aryajur,
I have one question abt your approach. if I'm not mistaken, boost-buck converter will convert to higher voltage. we design the boost-buck converter with a componenet specification to match certain criteria, for example to convert the voltage from +5V to +20V, but for a design where we alrady have 12V supply (stored in the capacitor), the boost-buck converter will up the output voltage higher and may probably damage other components.
so I'm quite confius here, because to design a boost-buck converter, we need to specify the Vin and intended Vout. My inteded Vout is +12V (this is the voltage that will be used for other blocks in the controller) and the input from car battery is +12V.