I would like to know whether R24 has been correctly applied to pulldown the EN pin when none of the diodes are switched on.

The idea here is that voltages higher than 3.3V may be applied from any of 3 sources to bring the EN pin high. The zener forces these voltages down to the maximum pin tolerance (3.3V). When none of the diodes are switched on, the EN pin must be held low.

Proposed schematic

Alternatively I could put a high value pulldown in parallel with the zener.

Alternative schematic

Edit: A high level requires 1.7V and low less then 0.5V. Pin leakage current is 20nA. Input voltages range from 3.3V to 5V when active otherwise float.

  • \$\begingroup\$ Define inputs better. V+ max, and min ( open circuit or low Z 0V) and why two 10k's? \$\endgroup\$ – Sunnyskyguy EE75 Apr 22 at 12:47
  • \$\begingroup\$ Updated information. Series 10K chosen to limit the zener current to just above Iz to minimise energy loss. \$\endgroup\$ – Rob Apr 22 at 13:12
  • \$\begingroup\$ 1st cct is better OK for Voh margin, yet both may work.. 3.3V-Vf vs 3.3-Vf(47/57) for Vf= diode drop. but caution if external 5V is applied before Vdd on IC with EN as it will try to power up the card with that ( if not common) \$\endgroup\$ – Sunnyskyguy EE75 Apr 22 at 13:16

The first circuit is probably better, since the second creates a voltage divider. Warning: low voltage zeners have very soft "knees", i.e. they start conducting significantly well before the rated voltage. We will need to know the characteristics of the inputs and enable signal to know if this will work.

If the enable is a standard CMOS input, it will probably have a worst case threshold of 70% of Vcc, or, 2.3V.

The zener V-I curve that I quickly found shows the voltage drop to be 20% less at 1 mA. R17 may limit the current to a lot less than 1 mA, unless your circuit input has a high voltage. We need to know the characteristics of the input. At what input threshold do you expect the enable to be logic 1?

Edit: OK, if your input is 3.3 - 5V, then R17 is way too high. If you lower R17 it to 1k it may work. I can't find any zener I-V curves that show currents less than 1 mA, so knowing what happens when your input is near 3.3V is difficult to determine.

And I agree with sunnyskyguy, R24 isn't necessary.

  • \$\begingroup\$ Information updated. Thank you for warning about the knee - the inputs are either fully on between 3.3V and 5V or floating so I don't think early breakdown of the zener will be much of a problem. \$\endgroup\$ – Rob Apr 22 at 13:16
  • \$\begingroup\$ Just replace the Zener with a TL431. \$\endgroup\$ – CL. Apr 23 at 6:38

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