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LM5165: RT resistor value

Trevor White
Intellectual 270 points
Part Number: LM5165

Hi, I have found that in a production batch the resistor for RT(pin 5) to ground has been left off. I am trying to decide if I need to sort this out or if I can get away with leaving that particular batch of boards working without the resistor. I tested a board by removing the resistor and the circuit appeared to perform still but I am not completely sure of the possible problems I might face with RT open circuit.

Can anyone enlighten me on the potential problems I might face? The circuit is supplying a processor with 3V3 from a 48V input. Current is approx. 50mA output.

Thanks

Trev

  • 0
    TI__Mastermind 39790 points
    The RT resistor sets the Switching frequency in COT operation. The frequency determines the peak to peak ripple current in the inductor. By leaving the RT pin open, you will set the frequency to it lowest frequency - See equation 2 in the datasheet. You will not only have an error in your intended set frequency (When RT is installed) but you will also get a lot of frequency variation from part to part and environmental changes. I strongly advise installing RT resistor to Gnd.