You can use a comparator. For example
.1. On input V+ pin use your sawtooth signal
.2. On input V- pin use a fixed voltage as threshold
The output of the comparator will be HIGH if the V+ > V - and LOW is V- > V +. So you will get a square signal on the same frequency as the sawtooth signal. The duty cycle (how much % is HIGH / LOW) can be adjusted by increasing/decreasing the voltage on your V- pin.
Good luck.