Texas Instruments UC2845AQD8

Overview

UC2845AQD8 from Texas Instruments is a current-mode PWM controller IC designed for off-line and DC-DC power converters, featuring an 8-pin SOIC package, UVLO turn-on threshold of 8.4 V (typ), maximum duty cycle of 48% (typ), and operation across –40°C to 125°C. It delivers high-current totem-pole output (±1 A), pulse-by-pulse current limiting, and internal 5-V bandgap reference with ±10 mV total variation over line/load/temperature.

For engineers reviewing the UC2845AQD8 datasheet, UC2845AQD8 pinout, UC2845AQD8 application, or UC2845AQD8 equivalent, this page provides verified technical context, real-world design meaning of key specs, validated pin functions, and two confirmed alternative parts for <50% duty-cycle fixed-frequency current-mode SMPS designs requiring industrial temperature range support.

Technical Context

The UC2845AQD8 implements a current-mode control architecture with a dedicated ISENSE input for cycle-by-cycle peak current limiting, an error amplifier (COMP) with 90 dB open-loop gain and 1 MHz unity-gain bandwidth, and an oscillator driven by external RT/CT components enabling frequency tuning from ~47 kHz to >500 kHz. Its UVLO hysteresis (8.4 V on / 7.6 V off) ensures reliable start-up under brownout conditions.

Unlike UC2842A/UC2843A variants, UC2845AQD8 integrates a toggle flip-flop in its PWM logic to enforce ≤50% maximum duty cycle-critical for forward and push-pull topologies-and features trimmed oscillator discharge current (8.3 mA typ) and low start-up current (<0.5 mA), reducing auxiliary supply burden in high-efficiency offline adapters.

Key Specifications

Pinout & Package

UC2845AQD8 uses an 8-pin SOIC (D) package measuring 4.90 mm × 6.00 mm with 1.27 mm lead pitch and 1.75 mm max height. Pin 1 is marked by a beveled corner or dot; device orientation follows JEDEC MS-012AB standard.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UC2845AQD8, UC2845AQDR offers identical control functionality in a larger 14-pin package for improved grounding, while UC3845BNG lacks the hardware-enforced 50% duty limit and industrial temperature rating-making it unsuitable for forward converters operating in harsh environments.

Availability

UC2845AQD8 is available at Aetrix Electronics and suitable for industrial off-line PSUs, isolated DC-DC forward converters, high-efficiency battery chargers, and programmable lab power supplies requiring stable component supply across extended temperature ranges.

Supply support for UC2845AQD8 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 IC design.

The UC284x family-including UC2845AQD8-is engineered specifically for cost-sensitive, high-reliability current-mode switch-mode power supplies targeting industrial, telecom, and consumer power adapter applications.

FAQ

What is the maximum switching frequency supported by UC2845AQD8?

The UC2845AQD8 oscillator operates up to 500 kHz under specified conditions. Typical frequency is set via external RT and CT components, with TI recommending RT between 5 kΩ and 100 kΩ and CT between 470 pF and 4.7 nF. At RT = 10 kΩ and CT = 3.3 nF, the UC2845AQD8 achieves approximately 52 kHz; higher frequencies require smaller CT values but must maintain stable oscillation and adequate dead time for the 48% max duty cycle constraint.

Does UC2845AQD8 support slope compensation for duty cycles above 50%?

No-UC2845AQD8 does not support slope compensation because its internal logic enforces a hardware-limited maximum duty cycle of 48% (typ). This fixed limit eliminates the need for external ramp injection and is intentional for forward and push-pull topologies where duty cycle >50% risks transformer saturation. For applications requiring >50% duty, use UC2842AQD8 or UC2843AQD8 instead.

What is the purpose of the VREF pin on UC2845AQD8?

The VREF pin on UC2845AQD8 provides a precision 5.0 V ±10 mV output derived from an internally trimmed bandgap reference. It supplies bias current to the RT/CT network, powers optocoupler LEDs in isolated feedback paths, and can serve as a stable reference for external monitoring or control circuitry-delivering up to 20 mA while maintaining low noise (<50 µV RMS).

Can UC2845AQD8 replace UC2844AQD8 in an existing design?

UC2845AQD8 and UC2844AQD8 share identical pinout, package, and oscillator characteristics, but differ critically in UVLO thresholds (UC2845AQD8: 8.4 V on / 7.6 V off; UC2844AQD8: 16 V on / 10 V off) and max duty cycle (both limited to ≤50%). Swapping requires verifying that the lower UVLO allows reliable start-up at the target input voltage and that the system does not rely on UC2844AQD8's higher turn-on threshold for brownout immunity.

How is current limiting implemented in UC2845AQD8?

UC2845AQD8 implements pulse-by-pulse current limiting via its ISENSE pin: when the sensed voltage exceeds 1 V (internally clamped), the PWM comparator immediately terminates the current switching cycle. This cycle-by-cycle action protects the main power switch during overload or short-circuit events. The 1-V clamp threshold is fixed and temperature-compensated, ensuring consistent trip points from –40°C to 125°C without external calibration.