Shown on this slide are the benefits of the constant frequency control. It was mentioned previously that the basic hysteretic regulator had an uncontrolled frequency range where the frequency varies considerably. The goal in designing the controller is to maintain the simplicity of the hysteretic regulator but with a predictable operating frequency. The circuit shown starts with a basic hysteretic regulator, with an added fixed on-time block, hence the constant on-time term. The first expression at the top is an equation to calculate a buck regulator switching frequency. Next, the on-time term is substituted. Set the on-time inversely proportional to the input voltage, as shown in the middle expression, with the programming resistor, RON, in the numerator. Then substitute this TON equation for TON in the first equation, and solve for the frequency. It follows then that the frequency is proportional to the output voltage, which is a constant, as are the K and RON terms. Shown on the right of the slide is a plot of switching frequency versus input voltage. To generate this plot, the input voltage was swept, from 15 to about 75 V, and it can be seen that the frequency remains fairly constant.