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CD4046B: Connection for pin 15, "Zener?"

Brian Goldberg
Prodigy 60 points
Part Number: CD4046B

I am working on a design with this PLL chip, and I don't know what I am expected to hook up Pin 15 to. It appears to be an internal zener diode and the datasheet says it's to assist in supply regulation.

What should I connect pin 15 to?

Thanks

 

  • 0
    TI__Guru* 96591 points

    Hey Brian, welcome to the forums!

    I honestly couldn't tell you why that Zener diode was included with these devices. I've never seen it actually used in a design.

    I'd recommend to just terminate that pin to ground or leave it floating -- either way, it won't have an impact on performance of the device.

  • 0 in reply to Emrys Maier
    Prodigy 60 points

    Right. I won't be connecting it, but I still don't quite understand another related element of the design.

    If the Zener Diode is tied to Vss, that suggests that Vss would have to be above ground potential to drive the zener, or below. How does the rest of the circuit operate if Vss is being pulled to to a potential that is > or < ground?

  • 0 in reply to Brian Goldberg
    TI__Guru* 96591 points

    Zener diodes are intended to be reverse biased - this is the appropriate circuit to make a shunt voltage reference with a Zener diode:

    The resistor has to be small enough to provide enough current to keep the Zener 'on' and drive whatever load is connected at "Vref"