UC3843 - Fixed Frequency Current-Mode PWM Controller

Overview

The UC3843 is part of the UC3842/UC3843/UC3844/UC3845 family of fixed-frequency current-mode PWM controllers. These integrated circuits are specifically optimized for off-line and DC-to-DC converter applications, offering a robust solution for power supply designs. The UC3843 provides precise control over switching power supplies while maintaining high efficiency and reliability.

This controller features a trimmed oscillator for precise duty cycle control, a temperature-compensated reference, and a high-gain error amplifier. The current-mode architecture provides inherent cycle-by-cycle current limiting, improved load transient response, and easier loop compensation compared to voltage-mode controllers.

Key Features

• Low start-up current (<1mA typical)
• Automatic feed-forward compensation
• PWM latch for single pulse per period operation
• Internal trimmed bandgap reference
• Undervoltage lockout (UVLO) with hysteresis
• Maximum duty cycle clamp
• High current totem pole output (1A peak)
• Operating frequency up to 500kHz
• Error amplifier with wide bandwidth
• Current sensing with leading-edge blanking

Applications

The UC3843 is designed for a wide range of power conversion applications, including:

• Off-line switching power supplies
• DC-to-DC converters
• Battery chargers
• Power modules
• LED drivers
• Industrial power systems
• Telecom power supplies
• Distributed power systems

The device's current-mode control architecture makes it particularly suitable for applications requiring fast transient response and inherent pulse-by-pulse current limiting.

Electrical Characteristics

Absolute Maximum Ratings

• Supply Voltage (V_CC): 30V
• Output Current (Source or Sink): ±1A (peak)
• Error Amplifier Output Current: 10mA
• Power Dissipation: 1W
• Operating Junction Temperature: -40°C to +150°C
• Storage Temperature: -65°C to +150°C

Recommended Operating Conditions

• Supply Voltage (V_CC): 12V to 25V
• Oscillator Frequency Range: 10kHz to 500kHz
• Timing Capacitor Range: 0.001µF to 0.1µF
• Timing Resistor Range: 1kΩ to 100kΩ
• Operating Temperature Range: 0°C to +70°C (Commercial), -40°C to +85°C (Industrial)

Electrical Specifications (Typical at 25°C, V_CC = 15V)

• Start-up Current: 0.5mA
• Operating Current: 11mA
• Reference Voltage: 5V ±1%
• Reference Load Regulation: 0.2% (0mA to 20mA)
• Reference Line Regulation: 0.01%/V (V_CC = 12V to 25V)
• Error Amp Input Bias Current: -0.2µA
• Error Amp Voltage Gain: 90dB
• Error Amp Unity Gain Bandwidth: 1MHz
• Current Sense Input Threshold: 1V
• Output Rise Time: 50ns (1nF load)
• Output Fall Time: 50ns (1nF load)

Functional Description

UVLO and Start-up

The UC3843 features an undervoltage lockout circuit with 6V turn-on and 4.9V turn-off thresholds (UC3843 variant). This hysteresis prevents erratic operation during power-up and power-down sequences. The low start-up current allows for simple bootstrap start-up circuits.

Oscillator

The oscillator frequency is programmed by an external resistor (R_T) and capacitor (C_T). The capacitor charges through R_T and discharges through an internal current source, generating a sawtooth waveform. The discharge current is temperature-compensated to maintain consistent frequency over temperature.

Current Sense and PWM Operation

The current sense input (pin 3) provides cycle-by-cycle current limiting. A leading-edge blanking circuit prevents false triggering during MOSFET turn-on. The PWM comparator uses both the current sense signal and error amplifier output to determine the pulse width.

Error Amplifier

The high-gain error amplifier features a accessible output (pin 1) and inverting input (pin 2) for flexible compensation. The non-inverting input is internally connected to the 5V reference.

Package Information

The UC3843 is available in several package options:

• 8-Pin DIP (N): 0.300" wide plastic dual in-line package
• 8-Pin SOIC (D): 0.154" wide small outline integrated circuit
• 14-Pin SOIC (D): With additional NC pins for improved thermal performance

Pin Configuration (8-Pin Package)

• Pin 1 (COMP): Error amplifier output and compensation node
• Pin 2 (FB): Error amplifier inverting input and feedback input
• Pin 3 (ISENSE): Current sense input for PWM control
• Pin 4 (RT/CT): Oscillator timing components connection
• Pin 5 (GND): Ground reference
• Pin 6 (OUTPUT): PWM output driver
• Pin 7 (VCC): Supply voltage input
• Pin 8 (VREF): 5V reference output

Design Considerations

Layout Guidelines

Proper PCB layout is critical for optimal performance:

• Keep high current paths short and wide
• Place timing components close to the IC
• Use a ground plane for noise immunity
• Separate power and signal grounds
• Minimize loop areas in high di/dt paths

Thermal Considerations

While the UC3843 has modest power dissipation, thermal management is important in high ambient temperature environments:

• Use copper area for heat dissipation
• Consider using the SOIC package for better thermal performance
• Ensure adequate airflow in enclosed designs

Conclusion

The UC3843 current-mode PWM controller provides a reliable, high-performance solution for a wide range of power conversion applications. Its combination of precise control features, robust protection circuits, and flexible design options make it an excellent choice for both offline and DC-DC converter designs. The device's current-mode architecture offers inherent advantages in loop stability and transient response