Analog Devices Inc. LT3781IG#PBF

Overview

LT3781IG#PBF from Analog Devices (formerly Linear Technology) is a high-voltage synchronous forward controller IC for isolated and nonisolated DC/DC converters, supporting input voltages up to 72V, fixed-frequency current-mode control up to 350kHz, and dual N-channel MOSFET drive with ≤50% duty cycle enforcement. It integrates 5V reference (±1%), undervoltage lockout (8.4V/14.5V thresholds), soft-start, and fault protection for telecom power supplies and battery backup systems.

For engineers reviewing the LT3781IG#PBF datasheet, LT3781IG#PBF pinout, LT3781IG#PBF application, or LT3781IG#PBF equivalent, key selection considerations include bootstrap gate drive capability (VBST up to 90V), synchronized multi-controller phase alignment, adaptive current-sense blanking, thermal/overvoltage fault latching, and 20-lead SSOP package compatibility with quarter-brick footprints.

Technical Context

The LT3781IG#PBF employs fixed-frequency current-mode control with internal T-flip-flop clock generation running at twice the switching frequency, enforcing ≤50% maximum duty cycle to prevent transformer saturation. Its error amplifier features 1.25V internal reference, 72dB gain, and 1MHz GBW for stable loop compensation in isolated topologies.

Fault detection includes independent OVLO (1.25V threshold, 40mV hysteresis), THERM (1.25V threshold), and 5VREF overcurrent shutdown, all triggering graceful soft-start reset. The VBST–BSTREF UVLO (7.0V rising edge) gates TG driver activation to avoid linear-region operation during bootstrap charging.

Key Specifications

Pinout & Package

LT3781IG#PBF is housed in a 20-lead plastic SSOP (G package) with exposed pad for thermal enhancement. Pin spacing is 0.65mm, body dimensions are 7.5mm × 5.6mm, and recommended PCB footprint follows Linear Technology's AN133 guidelines.

Key Features

Applications

Equivalent & Alternatives

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

Compared with LT3748IMSE#PBF and UCC2891DR, the LT3781IG#PBF provides superior system integration via its 5VREF, dedicated thermal/OVLO pins, and adaptive blanking-making it optimal for high-reliability, optocoupler-regulated forward converters where startup control and fault robustness are critical.

Availability

LT3781IG#PBF is available at Aetrix Electronics and suitable for telecom rectified bus supplies, lead-acid battery backup systems, and industrial distributed power converters requiring stable component supply, long-term lifecycle support, and guaranteed extended temperature performance.

Supply support for LT3781IG#PBF 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 acquired Linear Technology in 2017 and maintains full product support, datasheet documentation, and application engineering for legacy Linear parts including the LT3781 series.

The LT3781 product line was designed specifically for high-power synchronous forward DC/DC converters in telecom, industrial, and automotive applications demanding high input voltage range, precise regulation, and robust fault management.

FAQ

What is the maximum operating frequency of the LT3781IG#PBF?

The LT3781IG#PBF supports a maximum switching frequency of 350kHz when synchronized externally via the SYNC pin. In free-run mode, oscillator frequency is set by the FSET RC network and typically ranges from 70kHz to 350kHz. The internal oscillator runs at twice the switching frequency to enforce ≤50% duty cycle, ensuring safe transformer operation in forward topologies. Frequency programming error is ±5% for f_OSC ≤ 500kHz.

Does the LT3781IG#PBF require an external bootstrap diode and capacitor?

Yes, the LT3781IG#PBF requires an external Schottky diode (cathode to VCC, anode to VBST) and bootstrap capacitor (0.1µF to 1.0µF) between VBST and BSTREF pins to generate the boosted gate drive voltage for the high-side MOSFET. The diode must have reverse breakdown voltage exceeding the input voltage, and the capacitor must be ≥100× the total TG gate capacitance. This bootstrap circuit is essential for driving N-channel FETs in the high-side position of forward converters.

How does the LT3781IG#PBF handle thermal faults?

The LT3781IG#PBF monitors system temperature via the THERM pin: when voltage exceeds 1.25V (with 40mV hysteresis), it triggers a General Fault Condition (GFC), disabling all switching outputs and discharging the soft-start capacitor. Recovery only begins once the SS pin voltage falls below 225mV, ensuring zero-output-current state before restart. This latched, graceful recovery prevents thermal runaway and supports thermistor-based sensing with the integrated 5VREF supply.

Can the LT3781IG#PBF be used in nonisolated forward configurations?

Yes, the LT3781IG#PBF supports both isolated and nonisolated forward topologies. In nonisolated mode, the VFB pin connects directly to the output via resistor divider, VC drives local compensation, and optocoupler isolation is omitted. The IC's current-mode control, adaptive blanking, and ≤50% duty cycle enforcement remain fully functional. Layout must still separate SGND and PWRGND planes to maintain stability, and the THERM/OVLO pins retain their protection roles.

What is the purpose of the 5VREF pin on the LT3781IG#PBF?

The 5VREF pin on the LT3781IG#PBF provides a precision ±1% 5V reference output capable of sourcing up to 20mA. It powers external circuitry including the FSET timing resistor, OVLO and THERM fault-sense dividers, optocoupler LEDs, and auxiliary bias rails. Internally short-circuit protected to ~45mA, exceeding this current disables switching and resets soft-start-enabling self-protecting bias design without external regulators.