Texas Instruments UC3842AD8

Overview

UC3842AD8 from Texas Instruments is a current-mode PWM controller IC designed for off-line and DC-DC switch-mode power supplies. It features 500 kHz maximum switching frequency, 16 V UVLO turn-on threshold, 10 V UVLO turn-off threshold, 8.3 mA trimmed oscillator discharge current, and 1 A totem-pole output drive capability - enabling direct MOSFET gate driving in flyback and forward converter topologies.

For engineers reviewing the UC3842AD8 datasheet, UC3842AD8 pinout, UC3842AD8 application, or UC3842AD8 equivalent, key selection considerations include its 8-pin SOIC (D) package, peak-current-mode control architecture, pulse-by-pulse current limiting, undervoltage lockout with hysteresis, and compatibility with TI's WEBENCH® Power Designer for custom SMPS design.

Technical Context

The UC3842AD8 implements fixed-frequency peak-current-mode control using an internal error amplifier (80 dB open-loop gain), a precision 5 V bandgap reference (±1% over temperature), and a programmable oscillator driven by external RT/CT components. Its current-sense comparator clamps input at 1 V and supports slope compensation for duty cycles >50%.

During undervoltage lockout, the device draws only 0.3–0.5 mA startup current and forces OUTPUT low while sinking up to 10 mA at ≤1.2 V - preventing parasitic MOSFET turn-on during power-up. The 1 A totem-pole output drives external power switches directly without buffer stages.

Key Specifications

Pinout & Package

UC3842AD8 is housed in an 8-pin SOIC (D) package measuring 4.90 mm × 6.00 mm, optimized for surface-mount assembly and thermal performance in power supply PCBs.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UC3842BD8, UC3842AD8 trades minor reference and oscillator tolerances for lower cost in industrial-grade power supplies; compared with TL494CN, it integrates current sensing and gate drive for simpler flyback/buck designs but lacks dual outputs and voltage-mode flexibility.

Availability

UC3842AD8 is available at Aetrix Electronics and suitable for industrial switch-mode power supplies, offline AC-DC adapters, and DC-DC buck converters requiring stable component supply, long-term lifecycle support, and traceable sourcing.

Supply support for UC3842AD8 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 specializing in analog and embedded processing technologies, with leadership in power management ICs and design tools.

The UC384x family was developed specifically for cost-sensitive, high-reliability current-mode switch-mode power supplies - targeting off-line AC-DC, DC-DC, and battery-powered converter applications where integration, efficiency, and robustness are essential.

FAQ

What is the maximum operating frequency of the UC3842AD8?

The UC3842AD8 supports a maximum oscillator frequency of 500 kHz, which translates to 500 kHz PWM switching in UC3842A/UC3843A variants. This high-frequency capability allows smaller magnetic components and faster transient response in compact power supply designs. The actual frequency is set by external RT and CT components per the oscillator equation, and the trimmed 8.3 mA discharge current ensures consistent timing across production lots. UC3842AD8 maintains full functionality up to this rated frequency under recommended operating conditions.

Does the UC3842AD8 require an external gate driver?

No, the UC3842AD8 does not require an external gate driver. Its integrated totem-pole output stage can source and sink up to 1 A of peak current, enabling direct drive of medium-power MOSFET gates commonly used in sub-100 W flyback, buck, and forward converters. A series gate resistor is still required to control dV/dt and suppress ringing, but no buffer or level-shifting stage is needed. This integration reduces BOM count and layout complexity compared to controllers requiring external drivers.

What is the purpose of the VREF pin on the UC3842AD8?

On the UC3842AD8, the VREF pin provides a precision 5.0 V ±1% (typical) bandgap reference output capable of supplying up to 10 mA. It serves three key roles: biasing the RT/CT timing network, powering external opto-isolator circuits in isolated supplies, and acting as a stable reference for feedback divider networks. A 1 µF high-frequency bypass capacitor is mandatory at this pin to ensure stability and low-noise operation. The VREF output is short-circuit protected at ~100 mA and remains active only when the device is out of undervoltage lockout.

How does the UC3842AD8 implement current limiting?

The UC3842AD8 implements hardware-based pulse-by-pulse current limiting via its ISENSE pin, which accepts a voltage signal from a current-sense resistor. The internal comparator clamps this input at 1 V - meaning any sensed voltage exceeding 1 V immediately terminates the current PWM pulse. This cycle-by-cycle action protects the power switch during overloads or short circuits without relying on software or external latches. The error amplifier output (COMP) modulates the current limit threshold indirectly, allowing regulated current control in constant-current applications like battery charging.

What is the UVLO behavior of the UC3842AD8 during power-up?

During power-up, the UC3842AD8 remains in undervoltage lockout (UVLO) until VCC reaches 16 V (turn-on threshold). In UVLO mode, the device draws only 0.3–0.5 mA, holds OUTPUT low, and disables all internal circuitry except the UVLO comparator. Once VCC crosses 16 V, the 5 V reference rises, the oscillator starts, and normal PWM operation begins. If VCC later drops below 10 V (turn-off threshold), the device re-enters UVLO, forcing OUTPUT low again to prevent erratic switching - ensuring clean, controlled start-up and brown-out recovery.