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Fairchild's FAN7688 advanced secondary side LLC resonant converter controller with synchronous rectifier control

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. It 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 a better 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.

Features        
  • Secondary side PFM controller for LLC resonant converter with synchronous rectifier control
  • Charge current control for better transient response and easy feedback loop design
  • Adaptive synchronous rectification control with dual edge tracking
  • Closed-loop soft-start
  • Wide operating frequency (39 kHz to 690 kHz)
  • Green functions to improve light-load efficiency
    • Symmetric PWM control at light-load to limit the switching frequency while reducing switching losses
    • Disabling SR at light-load condition
  • Wide operating temperature range: -40°C to 125°C
  • Protection functions with auto-restart
    • Over-current protection (OCP)
    • Output-short protection (OSP)
    • Non-zero-voltage switching prevention (NZS) by compensation cutback (frequency shift)
    • Power limit by compensation cutback (frequency shift)
    • Over-load protection (OLP) with programmable shutdown delay time
    • Over-temperature protection (OTP)
  • Programmable dead times for primary side switches and secondary side synchronous rectifiers
  • VDD under-voltage lockout (UVLO)
  • SOP-16L package
Applications          
  • Desktop ATX, desktop-derived server, blade server, and telecom power supplies
  • Intelligent 100 W to 2 kW+ off-line power supplies
  • Industrial power
  • High-efficiency isolated DC-DC converters
  • Large screen display power

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