UC3845

UC3845 Current-Mode PWM Controller Technical Summary

Overview

The UC3845 is a fixed-frequency current-mode pulse-width modulation (PWM) controller IC designed primarily for off-line and DC-to-DC converter applications. As part of the UC384x family (which includes UC3842, UC3843, and UC3844 variants), it provides precise control over power conversion circuits with its current-mode architecture that offers improved line regulation and simplified loop compensation compared to voltage-mode controllers.

This device is particularly optimized for systems requiring high-performance switching power supplies, featuring a trimmed oscillator for precise duty cycle control, a temperature-compensated reference, and high-current totem pole output ideal for driving power MOSFETs. The UC3845 variant specifically includes a maximum duty cycle clamp at 50%, making it suitable for forward converter topologies.

Key Features

• Low Start-up Current: Typically 0.5mA, enabling efficient system initialization
• Precision Current Sensing: Enhanced noise immunity for reliable operation
• Maximum Duty Cycle Clamp: Fixed at 50% for forward converter compatibility
• Under-Voltage Lockout (UVLO): With hysteresis for stable operation during power transitions
• High-Frequency Operation: Supports switching frequencies up to 500kHz
• Current-Mode Control: Provides inherent line feedforward, cycle-by-cycle current limiting, and simplified feedback loop compensation
• Trimmed Oscillator: ±2% initial accuracy for precise timing control
• High-Current Output: 1A peak output current for direct MOSFET driving
• Error Amplifier: Features wide bandwidth (typ. 1MHz) with access to compensation pin
• Latching PWM: For single-pulse metering protection

Applications

The UC3845 is widely used in various power conversion applications including:

• Off-line switching power supplies (AC/DC converters)
• DC-DC converters and power modules
• Forward converter topologies
• Battery chargers and power adapters
• Distributed power systems
• Industrial power supplies
• Telecommunications power systems
• LED drivers and lighting systems
• Computer peripheral power supplies

Electrical Characteristics

Package Information

The UC3845 is available in several industry-standard packages:

• 8-Pin DIP (PDIP): Plastic dual in-line package, suitable for through-hole mounting
• 8-Pin SOIC (SO): Small outline integrated circuit package for surface mount applications
• 14-Pin SOIC (SO-14): With additional NC (no connect) pins for improved thermal performance

Pin Configuration (8-Pin Package):

1. COMP: Error amplifier output and compensation node
2. FB: Feedback input for voltage regulation
3. ISENSE: Current sense input for cycle-by-cycle current limiting
4. RT/CT: Timing resistor/capacitor connection for oscillator frequency setting
5. GND: Ground reference for the IC
6. OUTPUT: Totem pole output for driving power MOSFET
7. VCC: Positive supply voltage input
8. VREF: 5V reference output for external circuitry

Functional Description

The UC3845 operates as a current-mode controller where the output pulse width is determined each cycle by both the error amplifier voltage (representing the output voltage error) and the instantaneous current sense signal (representing the switch current). This dual-loop control provides inherent line voltage feedforward and cycle-by-cycle current limiting.

The oscillator generates a sawtooth waveform that is compared with the error amplifier output to determine the pulse width. The current sense comparator provides cycle-by-cycle current limiting by terminating the output pulse when the current sense signal reaches the threshold set by the error amplifier output.

The under-voltage lockout circuit ensures the IC remains in a low-current state until the supply voltage reaches the start threshold (typically 8.