It's a reasonable approach if: 1) you have a good idea of how long the power will be out, and 2) if power is out too long, your system must reset itself and restart gracefully when power returns, and 3) repetitive resetting will not damage the system (sort of same as 2)
You can use with a large capacitor with the numbers you provided. Here's one way to work out the size of the capacitor.
There are two simple formulas you can use:
- Q = CV is the electrical charge stored for capacitance C with voltage V in unit of Coulombs
- Definition of Coulomb: 1 Coulomb = 1 Ampere * 1 second, or the amount of charge transferred if a current of 1 Amp flowed for 1 second.
So if your MCU circuit must be powered for 1/2 second at 80 mA, that is 0.08A * 0.5s = 0.04 Coulombs of charge transferred during that time.
For now let's assume your DC-DC converter has 100% efficiency, and let's say it can run down to 6V input and still maintain 3.3V output. So when the power cuts, the capacitor starts at 24V and discharges down to 6V before the DC-DC converter cuts out and we want enough capacitance at the input so that this takes 0.5 seconds.
Using Q=CV, we have 0.04 = C*(24 - 6). Solving for C, we get C = 2,222 microFarads.
So let's say our DC-DC converter is 85% efficient, so we bump this up: 2222/.85 = 2614 uF.
So you need a large capacitor but it is within reason.
One other important thing to consider is charging a 2,700 uF capacitor. There will be very high inrush current when the 24V comes back on. This current will be limited by the diode resistance, supply + wire impedance, and capacitor impedance. Read the datasheets for the diode, capacitor, and your power supply. Diodes and capacitors will have limits on surge current and how long the surge can last. Capacitors have a repetitive peak current spec. Your 24V supply may not handle the heavy load gracefully (e.g. if it has foldback current limiting it may restart several times).
So yes your approach is feasible but issues such as high inrush current, MCU's reset circuit, and ability of downstream circuits to handle power cycling will require great consideration.