UC3845 Current Mode PWM Controller IC DIP-8

UC3845 Current Mode PWM Controller IC DIP-8 is a foundational integrated circuit specifically optimized for implementing cost-effective, high-performance current-mode Pulse Width Modulator (PWM) control in switching power supplies. This controller IC offers critical features like an internal temperature-compensated reference, a high-gain error amplifier, and a current-sensing comparator, simplifying the design of power conversion circuits. The UC3845 variant is tailored for operation with duty cycles exceeding 50%, making it the ideal choice for demanding topologies like boost, flyback, and forward converters. Therefore, designers frequently select this IC when they need precise regulation, fast transient response, and built-in protection mechanisms in their power electronics designs.

Key Features:

• Current-Mode Control: Provides superior line regulation, simplified loop compensation, and excellent pulse-by-pulse current limiting capability compared to voltage-mode control.
• High Operating Frequency: Capable of operating up to 500 kHz, allowing the use of smaller external components and increasing power density.
• Optimized for >50% Duty Cycle: The UC3845 specific version is internally configured to support and stabilize power supply topologies requiring duty cycles above 50%.
• Low Start-up Current: Requires only ≤ 1 mA of current during initial start-up, minimizing the size and cost of the external starting components.
• Under-Voltage Lockout (UVLO): Incorporates UVLO with hysteresis to prevent erratic operation at low supply voltages, enhancing system reliability.

Technical Specifications:

• Maximum Duty Cycle: Limited to 100% (Optimized for D ≥ 50%)
• UVLO Turn-On/Turn-Off Thresholds: V_ON ≈ 16 V / V_OFF ≈ 10 V (Typical)
• Operating Frequency: User-programmable via external R_T and C_T up to 500 kHz.
• Output Peak Current (Source/Sink): ±1 A (Capable of driving power MOSFET gates directly).
• Reference Voltage (V_REF): 5.0 V (±2% precision, temperature compensated).
• Operating Supply Voltage Range (V_CC): 10 V to 30 V.

Mechanical Specifications:

• Package Type: DIP-8 (Dual In-line Package, 8-Pin Through-Hole)
• Mounting: Through-Hole Technology (THT)
• Lead Finish: Sn/Pb (Tin/Lead) or Pb-Free plating options.
• Thermal Resistance (Θ_JA): Typically 100 °C/W (Junction to Ambient).

Dimensions:

• Body Length: ≈ 9.9 mm
• Body Width: ≈ 6.4 mm
• Pin Pitch (Between pins): 2.54 mm (0.1 inch)
• Row Spacing (Between rows): 7.62 mm (0.3 inch)
• Standard Pin Thickness: ≈ 0.46 mm

Pinout and Wiring:

• Pin 1 (COMP): Compensation network connection (Output of the Error Amplifier).
• Pin 2 (VFB): Voltage Feedback input (Inverting input of the Error Amplifier). Connect to the output voltage divider.
• Pin 3 (ISENSE): Current Sense input. Connect to the low-side shunt resistor for primary current sensing.
• Pin 4 (RT/CT): Timing Resistor and Capacitor. Connect external R/C components to set the oscillator frequency.
• Pin 5 (GND): Signal and Power Ground connection.
• Pin 6 (OUT): High-current totem-pole output used to drive the gate of the external power MOSFET.
• Pin 7 (VCC): Input supply voltage for the IC.
• Pin 8 (VREF): 5.0 V precision reference output. Use this to supply the Error Amplifier and R/C timing network.

Datasheet Reference:

• Download Texas Instruments UC3845 Current Mode PWM Controller Datasheet

Commonly Used in:

• Off-Line Power Supplies: The IC provides the core control for isolated AC-DC converters, consequently, they are found in chargers and wall adapters.
• DC-DC Converters: Widely employed in non-isolated boost and buck-boost converters for industrial and automotive applications.
• LED Drivers: Controlling the power stage in high-power LED lighting systems that require constant current regulation.

Applications:

• Switching Power Adapters: Forming the heart of switch-mode power supplies (SMPS) for consumer electronics.
• Battery Chargers: Implementing precise charging profiles with robust current and voltage limits.
• Uninterruptible Power Supplies (UPS): Managing the charging and regulation stages within smaller UPS units.
• Inverters: Providing the PWM signal for DC-AC power inversion systems.

Equivalent Models:

• UC3842 / UC3843 / UC3844: These are variants within the same family, however, the UC3842 and UC3843 limit the duty cycle to 50%. Always check the specific duty cycle rating required for your topology.
• KA3845 / SG3845: Direct functional and pin-compatible clones manufactured by companies like Fairchild/ON Semi and Silergy.
• TL3845: A lower-cost version often used in less demanding commercial applications.

Package Includes:

• 1 x UC3845 Current Mode PWM Controller IC DIP-8