Texas Instruments UC3845ADG4

Overview

UC3845ADG4 from Texas Instruments is a current-mode PWM controller IC designed for fixed-frequency DC-DC and off-line switch-mode power supplies. It delivers 1-A totem-pole gate drive output, features pulse-by-pulse current limiting, operates up to 500 kHz, and includes an internally trimmed 5-V bandgap reference - enabling stable regulation in flyback, forward, and boost converters powering industrial PSUs and battery-operated systems.

For engineers reviewing the UC3845ADG4 datasheet, UC3845ADG4 pinout, UC3845ADG4 application, or UC3845ADG4 equivalent, key selection criteria include its 8.4-V UVLO turn-on threshold, <0.5 mA start-up current, 50% maximum duty cycle, oscillator discharge current trimmed to 8.3 mA, and SOIC-8 package compatibility with standard PCB layouts for cost-sensitive power supply designs.

Technical Context

The UC3845ADG4 implements peak-current-mode control using a high-gain (80 dB) error amplifier, a precision oscillator with programmable RT/CT timing, and a current-sense comparator clamping input at 1 V. Its internal logic enforces 50% maximum duty cycle via dedicated latch architecture - distinguishing it from UC3842A/UC3843A variants.

During undervoltage lockout (UVLO), the device draws only 0.3–0.5 mA and actively sinks ≥10 mA at ≤1.2 V on the OUTPUT pin to prevent parasitic MOSFET turn-on. The oscillator's trimmed 8.3-mA discharge current ensures accurate duty-cycle programming across temperature and process variation.

Key Specifications

Pinout & Package

UC3845ADG4 is housed in an 8-pin SOIC (D) package measuring 4.90 mm × 6.00 mm, with gull-wing leads and standard JEDEC MS-012AC footprint. Thermal resistance R_θJA is 117.4°C/W, supporting operation up to +70°C ambient without forced airflow.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar current-mode PWM controller applications.

Compared with UC3845ADG4, UC3845BDG4 offers tighter reference and oscillator specs for higher-precision regulation, while NCP1207ADR2G trades programmability for integrated protections and lower EMI - making UC3845ADG4 optimal for cost-sensitive, design-flexible DC-DC and off-line flyback implementations requiring adjustable frequency and duty cycle.

Availability

UC3845ADG4 is available at Aetrix Electronics and suitable for industrial power supplies, battery-operated PSUs, and off-line flyback converters requiring stable component supply, long-term lifecycle support, and consistent SOIC-8 packaging across production batches.

Supply support for UC3845ADG4 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a global semiconductor leader specializing in analog, embedded processing, and power management technologies, with decades of expertise in power conversion ICs.

The UC384x family was engineered specifically for cost-effective, high-reliability current-mode switch-mode power supplies - targeting industrial, telecom, and consumer power applications where predictable startup, tight duty-cycle control, and robust fault handling are essential.

FAQ

What is the UVLO threshold for UC3845ADG4?

The UC3845ADG4 features an undervoltage lockout with a turn-on threshold of 8.4 V and turn-off threshold of 7.6 V, providing 0.8-V hysteresis. This narrow window is optimized for regulated DC-DC converter inputs, ensuring stable startup and shutdown behavior without oscillation during brownout conditions. The specification is guaranteed across the full operating temperature range (0°C to 70°C).

Does UC3845ADG4 support 100% duty cycle operation?

No, UC3845ADG4 limits maximum duty cycle to 50% by internal logic - a key differentiator from UC3842A/UC3843A variants. This hard limit prevents subharmonic oscillation in continuous-conduction-mode flyback topologies and is enforced regardless of RT/CT values. Designers requiring >50% duty cycle must select UC3842A or UC3843A instead.

What is the purpose of the VREF pin on UC3845ADG4?

The VREF pin on UC3845ADG4 provides a stable 5.0-V ±100-mV output used to bias external circuitry such as optocoupler LEDs or voltage-divider networks. It is short-circuit protected (≈100 mA) and rated for ≤10 mA continuous load. A 1-µF ceramic capacitor must be placed directly at the pin to suppress high-frequency noise and ensure regulator stability.

Can UC3845ADG4 drive a MOSFET directly without a gate driver?

Yes, UC3845ADG4 integrates a ±1-A totem-pole output stage capable of directly driving medium-power N-channel MOSFETs in ≤100-W converters. Its low saturation voltage (<0.4 V at 20 mA sink) and fast rise/fall times (25–150 ns) eliminate the need for external buffers in most applications - though a series gate resistor (e.g., 10–22 Ω) is recommended to damp ringing and limit peak current.

How does UC3845ADG4 implement pulse-by-pulse current limiting?

UC3845ADG4 compares the ISENSE voltage (from a shunt resistor) against an internal 1-V clamp threshold on every switching cycle. When the sensed voltage reaches 1 V, the PWM latch resets immediately, terminating that pulse. This cycle-by-cycle action provides instantaneous overcurrent protection without system shutdown, allowing continued operation under transient overload - a core feature of peak-current-mode control.