Texas Instruments UC3845AD8G4

Overview

UC3845AD8G4 from Texas Instruments is a current-mode PWM controller IC designed for off-line and DC-DC switch-mode power supplies. It features 8.3 mA trimmed oscillator discharge current, <0.5 mA start-up current, undervoltage lockout with 8.4 V turn-on/7.6 V turn-off hysteresis, 500 kHz max switching frequency, and a 1 A totem-pole output stage driving MOSFET gates directly in flyback and forward converters.

For engineers reviewing the UC3845AD8G4 datasheet, UC3845AD8G4 pinout, UC3845AD8G4 application, or UC3845AD8G4 equivalent, key selection criteria include its 50% maximum duty cycle limit, internal 5 V ±1% reference, pulse-by-pulse current limiting, and SOIC-8 package compatibility with board-level thermal management for industrial PSU and battery-operated converter designs.

Technical Context

The UC3845AD8G4 implements peak-current-mode control using an internal error amplifier (80 dB open-loop gain, 1 MHz unity-gain bandwidth) and a dedicated current-sense comparator clamping input at 1 V. Its oscillator uses external RT/CT components with factory-trimmed 8.3 mA discharge current to ensure accurate timing and stable 50% max duty cycle across temperature and supply voltage.

During undervoltage lockout (UVLO), the device draws only 0.3–0.5 mA and forces OUTPUT low while sinking ≥10 mA at ≤1.2 V - preventing parasitic FET turn-on during startup. The 1-A totem-pole output drives gate capacitance directly without external buffers, supporting fast rise/fall times (<150 ns with 1 nF load).

Key Specifications

Pinout & Package

UC3845AD8G4 is housed in an 8-pin SOIC (D) package measuring 4.90 mm × 6.00 mm, rated for 0°C to 70°C operation. Thermal resistance is R_θJA = 117.4°C/W.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UC3845D8, UC3845AD8G4 offers tighter oscillator discharge tolerance for improved duty cycle stability; compared with NCP1014P, it provides full external gate drive flexibility and programmable frequency but requires discrete MOSFET and layout attention for EMI control.

Availability

UC3845AD8G4 is available at Aetrix Electronics and suitable for industrial power supply, battery-operated PSU, and DC-DC converter module applications requiring stable component supply, long-term lifecycle support, and traceable sourcing.

Supply support for UC3845AD8G4 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 company delivering analog and embedded processing solutions for industrial, automotive, and personal electronics markets.

The UCx84xA product line was designed specifically for cost-sensitive, high-reliability current-mode switch-mode power supplies - emphasizing low start-up current, precise oscillator timing, and robust gate drive in standard packages.

FAQ

What is the maximum operating temperature range for UC3845AD8G4?

The UC3845AD8G4 is specified for operation from 0°C to 70°C ambient temperature. This commercial-grade rating aligns with its intended use in industrial power supplies and battery-operated equipment where enclosure temperatures remain within this range. Junction temperature must not exceed 150°C under any condition, and thermal design should account for R_θJA = 117.4°C/W in SOIC-8 packaging. Derating is required above 70°C ambient.

Does UC3845AD8G4 support voltage-mode control, or is it current-mode only?

The UC3845AD8G4 is fundamentally a current-mode PWM controller but can be configured for voltage-mode operation. By connecting the COMP pin directly to the FB pin, the internal error amplifier regulates duty cycle directly without current-sense feedback. However, pulse-by-pulse current limiting remains active via the ISENSE pin, preserving overcurrent protection. This hybrid configuration is documented in TI's application notes for specific DC-DC topologies.

What is the purpose of the RT/CT pin on UC3845AD8G4, and what component values are recommended?

The RT/CT pin on UC3845AD8G4 sets oscillator frequency via an external resistor (RT) to VREF and capacitor (CT) to GND. Texas Instruments recommends RT between 5 kΩ and 100 kΩ and CT between 470 pF and 4.7 nF. For example, RT = 10 kΩ and CT = 3.3 nF yields ~100 kHz operation. The 8.3 mA trimmed discharge current ensures predictable timing, making UC3845AD8G4 less sensitive to component tolerances than earlier UC384x variants.

How does the undervoltage lockout (UVLO) function differ between UC3845AD8G4 and UC3842AD8G4?

UC3845AD8G4 has an 8.4 V turn-on and 7.6 V turn-off UVLO threshold, providing narrow hysteresis optimized for regulated DC inputs like battery or intermediate bus voltages. In contrast, UC3842AD8G4 uses 14.5–17.5 V turn-on and 8.5–11.5 V turn-off, targeting wide-range offline AC/DC applications. This difference reflects distinct system-level power sequencing requirements - UC3845AD8G4 avoids false triggering during DC rail droop, while UC3842AD8G4 accommodates high-voltage startup transients.

Can UC3845AD8G4 drive a MOSFET directly without an external gate driver?

Yes, UC3845AD8G4 can drive a MOSFET directly without an external gate driver. Its totem-pole OUTPUT pin sources and sinks up to ±1 A, sufficient for gate capacitances up to ~2 nF at 500 kHz. Designers must include a series gate resistor (e.g., 10–22 Ω) to damp ringing and limit peak current. Layout best practices - short traces, solid ground plane, and local decoupling - are essential to maintain switching integrity and minimize EMI in UC3845AD8G4-based designs.