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LTC366a Spread Spectrum Clock input and Frequency Setting


LTC3633a has provision for feeding in an external clock (MODE/SYNC pin), and also a pin to set a fixed frequency clock with a set resistor (RT pin). I would like to feed in a spread spectrum clock to the MODE/SYNC pin to try to spread noise out over a greater frequency range and I would like to know what should be done with the RT pin in this case as I cannot tell from the datasheet (attached)

Please advise in this regard, or if I have missed something in the datasheet please let me know. 


  • Hi,

    RT resistor should still be in the circuit.  What is the purpose to add the spread spectrum here? Is it because of the EMI? Please contact our local FAE, and we would like to know further about your real application and business. Thanks.

  • Hi Josh, 

    The purpose to add the spread spectrum is to reduce the EMI radiated at the specific switching frequency of the part, and to instead spread that energy over a larger spectrum so that its content at any given frequency (as measured by an appropriate antenna) is reduced. 

    Unfortunately it seems I can't attach a file in this reply, but I tried an LTSpice sim with a spread spectrum clock input fed from your LT6908-1 chip into the Mode/Sync pin of the LTC366a and the output of the part (measured at the switch node) did not seem to be spread at all, but rather stuck at the fixed frequency as set by the resistor on the RT pin. Is this expected behavior? How would the part know to override the set frequency set by the RT resistor with the clock given at the Mode/Sync pin?

  • You might send the file to me at

    Need to check if both the LT6908 and LTC3633 are set properly.