Analog Devices Inc. LT8311EFE#TRPBF

Overview

LT8311EFE#TRPBF from Analog Devices (formerly Linear Technology) is a secondary-side synchronous rectifier controller with integrated opto-coupler driver for isolated forward converters. It enables preactive-mode DCM operation without a pulse transformer, delivers 1.227V ±1.5% feedback reference, supports 3.7V–30V input, and drives N-channel MOSFETs for active-clamp reset topologies in 48V/industrial offline supplies.

For engineers reviewing the LT8311EFE#TRPBF datasheet, LT8311EFE#TRPBF pinout, LT8311EFE#TRPBF application, or LT8311EFE#TRPBF equivalent, key selection criteria include preactive vs. SYNC mode timing control, opto-driver output capability (10mA source), power-good monitoring window (±7%), soft-start slew rate control, and high-voltage CSW/FSW pin ratings up to 150V.

Technical Context

The LT8311EFE#TRPBF implements dual synchronous control schemes: preactive mode uses drain-sense timing (CSW/FSW pins) and internal prediction logic to drive CG/FG outputs without primary-side synchronization signals, while SYNC mode accepts external PWM edges via the SYNC pin to coordinate with primary-side ICs. Both modes support discontinuous and forced continuous conduction.

Its transconductance error amplifier (370 µmhos, 65 dB gain) interfaces with an opto-coupler through the COMP and OPTO pins, delivering precise voltage regulation via a 1.227V reference with 0.1% line/load regulation. The integrated 10mA opto-driver includes short-circuit protection and –5 to –6.2 V/V DC gain across its operating range.

Key Specifications

Pinout & Package

LT8311EFE#TRPBF is housed in a 20-lead plastic TSSOP (FE package) with exposed pad (Pin 21 = GND), optimized for high-voltage spacing. Pins 1, 3, 18, and 20 are internally trimmed for creepage/clearance compliance in >72V systems.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UCC24612DR and LM5045MTX/NOPB, the LT8311EFE#TRPBF uniquely combines preactive-mode transformerless operation, 150V-rated sense pins, and integrated 10mA opto-driver-making it optimal for compact, high-reliability isolated forward converters below 100W where secondary-side autonomy is essential.

Availability

LT8311EFE#TRPBF is available at Aetrix Electronics and suitable for industrial offline power supplies, automotive HV battery isolation, and telecom 48V isolated DC-DC converters requiring stable component supply and long-term lifecycle support.

Supply support for LT8311EFE#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, serving industrial, automotive, communications, and defense markets.

The LT8311 product line was designed specifically for secondary-side synchronous rectification in isolated forward converters-emphasizing transformerless preactive control, robust opto-feedback integration, and high-voltage operational safety in harsh environments.

FAQ

What is the function of the PMODE pin on the LT8311EFE#TRPBF?

The PMODE pin selects between two synchronous control modes: tying PMODE to GND enables preactive mode (no pulse transformer required), while connecting it to INTVCC enables SYNC mode (requires external PWM signal via pulse transformer). In preactive mode, the LT8311EFE#TRPBF uses CSW/FSW drain sensing to autonomously determine MOSFET timing; in SYNC mode, it synchronizes CG/FG outputs to primary-side edges. This dual-mode flexibility allows designers to optimize for simplicity or precision based on system requirements.

How does the LT8311EFE#TRPBF achieve transformerless operation in preactive mode?

The LT8311EFE#TRPBF achieves transformerless operation by directly sensing the drain voltages of the forward and catch MOSFETs via the CSW and FSW pins-both rated to 150V. Using internal prediction logic and current comparator blanking, it infers MOSFET conduction states and generates precisely timed CG/FG gate drive signals without needing a pulse transformer. This eliminates magnetic components, reduces EMI, and simplifies layout-making the LT8311EFE#TRPBF ideal for space-constrained isolated forward converters where reliability and BOM reduction are critical.

What is the purpose of the TIMER pin on the LT8311EFE#TRPBF?

The TIMER pin on the LT8311EFE#TRPBF sets an upper limit on the sum of the forward and catch MOSFET on-times per switching cycle-including dead time-via an external resistor to ground. If this timeout period is exceeded (e.g., during overload or short-circuit), synchronous conduction shuts down to protect the power stage. Recovery occurs automatically once the condition clears. This feature enhances system robustness without requiring external fault logic, and resistor values (e.g., 71.5kΩ for 255kHz) allow precise tuning of the timeout threshold for each LT8311EFE#TRPBF design.

Can the LT8311EFE#TRPBF drive standard opto-couplers like PC817 without external components?

Yes-the LT8311EFE#TRPBF's OPTO pin sources up to 18mA (typical 10.5–15mA) and includes short-circuit protection, enabling direct drive of common opto-couplers such as PC817, PC818, or TLP185 without external transistors or current-limiting resistors beyond the series LED resistor. Its 1V opto-driver reference and –5 to –6.2 V/V DC gain ensure stable closed-loop regulation across temperature and line variations. This integrated capability reduces component count and improves transient response compared to discrete opto-driver implementations.

What is the significance of the exposed pad (Pin 21) on the LT8311EFE#TRPBF package?

The exposed pad (Pin 21) on the LT8311EFE#TRPBF is electrically and thermally connected to GND and must be soldered directly to the PCB ground plane. It provides a low-thermal-resistance path (θJC = 10°C/W) from the die to the board, critical for dissipating heat generated by the internal gate drivers and opto-driver-especially under high-duty-cycle or high-ambient conditions. Failure to solder the pad compromises thermal performance and may cause premature thermal shutdown or reduced reliability. The pad also contributes to high-voltage creepage compliance in the FE package.