onsemi UC3845N

UC3845N onsemi Current Mode Controller Summary of Main Features

The onsemi UC3845N is a high-performance, fixed-frequency current mode controller specifically engineered for use in off-line and DC-to-DC converter applications. Supplied in a standard 8-pin dual-in-line plastic package (8-PDIP), it is well-suited for designers seeking a robust and cost-effective solution to implement boost, flyback, or step-up/step-down isolated and non-isolated power topologies. The UC3845N integrates the essential building blocks for reliable output voltage regulation, cycle-by-cycle current limiting, and precise control of power MOSFET drivers with minimal need for external components.

Fundamental Structure and Working Mechanism of the UC3845N

At the heart of the UC3845N is a current mode PWM architecture operating at fixed frequency, controlled by external timing components. The device incorporates an internal oscillator, high-gain error amplifier, current sense comparator, and a powerful totem pole output stage. The oscillator’s frequency is defined by external resistor (RT) and capacitor (CT) values, with the ability to adjust output deadtime between 50% and 70%. The current mode approach ensures fast transient response and inherent cycle-by-cycle current limiting, crucial for robust protection and reliable system operation. The integrated PWM latch ensures only one output pulse per oscillator cycle, greatly enhancing stability, especially in demanding converter topologies.

Primary Characteristics and Performance Benefits of the UC3845N

The UC3845N stands out due to its comprehensive set of features designed to streamline power supply design:

Current mode control with output switching frequency up to 500 kHz for fast transient response.

Adjustable output deadtime (50%-70%) enables fine-tuning of converter efficiency and noise performance.

Automatic feed forward compensation simplifies loop compensation in varying VIN conditions.

Latching PWM architecture provides secure cycle-by-cycle current limiting and single pulse metering for enhanced fault tolerance.

Precision internal reference (5V bandgap) delivers consistent reference voltage with tight tolerance.

High-current totem-pole output stage (±1.0A peak) allows direct driving of MOSFETs, reducing external buffering components.

On-chip undervoltage lockout (UVLO) with hysteresis for both VCC and reference voltages, improving system robustness during start-up and abnormal conditions.

Low startup and operating current for high efficiency in energy-sensitive applications.

Direct compatibility with onsemi SENSEFET™ family for further streamlining high-performance switching regulator designs.

Typical Use Cases and Application Areas for the UC3845N

The UC3845N is optimized for a wide spectrum of power electronic conversions requiring tight control and robust protection. Typical engineering applications include:

Off-Line flyback and boost AC-DC converters in power supplies, adapters, and chargers.

Step-up and step-down isolated or non-isolated DC-DC converters for industrial, consumer, or communication equipment.

Systems requiring precise cycle-by-cycle current limiting and overload protection, such as battery charging circuits and distributed power architectures.

Frequency-locked converters synchronized to system clocks for noise-sensitive or EMI-regulated environments.

UC3845N Technical Details and Available Packages

Engineers benefiting from the UC3845N’s capabilities should note its key electrical and mechanical properties:

Input voltage (VCC) thresholds: UVLO turn-on at 8.5V and turn-off at 7.6V, ideal for low voltage DC-DC applications and off-line designs with efficient bootstrap startup.

Output drive capacity: ±1A peak current, suitable for direct MOSFET gate drive with fast edge rates (typ. 50 ns rise/fall).

Internal 5V reference trimmed to within ±2% at 25°C for UC384x family.

Oscillator frequency programmable up to 500 kHz; timing set via RT and CT.

Wide operating temperature: 0°C to +70°C for commercial grade.

Package options: 8-PDIP (UC3845N), with analogous devices available in 8-SOIC and 14-SOIC packages for layout flexibility.

Standard Circuit Implementation and Design Guidelines for the UC3845N

Designing with the UC3845N involves selection and tuning of external components for oscillator timing (RT, CT), compensation for the error amplifier, sensing resistor placement for peak current control, and appropriate PCB layout to mitigate high-frequency noise. The device includes features to simplify soft-start (via external circuits), output deadtime control, MOSFET gate damping, and current spike suppression (using RC filtering at the current sense input). For high reliability, implement separate ground returns for control and power paths and maintain bypass/wiring practices common in high-frequency switching regulators. Reference applications such as a 27W off-line flyback regulator and charge pump converter circuits exemplify the UC3845N’s flexibility across converter designs.

Alternative or Substitute Models Comparable to the UC3845N

When sourcing or qualifying alternatives for the UC3845N, product selection engineers can consider several equivalent models from the same family and related series:

onsemi UC3844 and UC3845 devices, which offer similar functionality with minor variations in UVLO thresholds and operating characteristics.

onsemi UC2844 and UC2845, which are designed for extended temperature range and tighter reference specification, suiting applications requiring higher precision or full industrial qualification.

Designers should cross-reference key parameters such as UVLO levels, reference voltage tolerance, oscillator frequency range, and output drive capability to ensure compatibility and optimal performance in the target application.

Conclusion

The onsemi UC3845N current mode controller remains a highly versatile and industry-proven solution for engineers developing flyback, boost, and other switched-mode power supplies. Its robust integrated feature set, flexibility in application, and straightforward external interface make it ideal for both new designs and legacy system maintenance. By understanding the architectural advantages, application fit, and available alternatives, engineers and procurement professionals can make informed decisions, ensuring reliable and efficient power management solutions using the UC3845N platform.