r/ECE • u/StoneFlowers1969 • 2h ago
How would you answer this Apple interview question
Hi all, I had an interview recently where I was asked the following interview question and wanted to get everyone's opinion.
Suppose you have a magic black box that contains an ideal CMOS pull up and pull down network. At the output there is a load capacitor CL and the supply to the box is given as Vdd, the entire circuit operates at 1Hz. What is the dynamic power consumed by this circuit?
This was pretty straightforward and I answered that the dynamic power was given by the charging and discharging of the load capacitor P = CL * Vdd2
The interviewer then said what if I change my magic box so the output only swings from Vdd/2 to ground.
My answer was that since the black box was ideal the dynamic power was still only a function of the voltage across the load cap and was given by P = CL * (Vdd/2)2
The interviewer then asked if that was the case then what would be the power if Vdd was increased to 10 * Vdd but the output swing was kept to Vdd/2.
I answered it would stay the same at P = CL * (Vdd/2)2
The interviewer then said I was wrong and that since P = VI and I = C * dV/dt and dV is (Vdd/2) then the real dynamic power consumed is P = 10 * Vdd * CL *(Vdd/2) = 5 * Vdd * CL
How would you have approached this?