Input output difference > Output switch saturation voltage + current limiting resistor threshold, about 2 V total.
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Correction, according to a AN920D, ton/(ton+toff) can't exceed about 0.86, this would increase the minimal input voltage for 12 V output to about 16 V.
only a few people will have the complete datasheet information in mind, thus for such questions it´s always a good idea to give a link to the datasheet. https://www.onsemi.com/pdf/datasheet/mc34063a-d.pdf
Another solution is to use:
Page11 --> claculation --> Step-down --> t_on/t_off formula.
The lower the input voltage the higher t_on/t_off.
I guess the maximum duty cycle is 100%, thus you can go simple and just set the
term below the dividing line = 0
--> V_in(min) -Vsat -V_out = 0
solved to: V_in(min) = V_out + V_sat
(ignoring the voltage drop at the shunt)
Alternatives:
* Look for application notes or design notes with a V_in(min) calculation
* Maybe the manufacturer provides selection tools or calculators for the MC633063A. Play with them to find the V_in limit.
Input output difference > Output switch saturation voltage + current limiting resistor threshold, about 2 V total.
--- Updated ---
Correction, according to a AN920D, ton/(ton+toff) can't exceed about 0.86, this would increase the minimal input voltage for 12 V output to about 16 V.