I've been working on a small project inspired by this question that uses a CMOS 555 timer as a power button controller (short press to turn on, 3s hold to turn off), the goal being to design a micropower circuit that can operate across a wide supply range. To do this, I use a CMOS 555 as a latching window comparator, and have separate RC networks connected to the THRES and TRIG inputs, as shown in the schematic.

While researching different CMOS 555 timer options, I found the datasheet for the TLC555 has the following disclaimer about the quiescent current:

These values apply for the expected operating configurations in which THRES is connected directly to DISCH or to TRIG.

Does this imply that the TLC555 will draw an excessive amount of quiescent current if THRES is not connected directly to DISCH or TRIG? If so, why would this be the case? Note that in the "idle" state TRIG will be close to the supply voltage and THRES will be close to ground.

CMOS 555 latching window comparator circuit

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    \$\begingroup\$ 555 is not micropower, not even close. it's mostly that internal voltage divider wasting energy. \$\endgroup\$ – Jasen Sep 17 at 7:54

If we look at the TLC555's internal block diagram:

enter image description here

We can see that the THRES input goes into a comparator which controls the Reset input of the flip-flop. So depending on how we connect THRES:

  • THRES = VDD: the flip-flop will continuously reset
  • THRES = GND: the flip-flop will never reset
  • THRES = the voltage across the timing capacitor: reset when the capacitor is charged to 2/3 * VDD

The most useful scenario is obviously the 3rd and that's also a typical usage case for this chip.

Never leave the THRES input floating as that indeed can lead to unexpected results and high quiescent power consumption! This is a CMOS chip and the THRES input is a very high impedance CMOS input so it must be connected to some well defined voltage.

In your application circuit, THRES is well defined by your RC circuit + MOSFET so in my opinion there should be no issue at all.

  • \$\begingroup\$ But aren't TRIG and THRES analig inputs, similar to what you would see in a CMOS op-amp or comparator. \$\endgroup\$ – Caleb Reister Sep 17 at 7:40
  • \$\begingroup\$ Yes they are, but they're "converted to digital" by the comparators. \$\endgroup\$ – Bimpelrekkie Sep 17 at 7:41

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