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There are a lot of ways to make a sawtooth generator. This is the simplest form. Usually a better version will have some control feedback to keep things constant.
The how? There are two ways to do it. Using a passive resistor to charge the cap leads to an exponential rise, while using constant current source leads to a more linear rise. The fall is very sudden, in a spike manner, discharging thr cap almost immediately. You can switch through the resistor and CCS to view the rise changing from linear to exponential. The rise is exponential because as the voltage on the capacitor rises the potential through the resistor decreases, and the current at which the capacitor is charged also falls (according to Ohm's law). Theoretically the voltage on the capacitor will never completely reach the Vcc, it will always remain slightly (even aV) below the maximum level, no matter how much time is given. With the constant current source the charging current always remains the same, thus the charging cycle is kept constant/linear.
Now to the why. These circuits are often used as a sampling clock generator in PWM circuits and in old TV's as the horizontal sweep generator. Other applications include precise timing due to the almost instantaneous switching.
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