UC3842 Fixed Frequency Current-Mode PWM Controller

Overview

The UC3842 (along with its variants UC3843, UC3844, and UC3845) is a fixed-frequency current-mode pulse-width modulation (PWM) controller IC designed specifically for off-line and DC-to-DC converter applications. This high-performance controller provides the necessary features to implement efficient power supply designs with minimal external components.

As a current-mode controller, the UC3842 offers improved line regulation, simplified loop compensation, and inherent pulse-by-pulse current limiting. The device operates at frequencies up to 500kHz, making it suitable for both low and high-frequency power conversion applications.

Key Features

• Low Start-up Current: Typically less than 1mA, enabling efficient system initialization
• Precision Duty Cycle Control: Provides accurate PWM output for stable power conversion
• Maximum Duty Cycle Clamp: Prevents excessive duty cycles that could damage power components
• Undervoltage Lockout (UVLO) with Hysteresis: Ensures reliable operation under varying input conditions
• High Current Totem Pole Output: Capable of directly driving power MOSFETs (up to 1A peak)
• Current-Mode Operation: Provides inherent cycle-by-cycle current limiting
• Adjustable Oscillator Frequency: Up to 500kHz operation
• Error Amplifier with Reference: Includes internal bandgap reference for precise regulation
• Latching PWM: Provides noise immunity and single-pulse metering

Applications

The UC3842 is widely used in various power conversion applications, including:

• Off-line switch-mode power supplies (SMPS)
• DC-to-DC converters
• Battery chargers
• Power factor correction circuits
• Flyback and forward converters
• Industrial power systems
• Telecommunications power supplies
• Computer peripheral power systems

Electrical Characteristics

Absolute Maximum Ratings

• Supply Voltage (Vcc): 30V
• Output Current (Source/Sink): ±1A (peak)
• Error Amp Input Voltage Range: -0.3V to Vcc
• Power Dissipation: 1W
• Operating Junction Temperature: -40°C to +150°C
• Storage Temperature Range: -65°C to +150°C

Recommended Operating Conditions

• Supply Voltage (Vcc): 12V to 25V
• Oscillator Frequency Range: Up to 500kHz
• Operating Temperature Range: 0°C to +70°C (Commercial), -40°C to +85°C (Industrial)
• Output Duty Cycle: 0% to 100% (UC3842/UC3843), 0% to 50% (UC3844/UC3845)

Key Performance Characteristics

• Start-up Current: <1mA (typical)
• Operating Current: 11mA (typical)
• Reference Voltage Accuracy: ±1% at 25°C
• Oscillator Frequency Tolerance: ±10%
• Error Amp Voltage Gain: 90dB (typical)
• Error Amp Unity Gain Bandwidth: 1MHz (typical)
• Output Rise Time: 50ns (typical) with 1nF load
• Output Fall Time: 50ns (typical) with 1nF load

Package Information

The UC3842 is available in several industry-standard packages:

8-Pin DIP (Dual In-line Package)

• Package Code: D
• Dimensions: 9.81mm x 6.35mm body
• Lead Pitch: 2.54mm
• Lead Count: 8
• JEDEC MS-001 standard

8-Pin SOIC (Small Outline Integrated Circuit)

• Package Code: D (SOIC-8)
• Dimensions: 4.90mm x 3.90mm body
• Lead Pitch: 1.27mm
• Lead Count: 8
• JEDEC MS-012 standard

14-Pin SOIC (Wide Body)

• Package Code: DW (SOIC-14)
• Dimensions: 8.65mm x 3.90mm body
• Lead Pitch: 1.27mm
• Lead Count: 14
• JEDEC MS-013 standard

Functional Description

Internal Block Diagram Overview

The UC3842 integrates several key functional blocks:

• Reference Voltage: Provides a stable 5V reference for internal and external use
• Oscillator: Sets the switching frequency via external timing components
• Error Amplifier: Compares feedback voltage with reference to generate error signal
• Current Sense Comparator: Provides cycle-by-cycle current limiting
• PWM Latch: Generates the PWM output signal based on error amp and current sense inputs
• Output Stage: High-current totem pole output capable of driving power MOSFETs
• Undervoltage Lockout (UVLO): Ensures proper operation only when supply voltage is adequate

Pin Configuration and Functions

• Pin 1 (COMP): Error amplifier output and compensation node
• Pin 2 (FB): Feedback input to error amplifier
• Pin 3 (ISENSE): Current sense input for cycle-by-cycle current limiting
• Pin 4 (RT/CT): Timing components connection for oscillator frequency setting
• Pin 5 (GND): Ground reference for the IC
• Pin 6 (OUTPUT): PWM output driver
• Pin 7 (VCC): Supply voltage input
• Pin 8 (VREF): 5V reference voltage output

Design Considerations

Start-up and UVLO Characteristics

The UC3842 features an undervoltage lockout circuit with hysteresis. The device begins operation when VCC reaches the start threshold (typically 16V) and continues operating until VCC falls below the stop threshold (typically 10V). This hysteresis prevents erratic operation during power-up and brownout conditions.

Current Sensing Implementation

Proper implementation of the current sense circuit is critical for reliable operation. The current sense input (Pin 3) should be connected to a current sense resistor or current transformer with appropriate filtering. The typical current limit threshold is 1V, and the input should be protected from noise spikes.

Frequency Setting

The oscillator frequency is set by external components connected to Pin 4 (RT/CT). The timing capacitor charges through the timing resistor, and discharges through an internal current source. Frequency can be calculated using the approximate formula: f ≈ 1.72/(RT × CT).

Conclusion

The UC3842 current-mode PWM controller provides a robust, feature-rich solution for a wide range of power conversion applications. Its combination of precise