# Output Capacitor Voltage Holding time during dropout

I have a buck converter.

• Vin = 12V

• Vout = 5V

• Iload = 0.5A

I have one dropout requirement. The input voltage will drop from 12V to 4V for 10s. Including ramp up/down time of 10ms. During this time, my 5V output should be stable.

Q: How to calculate the value for my input/output reservoir capacitors value so that I have constant 5V during dropout time. Is there any formula?

• Did you know C=Ic*dt/dV But was the 10s ramp with a 0.5A load? – Tony Stewart Sunnyskyguy EE75 Jul 25 '19 at 6:16
• yes. Can you please provide me the calculation. I am not getting – Newbie Jul 25 '19 at 7:00
• if the Vin must remain between 12v and 8volt, with input load of 0.25 amps, for 10 seconds, you need about 1 farad. – analogsystemsrf Jul 25 '19 at 7:41
• what is the formula you used? and what capacitor? input or output? can someone explain it a little more briefly and simple language – Newbie Jul 25 '19 at 8:14
• You won’t get a constant 5 volts so, be reasonable and think how much droop you can tolerate. – Andy aka Jul 25 '19 at 8:55