Electronics > Repair
LTC3850 Step Down Regulator
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2X:
Hi,
I have a PCB from a custom inverter and the problem is that the step-down regulator (LTC3850) doesn’t enable the Vout1 of 5V and only enable the Vout2 of 3.3V. At pins “RUN1” & “RUN2” I have 4.7V. I believe that the IC is faulty but I found a connection to the Ground at pin 25 (FREQ/PLLFLTR) and I would like your opinion if this is a short or a normal connection (if it was a short, I believe that I wouldn’t have the 3.3V). I checked the mosfets and the components around the IC and I didn’t find any short, except from the pin 25.
Because the PCB is over two layers (maybe I am missing something) I found that at the pin 25 is connected a ceramic SMD capacitor 1.5nF (measured value) with a parallel 0ohm resistor in parallel (picture 4). If I remove the resistor the connection to the ground is gone.

Datasheet LTC3850:
https://www.analog.com/media/en/technical-documentation/data-sheets/ltc3850-3850-1.pdf
fzabkar:
Are you sure there isn't a second capacitor between the zero-ohm resistor and ground? Otherwise it doesn't make sense to populate a capacitor if it is to be bypassed by a shorting link. :-?
2X:

--- Quote from: fzabkar on July 26, 2024, 06:56:58 pm ---Are you sure there isn't a second capacitor between the zero-ohm resistor and ground? Otherwise it doesn't make sense to populate a capacitor if it is to be bypassed by a shorting link. :-?

--- End quote ---

The same thought I have too.. why to short a capacitor with a resistor? Any thoughts are welcomed.
Finally, I replaced the chip LTC3850 and the PCB works ok.

P.S. Thank you very much for yoru reply.
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