Run a astable 555 timer at 100kHz (it is very near its limit) and you will get the sawtooth waveform of the timing capacitor. You can use a opamp buffer/amplifier to prevent loading of this signal.
If you have to make your own, the basic building blocks are: Constant current source, flip-flop, 2 comparators, a capacitor, and resistor.
Charge the cap from the current source, detect the upper voltage trip with the comparator, flip the flip-flop and discharge the cap through a resistor while detecting the lower voltage trip. Flip the flip-flop and start the charge cycle again..
You have now effectively made your own 555!
At 100kHz you will need a fairly fast comparator like the LM311