Fairchild’s FAN7688 is an advanced pulse frequency modulated (PFM) controller for LLC resonant converters with synchronous rectification (SR) that offers best-in-class efficiency for isolated DC-DC converters.

The device employs a current mode control technique based on a charge control, where the triangular waveform from the oscillator is combined with the integrated switch current information to determine the switching frequency. This provides superior control-to-output transfer function of the power stage simplifying the feedback loop design while allowing true input power limit capability. Closed-loop soft-start prevents saturation of the error amplifier and allows monotonic rising of the output voltage regardless of load condition. A dual edge tracking adaptive dead time control minimizes the body diode conduction time, thus maximizing efficiency.