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What sets the switching frequency of a pulse-width modulated (PWM) controller?

April 26, 2017controller Frequency modulated pulse-width PWM switching

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What sets the switching frequency of a pulse-width modulated (PWM) controller?

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Many PWM controller ICs operate at a fixed switching frequency, set by a single external resistor or capacitor. To set a desired oscillator frequency, use the equation in the controller datasheet to calculate the resistor value. For example, see the equation in LM5025 datasheet. Some PWM controller ICs operate with fixed on-time or fixed-off time, often controlled by an external resistor (see the LM5010 for example). The resulting switching frequency varies with input voltage. Other PWM controller ICs operate in a completely hysteretic mode. The switching frequency of a hysteretic converter depends on values of the output inductor and capacitor, as well as operating voltages and currents. An example is the LM3485 hysteretic controller.