Analog Devices Inc. LT1681ESW#TRPBF

Overview

LT1681ESW#TRPBF from Analog Devices (formerly Linear Technology) is a high-voltage synchronous forward controller IC designed for dual-transistor isolated DC/DC converters. It implements fixed-frequency current-mode control up to 350kHz, enforces ≤50% duty cycle to prevent transformer saturation, drives external N-channel MOSFETs, and operates with input voltages up to 72V. It is used in telecom power supplies and battery backup systems requiring robust fault protection.

For engineers reviewing the LT1681ESW#TRPBF datasheet, LT1681ESW#TRPBF pinout, LT1681ESW#TRPBF application, or LT1681ESW#TRPBF equivalent, key selection considerations include its 20-lead SOIC-W package, 1.25V error amplifier reference, 150mV current sense threshold, 5VREF output (±1% over temperature), and integrated soft-start with 225mV reset threshold.

Technical Context

The LT1681ESW#TRPBF employs a fixed-frequency current-mode architecture where the internal oscillator runs at twice the switching frequency (e.g., 700kHz oscillator for 350kHz operation). Its T-flip-flop logic ensures ≤50% maximum duty cycle, preventing transformer core saturation during transient conditions.

Fault detection is centralized via the General Fault Condition (GFC) system, which responds to undervoltage lockout on VCC (8.4V rising threshold), overvoltage on OVLO/THERM (1.25V rising threshold), IMAX overcurrent (360mV), and 5VREF overload - all triggering soft-start capacitor discharge and graceful recovery.

Key Specifications

Pinout & Package

The LT1681ESW#TRPBF is housed in a 20-lead plastic wide SOIC (SW) package with 1.27mm pitch and ≥1.27mm creepage/clearance for high-voltage isolation compliance.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar synchronous forward controller applications.

Compared with UCC2891DR and MAX5072AESA+, the LT1681ESW#TRPBF uniquely combines 350kHz sync capability, integrated 5VREF, transformer saturation protection, and dual-driver timing control-making it the only option qualified for isolated synchronous forward converters demanding coordinated multi-controller phase alignment and robust fault recovery.

Availability

LT1681ESW#TRPBF is available at Aetrix Electronics and suitable for telecom power supplies, lead-acid battery backup systems, and industrial motor drive PSUs requiring stable component supply, long-term lifecycle support, and guaranteed parametric performance across –40°C to 85°C.

Supply support for LT1681ESW#TRPBF 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

Analog Devices, Inc. (acquired Linear Technology in 2017) is a global leader in high-performance analog, mixed-signal, and power management semiconductors.

The LT1681ESW#TRPBF belongs to Linear's legacy high-voltage controller product line, engineered specifically for isolated synchronous forward DC/DC converters in telecom, industrial, and transportation applications demanding reliability under wide-input-voltage and high-temperature conditions.

FAQ

What is the maximum operating frequency of the LT1681ESW#TRPBF?

The LT1681ESW#TRPBF supports a maximum synchronized operating frequency of 350kHz. Its internal oscillator runs at twice this rate (700kHz) to generate the 50% duty-cycle clock. Free-run oscillator frequency is programmable up to 700kHz, but practical converter design limits effective switching to 350kHz due to minimum on-time and layout constraints. The LT1681ESW#TRPBF achieves this via TTL-compatible SYNC pin with frequency doubling decoder logic.

Does the LT1681ESW#TRPBF include an integrated 5V reference, and what is its accuracy?

Yes, the LT1681ESW#TRPBF integrates a precision 5V reference (5VREF pin) with ±2% initial accuracy (4.85V to 5.15V) over the full –40°C to 85°C temperature range. It delivers up to 20mA continuous load current and features short-circuit protection limiting output to ~45mA. This reference powers optocouplers, feedback circuitry, and external bias networks - eliminating need for discrete references in isolated forward designs. The LT1681ESW#TRPBF specifies 1Ω output impedance for stable regulation under dynamic loading.

How does the LT1681ESW#TRPBF prevent transformer saturation in forward converters?

The LT1681ESW#TRPBF prevents transformer saturation through three coordinated mechanisms: (1) hardware-enforced ≤50% maximum duty cycle via internal T-flip-flop logic; (2) dedicated IMAX pin monitoring primary current with 360mV threshold and 130ns disable delay; and (3) adaptive blanking via BLKSENS pin that suppresses false current-sense triggers during MOSFET turn-on. These features operate independently of loop compensation and remain active during startup, fault recovery, and transient events - ensuring core reset integrity across all operating conditions. The LT1681ESW#TRPBF datasheet confirms this behavior applies to all variants including LT1681ESW#TRPBF.

What fault protection features are built into the LT1681ESW#TRPBF?

The LT1681ESW#TRPBF incorporates a unified General Fault Condition (GFC) system that disables switching and initiates soft-start reset upon detection of: VCC undervoltage (8.4V rising threshold), 5VREF overload, OVLO/THERM overvoltage (1.25V rising threshold), IMAX overcurrent (360mV), or SHDN assertion. All faults trigger simultaneous discharge of the SS capacitor to 225mV, enforcing zero-current state before recovery. The LT1681ESW#TRPBF also includes independent UVLO on VBST–BSTREF (7.0V rising) to protect high-side gate drive during bootstrap charging. No external fault logic is required.

Can the LT1681ESW#TRPBF be used in non-isolated forward or buck-derived topologies?

Yes, the LT1681ESW#TRPBF supports both isolated and non-isolated forward topologies per its official datasheet. In non-isolated configurations, the VFB pin connects directly to output divider, VC drives current-sense comparator, and synchronous rectification remains functional using SG/BG outputs. However, BSTREF and VBST must still be properly configured for high-side drive - typically via bootstrap capacitor from VCC to BSTREF when using N-MOSFETs. The LT1681ESW#TRPBF's 72V input capability and 5VREF make it suitable for high-input-voltage buck-derived designs where isolation is unnecessary but efficiency and fault robustness are critical.