Analog Devices Inc. LT1681ISW#TRPBF

Overview

LT1681ISW#TRPBF from Analog Devices (formerly Linear Technology) is a high-voltage synchronous forward controller IC for dual-transistor DC/DC converters, supporting input voltages up to 72V, fixed-frequency current-mode control up to 350kHz, and 50% maximum duty cycle enforcement to prevent transformer saturation. It drives external N-channel MOSFETs and integrates soft-start, input overvoltage/undervoltage lockout, thermal shutdown, and primary-side saturation protection.

For engineers reviewing the LT1681ISW#TRPBF datasheet, LT1681ISW#TRPBF pinout, LT1681ISW#TRPBF application, or LT1681ISW#TRPBF equivalent, key selection considerations include its 20-lead wide SO package, VCC operating range (9–18V), 1.25V error amplifier reference, 150mV current sense threshold, and synchronized phase control for multi-controller systems.

Technical Context

The LT1681ISW#TRPBF implements fixed-frequency current-mode control with internal T-flip-flop logic enforcing ≤50% duty cycle. Its oscillator runs at twice the switching frequency and supports synchronization via TTL-compatible SYNC input up to 350kHz, with phase-locking capability for coordinated multi-converter operation.

Fault management includes independent OVLO, THERM, and IMAX inputs triggering general fault condition (GFC) response-disabling outputs and resetting soft-start capacitor to 225mV. Adaptive blanking via BLKSENS pin prevents false current-sense triggering during MOSFET turn-on transitions.

Key Specifications

Pinout & Package

LT1681ISW#TRPBF is housed in a 20-lead wide SOIC (SW) package with creepage-optimized pin spacing 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 LM5035MTX/NOPB, the LT1681ISW#TRPBF uniquely combines programmable synchronization, adaptive blanking, integrated 5VREF, and dedicated IMAX protection-making it optimal for high-reliability isolated forward converters requiring coordinated multi-rail timing and robust fault coverage.

Availability

LT1681ISW#TRPBF is available at Aetrix Electronics and suitable for telecom power systems, industrial DC supplies, automotive onboard chargers, and server PSU auxiliary rails requiring stable component supply and long-term production continuity.

Supply support for LT1681ISW#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 LT1681ISW#TRPBF belongs to Linear's legacy high-voltage controller product line, designed specifically for isolated synchronous forward DC/DC converters in telecom, industrial, and automotive power applications demanding reliability, fault resilience, and layout simplicity.

FAQ

What is the maximum input voltage supported by the LT1681ISW#TRPBF controller?

The LT1681ISW#TRPBF itself operates from a 9V to 18V VCC supply, but it controls external N-channel MOSFETs enabling system-level input voltage operation up to 72V DC. The VBST pin supports up to 90V referenced to BSTREF, allowing high-side gate drive in high-input applications. Absolute maximum ratings must never be exceeded in design.

Does the LT1681ISW#TRPBF integrate a voltage reference, and what is its accuracy?

Yes, the LT1681ISW#TRPBF integrates a precision 5V reference (5VREF pin) with ±1.5% initial accuracy over temperature (4.85V to 5.15V, –40°C to 85°C) and 1Ω output impedance. It can source up to 20mA and is short-circuit protected to 45mA. This eliminates the need for an external reference in most auxiliary bias applications.

How does the LT1681ISW#TRPBF prevent transformer saturation in forward converters?

The LT1681ISW#TRPBF prevents transformer saturation through hardware-enforced 50% maximum duty cycle operation, implemented via internal T-flip-flop logic that toggles the oscillator at twice the switching frequency. This ensures balanced volt-second product across the transformer primary winding regardless of load or line conditions.

What is the purpose of the BLKSENS pin on the LT1681ISW#TRPBF, and how is it used?

The BLKSENS pin on the LT1681ISW#TRPBF enables adaptive current-sense blanking: it monitors the gate voltage of the bottom-side primary FET and disables the SENSE amplifier during turn-on transitions. This prevents false overcurrent triggering due to gate-drive-induced voltage spikes, eliminating the need for fixed-delay circuits or complex timing adjustments.

Can the LT1681ISW#TRPBF be synchronized to an external clock, and what is the maximum frequency?

Yes, the LT1681ISW#TRPBF supports external synchronization via its TTL-compatible SYNC pin, accepting input frequencies up to 350kHz. The internal oscillator doubles this frequency to maintain proper timing relationships, and the SYNC decoder forces phase-lock between multiple controllers-enabling EMI reduction and coordinated multi-rail operation.