support at least 14A continuous.

    Now the actual problem.
    Your P-Channel MOSFET is designed in backwards. The body-diode should be blocking voltage from flowing from the input to the output. The voltage drop you see if the drop across this diode and the reason it is not working is because the P-FET can not switch using the the PGATE signal due to how the MOSFET is oriented. Flip Pin 3 and Pin 4 and the converter should work.

    Let me know how these adjustments work and if there are any questions.
  • 0 in reply to Ryan May
    Prodigy 30 points
    Hello Ryan May,

    Thanks you very much for the answer. For the diode i didn't know that trick, but i'm very glad to learn it. For output capacitors' polarity it was only a drawing error, when i mounted them I have checked them twice to be sure. The main problem of the circuit was that i have chosen the wrong code from the library so as you said i have to flip those pins (i bought a mosfet that can support -20A continuosly but in the schematic the code was wrong).

    Again, thank you for those tip and tricks. And i'm sorry to make you lost your time by answering me, i thought to have deleted this question.
    Now i'm re-routing all with the right mosfet, and then i will try it.

    Simone RIghetti.
  • 0
    TI__Genius 11490 points

    Is this still open?

  • 0 in reply to Vijay86929
    Prodigy 30 points
    I think I closed it, didn't I?
  • 0 in reply to simone righetti
    TI__Genius 13700 points
    Hello Simone,

    Thank you for your kind words. I am happy to help.