Analog Devices Inc. LTC1698IS#PBF

Overview

LTC1698IS#PBF from Analog Devices (formerly Linear Technology) is a precision secondary-side synchronous rectifier controller for isolated forward converters, designed to pair with the LT3781 primary-side controller. It drives dual external N-channel MOSFETs (forward and catch), delivers ±0.8% output voltage accuracy, supports 50–400 kHz synchronization frequency, and integrates a 3.3V/10mA auxiliary supply - enabling high-efficiency, space-constrained telecom and industrial DC/DC systems.

For engineers reviewing the LTC1698IS#PBF datasheet, LTC1698IS#PBF pinout, LTC1698IS#PBF application, or LTC1698IS#PBF equivalent, key selection considerations include its pulse-transformer-synchronized dual gate drive, optocoupler feedback driver with 5 V/V gain, programmable ±5% output margining, and integrated overvoltage/power-good monitoring for isolated power supplies.

Technical Context

The LTC1698IS#PBF implements a secondary-side control architecture where timing is derived exclusively from the LT3781 via SYNC input (±14V tolerant, 50–400 kHz), eliminating need for local oscillator or transformerless sensing. Its error amplifier regulates VFB to 1.233 V with 65–90 dB open-loop gain and 2 MHz unity-gain bandwidth, while the current-limit amplifier senses ISNS–ISNSGND differential voltage with –25 mV threshold and 2.2–5 millimho transconductance.

It features adaptive driver disable time-out (10–20 µs, proportional to fSYNC), dual rail-to-rail gate drivers (CG/FG) referenced to PGND, and an optocoupler driver (OPTODRV) with inverting 5 V/V DC gain, 1 MHz bandwidth, and –50 mA short-circuit capability - all operating from a single VDD supply (6–12.6 V) and internally regulated VAUX (3.135–3.465 V).

Key Specifications

Pinout & Package

Package: 16-lead SO (Small Outline) - RoHS-compliant, surface-mount, 1.27 mm pitch, JEDEC MS-012AC footprint.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UCC2897A and MAX17595, the LTC1698IS#PBF uniquely combines pulse-transformer synchronization, integrated 3.3 V/10 mA VAUX, and dedicated current-limit transconductance amplifier - making it optimal for compact, high-efficiency isolated forward converters requiring minimal secondary-side support components.

Availability

LTC1698IS#PBF is available at Aetrix Electronics and suitable for telecom power systems, industrial distributed power supplies, and automotive test equipment requiring stable component supply, long-term lifecycle support, and guaranteed performance over –40°C to +85°C.

Supply support for LTC1698IS#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) is a global leader in high-performance analog, mixed-signal, and power management semiconductors, serving precision instrumentation, industrial, automotive, and communications markets.

The LTC1698IS#PBF belongs to Linear's isolated power controller product line, engineered specifically for high-efficiency, low-noise secondary-side regulation in forward converter topologies - emphasizing robust synchronization, integrated housekeeping functions, and reliability in harsh environments.

FAQ

What is the operating temperature range for the LTC1698IS#PBF?

The LTC1698IS#PBF is rated for –40°C to +85°C ambient operation (industrial grade). Its electrical specifications - including VFB accuracy (±0.8%), VAUX regulation (3.135–3.465 V), and current limit threshold (–25 mV) - are guaranteed across this full range. The "I" suffix denotes industrial temperature qualification, distinct from the commercial-grade "E" variant.

Does the LTC1698IS#PBF require an external clock or oscillator?

No, the LTC1698IS#PBF does not require an external clock or oscillator. It derives all timing from the SYNC input signal, which is driven by the LT3781 primary controller via a pulse transformer. The device operates synchronously across 50–400 kHz without internal timing components, ensuring precise phase alignment and eliminating jitter from free-running oscillators.

Can the LTC1698IS#PBF be used without the LT3781 primary controller?

The LTC1698IS#PBF is designed explicitly for use with the LT3781 but can interface with other primary controllers that provide compatible SYNC signals - i.e., bipolar pulses (±5 V typical) with 75 ns minimum width and 50–400 kHz repetition rate. However, full functionality (e.g., adaptive time-out, optimal startup behavior) is validated only with LT3781 per the datasheet.

How does the MARGIN pin adjust output voltage in the LTC1698IS#PBF?

The MARGIN pin on the LTC1698IS#PBF connects to an internal 16.5 kΩ resistor biased at 1.65 V. Applying 0 V sinks 100 µA and lowers VFB by 5%; applying 3.3 V sources 100 µA and raises VFB by 5%. This linearly shifts the 1.233 V reference, changing regulated output voltage proportionally - enabling precise factory calibration or dynamic margin testing without resistor changes.

What protection features are built into the LTC1698IS#PBF?

The LTC1698IS#PBF integrates multiple hardware protections: undervoltage lockout (4 V threshold), overvoltage detection (OVPIN comparator with 1.233 V trip), power-good monitoring (PWRGD with 94% threshold and hysteresis), adaptive driver disable time-out (prevents negative output transients), and short-circuit protected OPTODRV and VAUX outputs - all operating independently of software or external supervision.