How to calculate the number of (CMOS inverter) buffer stages to drive capacitive load? In my case, I am driving a 1nF capacitive load with CMOS (inverter) buffer with the digital signal having an amplitude of 0-5V, and 1Mhz frequency.
rise time is 5ns.
Thanks.
You want it to change about 5V (10% to 90% I guess but ignore that for now) in 5ns, which is \$10^9\$ v/s, and the capacitor is \$10^{-9}\$F so you'll need an average current of 1A.
That's quite a bit of current and will require a significant driver such as a MOSFET gate driver (and be sure to read the fine print on how much current they actually pass, not just the datasheet blurb about '2A' or whatever- the current changes with the output voltage).
The charge stored in a capacitance C at voltage V is CV = 1n x 5 = 5 nC
Another measure of charge is It, current I flowing for time t.
If we want to shift 5 nC charge in 5 ns, we need a current of 1 amp.
One amp is not really CMOS buffer territory, you need a FET Gate driver.
