The UC3842BL is a fixed frequency current-mode PWM controller manufactured by UTC (Unisonic Technologies Co., Ltd.). It is optimized for high-performance, low-cost, off-line and DC-to-DC converter applications, offering the designer a cost-effective solution with minimal external components.

Applications:

• Switch-mode power supplies (SMPS)
• DC-DC converters
• Offline power supplies
• Battery chargers
• Uninterruptible power supplies (UPS)

Features:

• Low startup current: Minimizes power consumption during startup.
• Automatic feed-forward compensation: Improves line regulation.
• Internally trimmed reference with hysteresis: Provides a stable and accurate reference voltage.
• Current limiting: Protects the circuit from overcurrent conditions.
• Under-voltage lockout (UVLO) with hysteresis: Prevents operation at low input voltages.
• High current totem pole output: Provides fast switching speeds.
• Adjustable duty cycle: Allows for flexible design configurations.
• Fixed frequency operation: Ensures consistent performance.

Benefits:

• Improved Efficiency: Current-mode control inherently provides better efficiency than voltage-mode control.
• Simplified Design: Reduced component count simplifies the design process.
• Enhanced Reliability: Built-in protection features increase the reliability of the power supply.
• Cost-Effective Solution: Provides a high-performance solution at a low cost.
• Stable Operation: Automatic feed-forward compensation and internally trimmed reference ensure stable operation.

Additional Details:

The UC3842BL operates at a fixed frequency, which is typically around 50 kHz to 500 kHz, depending on the external components selected. It uses current-mode control, where the PWM duty cycle is controlled by the peak inductor current. This control method provides inherent line feed-forward compensation and cycle-by-cycle current limiting. The device also includes an under-voltage lockout (UVLO) circuit that prevents operation at low input voltages, ensuring that the power supply operates within its safe operating area.

The totem pole output driver can source or sink a significant amount of current, allowing it to directly drive a power MOSFET. An external resistor and capacitor network sets the oscillator frequency. The UC3842BL is typically available in an 8-pin DIP or SOIC package.

The device's current sensing is achieved through a current sense resistor connected in series with the power MOSFET's source. The voltage across this resistor is monitored and used to control the PWM duty cycle. The reference voltage is typically 5V and is very stable due to internal trimming.