7.3.2 Cycle-To-Cycle PFM

When the load does not vary over a wide range, ratio adaptive circuit techniques can be used to achieve cycle to cycle PFM regulation and lower ripple (or smaller output capacitors). The key to success is matching the duty cycle of the circuit closely to what is required by the input-to-output voltage ratio. This ratio then must be dynamically adjusted for input voltage changes (usually caused by batteries running down). The chosen ratio must allow most of the energy in each switching cycle to be delivered to the load and only a small amount to be stored. When the regulation limit is reached, the overshoot is small, and the system settles at an equilibrium point where it adjusts the off time in each switching cycle to meet the current requirements of the load. The off-time adjustment is done by exceeding the regulation limit during each switching cycle and waiting until the voltage drops below the limit again to start the next switching cycle. The current in the coil never goes to zero like it frequently does in the hysteretic operating mode of circuits with wide load variations or duty cycles that are not matched to the input/output voltage ratio. Optimizing the duty cycle for a given set of input/output voltages conditions can be done by referencing the circuit values in AN-1221 LM2623 General Purpose, Boost Converter Circuit.

.