While keeping the oscillator frequency relatively stable, this 555 based PWM controller features almost 0% to 100% pulse width regulation using the 100k variable resistor. To give a frequency range from about 170Hz to 200Hz , the frequency is dependent on the 100k pot and 100n.
PWM CONTROLLER circuit schematic

You can see the charging and discharging of the 100n cap is done through output pin 3, and this provide a push-pull symmetric drive for easy pulse-width setting. You can see two versions, the left side for grounded load, and the right side for ungrounded load. The grounded one use pin 7 to drive the transistor, while the ungrounded one use the same push-pull output pin3, this difference is needed because wen need an inverted phase to provide consistent potentiometer scale on both version. [Source: talkingelectronics.com]