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LMK1C1104: Unused output state

Part Number: LMK1C1104
Other Parts Discussed in Thread: LMK1C1103, LMK1C1102


I have one question regarding the LMK1C1104. 

The datasheet specifies that the unused outputs can be left floating, but are there any drawbacks to it compared to having a pulldown resistor on unused outputs (e.g current consumption, noise emission)? 



  • Tsugumi,

    With any CMOS driver, the optimal choice for unused outputs is leaving them floating. Any additional load will have some resistive or capacitive component, which will increase power consumption (resistive by DC power in the load during high duty cycle; capacitive by C * Fout * V^2 dissipating in the driver) and transients on the supply rails. At higher frequencies (10s or 100s of MHz), even small parasitic components can lead to large increases in power dissipation. As for noise and emissions, the trace length of a floating pin is too short to act as a good antenna for radiated noise at LVCMOS frequencies, and conducted emissions are as low as possible since there is less load to draw current from Vcc and therefore less supply ripple at the fundamental output frequency and its harmonics.

    Note that we also offer the LMK1C1102 and LMK1C1103, which are identical functionality to LMK1C1104 but with fewer outputs. Since the lower output count devices use the same die but do not bond out some pins, there is slightly less parasitic capacitance from the leadframe and bondwires, and an even shorter antenna for any unused outputs on the die. Power consumption and emissions for LMK1C1102/3 with all outputs used will be very slightly better than LMK1C1104 with unused outputs.


    Derek Payne

  • Hello Derek, 

    That does make sense, thank you so much !