• Transition Mode PFC Controller for Low Implementation Cost
• Industry Pin Compatibility With Improved Feature Set
• Improved Transient Response With Slew-Rate Comparator
• Zero Power Detect to Prevent OVP During Light Load Conditions
• Accurate Internal V_REF for Tight Output Regulation
• Two UVLO Options
• Overvoltage Protection (OVP), Open-Feedback Protection and Enable Circuits
• ± 750-mA Peak Gate Drive Current
• Low Start-Up and Operating Currents
• Lead (Pb)-Free Packages
• APPLICATIONS
  • Switch-Mode Power Supplies for Desktops, Monitors, TVs and Set Top Boxes (STBs)
  • AC Adapter Front-End Power Supplies
  • Electronic Ballasts

The UCC38050 and UCC38051 are PFC controllers for low-to-medium power applications requiring compliance with IEC 1000-3-2 harmonic reduction standard. It is designed for controlling a boost preregulator operating in transition mode (also referred to as boundary conduction mode or critical conduction mode operation). It features a transconductance voltage amplifier for feedback error processing, a simple multiplier for generating a current command proportional to the input voltage, a current-sense (PWM) comparator, PWM logic and a totem-pole driver for driving an external FET.

In the transition mode operation, the PWM circuit is self-oscillating with the turn-on being governed by an inductor zero-current detector (ZCD pin) and the turn-off being governed by the current-sense comparator. Additionally, the controller provides features such as peak current limit, default timer, overvoltage protection (OVP) and enable.

The UCC38050 and UCC38051, while being pin compatible with other industry controllers providing similar functionality, offer many feature enhancements and tighter specifications, leading to an overall reduction in system implementation cost. The system performance is enhanced by incorporation of zero power detect function which allows the controller output to shut down at light load conditions without running into overvoltage. The device also features innovative slew rate enhancement circuits which improve the large signal transient performance of the voltage error amplifier. The low start-up and operating currents of the device results in low power consumption and ease of start-up. Highly accurate internal bandgap reference leads to tight regulation of output voltage in normal and OVP conditions, resulting in higher system reliability. The enable comparator ensures that the controller is off if the feedback sense path is broken or if the input voltage is very low.

There are two key parameteric differences between UCC38050 and UCC38051. The UVLO turn-on threshold of UCC38050 is 15.8 V while for UCC38051 it is 12.5 V. Secondly, the g_M amplifier’s source current for UCC38050 is typically 1.3 mA while for UCC38051 it is 300 µA. The higher UVLO turn-on threshold of the UCC38050 allows quicker and easier start-up with a smaller V_CC capacitance while the lower UVLO turn-on threshold of UCC38051 allows the operation of the PFC chip to be easily controlled by the downsteam PWM controller in two-stage power converters. The UCC38050 g_M amplifier also provides a full 1.3-mA typical source current for faster start-up and improved transient response when output is low either at start-up or during transient conditions. The UCC38051 scales this source current back down to 300- µA typical source current to gradually increase the error voltage preventing a step increase in line currents at start-up but still provides good transient response. The UCC38051 is suitable for multiple applications including AC adapters where a two-stage power conversion is needed. The UCC38050 is suitable for applications such as electronic ballasts where there is no down-stream PWM conversion and the advantages of smaller V_CC capacitor and improved transient response can be realized.

Devices are available in either the industrial temperature range of –40°C to 105°C (UCC2805x) or commercial temperature range of 0°C to 70°C (UCC3805x). Package offerings are 8-pin SOIC (D) or 8-pin PDIP (P) packages.