SN74LVC1G123: 180 second timer (astable multivibrator)

The 74xx123 works by allowing the capacitor to charge through the resistor, and ending the pulse when the capacitor's voltage has reached a fixed threshold. (See the application note for details.)

The '123 itself has no upper limit on the component values. However, the accuracy of the charging time is influenced by

• the capacitance of the traces and the '123 itself (very small, so does not matter for large R/C values);
• the leakage current of the Rext/Cext pin (< 250 nA, so should not matter);
• the tolerances of the resistor and the capacitor (it's your choice how to balance this with cost);
• the leakage current of the capacitor (can be quite bad for electrolytic capacitors).

The leakage currents have a higher influence when the charging current is small, i.e., when the resistor has a large value. (If the leakage current happens to be as large as the charging current, the monostable will never time out.)

So in your application, you should probably choose a film capacitor. (Ceramic capacitors might be better at high temperatures, but are probably not available in sizes large enough for you.)

The SN74LVC1G123 includes a power-up reset circuit that prevents triggers during the power ramp, so long as the supply ramps up relatively quickly. It is recommended to keep the supply ramp faster than 200 µs/V to prevent issues.