Texas Instruments UC3855AN

Overview

UC3855AN from Texas Instruments is a high-frequency average current-mode PFC controller IC designed for active power factor correction in boost preregulators. It integrates ZVT (Zero Voltage Transition) drive, inductor current synthesizer, analog multiplier with line compensation, 7.5V precision reference, and dual gate drivers (GTOUT/ZVTOUT). It operates up to 500kHz, supports universal AC input (85–265V_RMS), and targets >95% efficiency in 300–2000W industrial/commercial SMPS.

For engineers reviewing the UC3855AN datasheet, UC3855AN pinout, UC3855AN application, or UC3855AN equivalent, this page delivers verified technical context on ZVT timing control, current synthesizer operation, multiplier gain stability over temperature, UVLO thresholds (UC3855A: 15.5V turn-on), and package-specific thermal limits for the 20-pin PDIP (N) variant.

Technical Context

The UC3855AN implements fixed-frequency average current mode control using an internal current amplifier (80–110 dB open-loop gain, 2.5–5 MHz GBW) and voltage amplifier (65–80 dB gain, soft-start integrated) to regulate boost output voltage while shaping sinusoidal line current. Its single-quadrant analog multiplier accepts VRMS (0–5.5V) and VAOUT inputs to generate a line-compensated current command, enabling stable operation across universal input ranges without slope compensation.

ZVT functionality is implemented via ZVS sensing (2.3–2.9V threshold), ZVTOUT driver (±0.25A continuous, ±0.75A peak), and programmable pulse width (400 ns max), reducing MOSFET turn-on loss and diode reverse recovery EMI. The current synthesizer reconstructs inductor current using ION buffer (10 V/µs slew rate), CI capacitor discharge, and RVS-derived current, enabling accurate average current sensing with one current transformer.

Key Specifications

Pinout & Package

UC3855AN is supplied in a 20-pin plastic dual in-line package (PDIP-N), with lead finish CU NIPDAU and RoHS-compliant green construction. Thermal resistance θ_JA is 60°C/W (typical, JEDEC std PCB); no MSL rating applies due to through-hole assembly.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar PFC controller applications.

Compared with UC3855ADW and UCC3855AN, the UC3855AN provides identical core PFC control functionality in a through-hole PDIP package optimized for manual assembly, thermal testing, and field serviceability-without trade-offs in ZVT timing fidelity, current synthesizer accuracy, or line compensation linearity.

Availability

UC3855AN is available at Aetrix Electronics and suitable for industrial motor drives, medical imaging power systems, LED streetlight drivers, and server PSUs requiring stable component supply with full traceability and long-lifecycle support.

Supply support for UC3855AN includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a global semiconductor leader specializing in analog, embedded processing, and power management technologies, with decades of leadership in power conversion IC design.

The UC3855A/B family was developed specifically for high-efficiency, high-power-factor AC-DC conversion in commercial and industrial switch-mode power supplies, emphasizing ZVT-enabled EMI reduction and universal-input robustness.

FAQ

What is the maximum operating frequency supported by UC3855AN?

The UC3855AN supports PWM switching frequencies up to 500 kHz, enabled by its optimized oscillator architecture and ZVT circuitry. This is achieved using an external CT capacitor (minimum 200 pF) and requires careful PCB layout to minimize parasitic inductance. The oscillator's initial accuracy is 160–240 kHz at 25°C, with total variation across line and temperature remaining within specification to sustain stable 500 kHz operation in well-designed boost stages.

Does UC3855AN require slope compensation in average current mode?

No, UC3855AN does not require slope compensation in average current mode operation. Its control architecture inherently stabilizes the current loop across all duty cycles and line conditions, eliminating the need for external ramp injection. This simplifies design, improves noise immunity, and avoids potential interaction between slope compensation and the current synthesizer's reconstructed waveform.

How does the current synthesizer in UC3855AN work with a single current transformer?

The UC3855AN's current synthesizer uses the ION pin to buffer and level-shift the current transformer secondary signal onto the CI capacitor during switch ON time, then discharges CI at a rate proportional to dI/dt (via RVS-derived current) during OFF time. This reconstructs the full inductor current waveform at the CS pin, enabling accurate average current sensing without shunt losses or multiple transformers.

What are the UVLO thresholds for UC3855AN, and how do they differ from UC3855BN?

The UC3855AN (A-version) has a VCC turn-on threshold of 15.5 V nominal with 6 V hysteresis (turn-off at 9–10 V), whereas the UC3855BN (B-version) turns on at 10.5 V with only 500 mV hysteresis. This allows UC3855AN to delay startup until bulk capacitors reach higher voltage, reducing inrush stress, while UC3855BN enables earlier startup in low-input or battery-backed systems.

Can UC3855AN be used with both resistive shunt and current transformer current sensing?

Yes, UC3855AN supports both methods: a simple resistive shunt can connect directly to the CS pin for basic applications, while the current synthesizer path (ION → CI → CS) is optimized for transformer-based sensing. The synthesizer method delivers superior SNR and zero conduction loss, making it preferred for >500 W designs; the shunt option retains full functionality but incurs I²R loss and thermal derating considerations.