The UC3845B is a high-performance, fixed-frequency, current-mode PWM (Pulse Width Modulation) controller designed specifically for off-line and DC-to-DC converter applications. It provides the necessary features to implement control of switch-mode power supplies (SMPS) with minimal external components. The "B" series offers enhanced performance with lower start-up current and tighter oscillator discharge current specifications.

Specifications

• Operating Voltage (Vcc): Up to 30V (Self-limiting by a 34V Zener).
• Output Current:  ± 1.0A (Peak).
• Oscillator Frequency: Up to 500kHz.
• Duty Cycle Range: 0% to 50% (Fixed internal toggle flip-flop).
• Start-up Threshold: 8.4V (Typical).
• Operating Threshold (after start-up): 7.6V (Typical).
• Start-up Current: 0.3mA (Typical).
• Package: SOIC-8 (Surface Mount).

Features

• Current-Mode Control: Provides inherent pulse-by-pulse current limiting and excellent load transient response.
• Trimmed Oscillator: Ensures precise frequency control and tight discharge current for dead-time adjustment.
• Under-Voltage Lockout (UVLO): Optimized for off-line applications, ensuring the IC only operates when sufficient voltage is available to drive the external MOSFET gate.
• High-Current Totem-Pole Output: Designed to directly drive the gate of power MOSFETs with high peak currents for fast switching.
• Double Pulse Suppression: Internal logic prevents the output from pulsing more than once per oscillator cycle, protecting the power stage.
• Error Amplifier: Features a high-gain error amplifier with a trimmed 2.5V internal reference.

Common Applications

• Off-line Flyback Converters: Commonly found in AC-DC adapters and PC power supplies.
• DC-DC Buck/Boost Converters: Used in industrial and automotive power systems.
• Forward Converters: Ideal for medium-power isolated supplies where duty cycle is limited to < 50>
• Battery Chargers: Precise current and voltage control for various battery chemistries.

Usage Tips

• Duty Cycle Limit: The UC3845B is hard-coded for a maximum 50% duty cycle. If your design requires up to 100% duty cycle (e.g., certain buck converters), use the UC3843 or UC3842 instead.
• Decoupling: Place a high-quality ceramic capacitor (0.1uF to 1.0uF) as close as possible between Pin 7 (Vcc) and Pin 5 (GND) to handle high-current switching noise.
• Current Sensing: To avoid false triggering due to noise spikes, add a small RC filter (e.g., 1k and 470pF) to the ISENSE pin.
• Heat Dissipation: Although the IC itself consumes little power, driving large MOSFETs at high frequencies can cause the package to warm up. Ensure adequate PCB traces for the GND pin to act as a heat sink.

Datasheet