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LM25085 dc-dc converter issue

Albin Gomes
Prodigy 60 points

I am trying to use the chip as a DC - DC conversion. Vin = 13 - 24 V, Vout = 12V (constant) 

There are a few issues I am facing:

1. Use of different DC sources gives me vary different outputs. One gives me 12V (desired) and the other 6V. I have checked the two power sources with other circuits and they work fine but when I try to use them for this specific circuit, it gives me the above mentioned values. 

2. The pfet that is part of the circuit shorts gate and source once I supply the source with some reference voltage. Initially the Gate would be, lets say at 9V input but after I connect the source to some supply, they short for some reason and they both end up with the same voltage. Tried the the same circuit with another pfet but the issue remains. 

Would appreciate any and all input that can be provided. 

Thanks. 

  • 0
    TI__Intellectual 2320 points

    Hello Albin,

    Can you please share your schematic when you get the chance. It would be easier to troubleshoot with the schematic.

    Regards,

    Sourav

  • 0 in reply to Sourav.Sen
    TI__Intellectual 2320 points
    Hello Albin,

    Thank you for sharing the schematic. First, I would start with adding an ESR resistor in series with the electrolytic output capacitor that you may have on board. The internal ESR resistor of the output capacitor may not be enough for a normal constant on-time operation. If you do not have enough ripple at the FB pin, the normal COT operation can be affected. Also take out the CFF from the board to ensure the part switches first (after adding the external ESR resistor) and then you can add that back later.

    Ideally during normal switching cycle, the PGATE should toggle between VIN (when the FET is OFF) to VIN-VCC (when the FET is ON). It seems in your case it seems like the part is not switching at all after you connect it to the power supply. Is that correct? Again, it is possible the part is not switching at all because there is not enough ripple at the FB pin. We need to ensure that first.

    What are the p-FET and the diode part number you are using in your board?

    Regards,
    Sourav