I need to design a circuit that outputs a 5V PWM with a frequency of 50Hz and a duty cycle between (5% and 10%). If you are familiar with R/C servos this's the typical signal to drive servo position. This circuit should output the PWM with 5% duty cycle until an event (another 5V TTL signal) change state from low to high. Once the event have been caught the PWM signal should change to 10% duty cycle. The accuracy in duty cycle should be better than 0,1%. It's not mandatory that the circuit reset to initials conditions after the input signal goes back to low (but it's welcome). My first approach was to use a small micro but unluckly this's not the case since I cannot use programmable devices in this application. I was thinking to use NE555 or NE556 IC but as far as I remember duty cycles below 50% and very low frequencies as those I need to manage are not very stable.
The LTC6992 is a pulse width modulation chip: -
The above picture shows it operating at 1 MHz. Here are some more top-level details that can be found in the data sheet: -
Minimum Duty Cycle at 0% or 5%
Maximum Duty Cycle at 95% or 100%
Frequency Range: 3.81Hz to 1MHz
2.25V to 5.5V Single Supply Operation
The only thing you have to do is set the input voltage you want that corresponds with 10% duty cycle.
Your only constraint was that you can't use a micro: -
My first approach was to use a small micro but unluckly this's not the case since I cannot use programmable devices in this application.
So this fits the bill given precisely what you have indicated. Note that goal post moving is not generally well-received.
You basically need to output either pulses of 1ms or 2ms in a 20ms grid. Your 0,1% accuracy calls for a crystal oscillator.
Use a 1MHz crystal, divide the clock by 1000 using a cascade of three CD4017 counters. You have a 1ms clock now. Use another CD4017 for the "1ms, zero" and "1ms, one" outputs you need and a single toggle flip-flop for discriminating the first 10ms from the second.
Feed these three inputs and your 1ms/2ms select into combinatorial logic. You are done.