The UCC3961 advanced primary-side startup controller is a unique solution that provides all the primary-side functions required for a single-ended, isolated, switch-mode power converter incorporating secondary-side PWM control. It is usable with a wide range of secondary control circuits and is especially well suited for systems where sophisticated handling of overload conditions is required.

Secondary-side control assumes that output voltage and current measurements are interfaced directly to an output ground-referenced PWM stage that develops the power switch command for the supply. This digital PWM command can then be transmitted to the primary-side power switch through a simple and low-cost isolating pulse transformer. With secondary-side control, it is much easier to monitor and control the system load with tightly coupled analog control loops. Load-oriented features such as output current sharing and synchronous rectification are implemented more easily.

The UCC3961 provides all the circuitry required on the primary side of a secondary-side controlled power supply. It features a free running 60-kHz to 360-kHz oscillator which is synchronizable to the secondary-side PWM signal and also has the ability to accept start/stop PWM commands from the isolating pulse-edge transformer (PET). The use of an extremely small and low-cost pulse transformer allows for higher converter bandwidth. This also eliminates the loop-gain variations due to initial accuracy and aging of an opto-coupler feedback element or the size penalty of a gate transformer. It also includes an undervoltage lockout circuit with 2-V hysteresis, a low-current startup with active low during UVLO, a soft-start capability, a 5-V reference and a high-current power output.

Advanced features of UCC3961 allows implementation of initial startup with optional high-voltage disconnect after starting, and input voltage monitoring with turnoff for either undervoltage or overvoltage conditions. Other features include power-switch current protection, pulse-by-pulse current limiting, shutdown after a programmable delay and continuous input volt*second clamp. The UCC3961 also provides a multi purpose, bidirectional shutdown pin (SD), that can be used in conjunction with the devices on the START pin, to modify the converters behavior in overload conditions. The possible configurations include continuous-peak current limiting, delayed or immediate shutdown with full cycle soft restart or fully latched overcurrent shutdown.

In a non-typical use, the UCC3961 can accommodate an analog feedback signal through an opto-isolator where the UCC3961 then can operate in voltage-mode control mode with primary-side peak current limiting.

The UCC3961 and the UCC2961 are available in the 14-pin SOIC (D) and PDIP (P) packages. For applications where only startup and control are desired, a simplified version of this product is offered in an 8-pin package as the UCC2960 and UCC3960.

• Operates with Secondary-Side PWM Control
• Isolated PWM Command through a Pulse Transformer
• Initial Free-Running Soft-Startup with Duty-Cycle Clamping
• Up to 400-kHz Synchronizable Switching Frequency
• High-Current FET Drive (1.5-A Sink, 0.75-A Source)
• Multi-Mode Overcurrent Protection with Restart, Latching, or Cycle-by-Cycle Current Limiting
• Programmable Volt-Second Clamp for Transformer Reset
• Undervoltage Lockout with 2-V Hysteresis
• Low-Current Startup with Optional Disconnect
• Programmable Overvoltage and Undervoltage Protection
• 8-pin Version Also Available (UCC3960)