Original question:
Please help explain some unexpected results.
I have 3570 ohm and 100pF in an astable LMC555CTPX oscillator. Datasheet Par. 9.6, Pg 16
My board and simulation operate at 1MHz, but the f=1/(1.4*Rt*Ct) results in the desired 2MHz.
What am I missing?
Thank you for your help.
Dave,
You are missing propagation delay because the simple formula doesn't include that. 2MHz is close to the maximum frequency which is just a typical value of 3 MHz which is almost entirely propagation delay. For 2 MHz try starting at 1k ohms and 370pF. May need to be lower capacitor value if using at 5V. Under 5V supply, might not get 2MHz at all.
Here is a more accurate astable formulais below. for 50% circuit RA + RB becomes RC + Ron(high) and RB + ron becomes RC + ron(low)
Unfortunately tPLH and tPHL are not data sheet specs. Also ron(high) > ron(low)
