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CD40109B: Wiring of CD40109

Part Number: CD40109B

Hi guys!

I have 2 questions concerning the level shifter CD40109!

Do I have to connect all unused pins to GND?

Are there any recommendations for wiring of in/outputs like parallel capacitors, series resistors or something like that?

Thanks a lot in advance!

Matthias

  • Hello Matthais,
    All unused _inputs_ need to be connected to either GND or VCC. Outputs can be left floating (ie disconnected).

    I'm not sure what you mean for I/O recommendations. Can you tell me about your application?
  • Hello Emrys,

    okay, thanks!

    I mean resistors like R73, R69 and R34 and capacitors like C45 and maybe capacitors from the supply inputs to ground.

  • Thanks for the schematic, that helps me a lot!

    The input resistor (10k) shouldn't cause any problems. The inputs are high impedance, and typically the only trouble with adding a large resistor will be with creating slow edges on the inputs from the RC circuit created (R73 + Cin of pin 3). 10k shouldn't cause any problems though.

    I would recommend against using series resistors on the supply pins - that can cause supply dips that might cause the device to behave erratically. That's not a requirement though - if it works in your system, it won't hurt the part.

    We recommend keeping output capacitance below 100pF just to prevent excessive current from the device. R34 shouldn't cause any ill effects .

    Is the 'alarm' connection going to a coil of some sort? I think your circuit is safe to do that, just curious.
  • Thanks for the answers!

    What do you mean by resistors on the supply pins? R69 on pin 2, which is the "enable"-pin?

    It can go out to some sort of coil but usually it is just a data-line for HIGH or LOW information signal. Why curious?

  • My apologies - I should have looked at the datasheet before commenting. I've just seen people connect series resistors with a supply pin before to limit current, and it can be problematic. The pin location threw me off :)

    With the enable pin, the leakage current (up to 1 uA) will probably generate some voltage drop on that 1M resistor though, you might consider reducing it to a 100k.

    As for the coil, I was wondering because sometimes people use these devices to drive coils and the sharp output current transient (ie going from on to off in ~10ns) will result in huge voltages across coils (V_L = L * di/dt) that aren't properly protected and can result in blown out IC's.