LT3748EMS#PBF

Analog Devices LT3748EMS#PBF — Primary‑Side Flyback PWM Controller (MSOP‑12)

The Analog Devices LT3748EMS#PBF is a primary‑side regulated flyback PWM controller designed to simplify isolated power designs by removing the need for an optocoupler or a third transformer winding. Built for boundary‑mode (critical‑conduction) operation, this controller targets compact, efficient isolated bias supplies and general‑purpose isolated DC/DC converters. Housed in a space‑saving MSOP‑12 package for surface‑mount assembly, the LT3748EMS#PBF helps engineers reduce bill of materials (BOM) cost and design complexity while maintaining isolated output regulation using primary‑side sensing.

Engineers, buyers, and sourcing teams will appreciate that this device is RoHS compliant (Pb‑free, #PBF), carries an Active lifecycle status per the manufacturer’s product page, and is available in tube packaging under the LT3748EMS#PBF ordering code, with a tape‑and‑reel companion ordering option (LT3748EMS#TRPBF) for automated assembly lines.

Why engineers choose the LT3748EMS#PBF

• No optocoupler required: Primary‑side sensing provides isolated regulation without an opto or auxiliary winding, reducing parts count and long‑term aging concerns associated with optocouplers.
• Boundary‑mode (critical conduction) operation: Well‑suited to compact magnetics and helps simplify transformer design for isolated flyback converters.
• One‑output simplicity: A focused feature set that efficiently addresses isolated bias and general‑purpose isolated rails.
• Proven LT3748 family: From Analog Devices’ LT series of controllers, widely used across industrial and communications infrastructure.

How primary‑side regulation (PSR) works in this controller

Primary‑side regulation leverages information available on the primary side of the transformer to infer the isolated output voltage. During the flyback interval, the reflected output voltage appears on the primary winding. By sampling this reflected voltage (in combination with the known transformer turns ratio) and monitoring primary current, the LT3748EMS#PBF estimates and regulates the secondary‑side output—without an optocoupler feedback path. This approach:

• Eliminates optocoupler aging and CTR variation as design variables.
• Simplifies transformer specification because no dedicated feedback winding is required.
• Reduces BOM and potential failure points, aiding long‑term reliability.

As with all PSR architectures, accurate transformer design (turns ratio, leakage, and magnetizing inductance) and careful sampling network layout are key to achieving robust regulation across line, load, and temperature.

Practical design considerations

When designing with the Analog Devices LT3748EMS#PBF, consider the following best practices to streamline development and first‑pass success:

• Transformer selection

• Choose a turns ratio that sets a reasonable reflected voltage while respecting switch and snubber limits.

• Balance magnetizing inductance to achieve desired boundary‑mode operation at nominal load without excessive current ripple at light load.

• Current sense and slope behavior

• Ensure accurate current sense resistor selection and tight layout around the sense path to minimize noise pickup and false tripping.

• Keep high‑di/dt loops compact to reduce ringing that can corrupt primary‑side voltage sampling.

• Snubbering and EMI

• Add an RC or RCD snubber (or clamp) across the primary winding or switch to manage leakage energy and maintain predictable sampling conditions.

• Pay attention to EMI by minimizing loop areas and using appropriate input filtering.

• Output regulation accuracy

• PSR accuracy depends on transformer turns ratio, diode drop/reflected voltage, and sampling network tolerances. Validate across temperature and load.

• Startup and biasing

• Verify start‑up path, bias supply behavior, and hold‑up during line dips as per datasheet guidance. Ensure that the controller’s bias rail is adequately decoupled and within recommended limits.

• Safety isolation and layout

• Meet creepage/clearance requirements for your safety class and isolation voltage. Maintain generous spacing between primary and secondary domains on the PCB and within the transformer design.

These considerations are broadly applicable to boundary‑mode flyback designs and help reap the benefits of the LT3748EMS#PBF’s primary‑side regulated control scheme.

Key specifications

• Brand / Manufacturer: Analog Devices
• Series: LT3748
• Function: Primary‑side regulated flyback PWM controller
• Topology: Flyback (primary‑side regulation)
• Operating mode: Boundary‑mode / critical‑conduction
• Number of outputs: 1
• Regulation method: Primary‑side sensing; no optocoupler required
• Package: MSOP‑12
• Mounting type: Surface Mount
• Pin count: 12
• Lead finish: Pb‑free (#PBF)

The following parameters are not specified in the provided data and should be confirmed in the datasheet:

• Input voltage (bias) range: Not specified here; see datasheet
• Gate driver capabilities and maximum duty cycle: Not specified here; see datasheet
• Switching frequency characteristics: Not specified here; see datasheet
• Protection features (UVLO, OVP/OTP, current limit details, soft‑start): Not specified here; see datasheet
• Operating temperature range and MSL: Not specified here; see datasheet

Refer to the Analog Devices datasheet for complete electrical characteristics and application examples.

Applications

The LT3748EMS#PBF is suited for a wide range of isolated power designs, including:

• Isolated flyback power supplies
• Bias supplies for isolated systems
• Industrial and communications equipment power
• General‑purpose isolated DC/DC converters

Its primary‑side regulation is especially attractive when cost, size, and long‑term reliability are priorities, and when load and line ranges are compatible with PSR regulation accuracy.

Lifecycle, sourcing, and approved alternates

• Lifecycle status: Active (per Analog Devices product page)
• Packaging: Tube (LT3748EMS#PBF); tape‑and‑reel variant available
• Approved alternate: LT3748EMS#TRPBF — functionally the same silicon in tape‑and‑reel packaging, ideal for high‑volume automated placement
• OCM source: Analog Devices product page

Sourcing notes for buyers and supply chain teams:

• When specifying this controller, include the full ordering code “LT3748EMS#PBF” in the BOM to ensure Pb‑free tube packaging is delivered to production.
• If your line is optimized for reels, consider “LT3748EMS#TRPBF” as a stocking equivalent to reduce repackaging steps and handling risk.
• For current lead times and inventory positions, refer to authorized Analog Devices distributors; availability is not included in the provided data.

Compliance and environmental

• RoHS: RoHS Compliant (Pb‑free, #PBF)
• Lead finish: Pb‑free (#PBF)
• Additional environmental declarations (e.g., REACH, halogen‑free), MSL rating, and moisture handling: Not specified in the provided data; consult manufacturer documents.

Compliance documentation can be found via the product page or requested from Analog Devices as needed for quality files.

Ordering information at a glance

• MPN: LT3748EMS#PBF (tube packaging)
• Alternate packaging: LT3748EMS#TRPBF (tape‑and‑reel)
• Package / case: MSOP‑12
• Mounting: Surface Mount

These ordering options carry the same electrical function; select based on your assembly flow and feeder requirements.

When to choose LT3748EMS#PBF over an optocoupler‑based approach

• You want to eliminate optocoupler aging and CTR drift from your regulation loop.
• You prefer a smaller, simpler BOM with fewer secondary‑side components.
• Your regulation targets are compatible with primary‑side feedback accuracy across expected line/load conditions.
• You value boundary‑mode operation for compact magnetics and straightforward transformer design.

Conversely, some designs with very tight regulation at extreme conditions or with multiple isolated rails may still favor secondary‑side feedback using an optocoupler; evaluate requirements accordingly.

Specifications summary

Below is a concise summary aligned with the provided product data. Items not present in the source are left as not specified.

• Controller type: Isolated flyback PWM controller with primary‑side regulation
• Outputs: 1
• Operating mode: Boundary‑mode (critical conduction)
• Feedback path: Primary‑side sensing; no opto required
• Package: MSOP‑12, Surface Mount
• Pins: 12
• Lead finish: Pb‑free (#PBF)
• Environmental: RoHS Compliant
• Electrical limits and protections: Not specified here; see datasheet

Frequently Asked Questions (FAQs)

• Does the LT3748EMS#PBF require an optocoupler for regulation?

• No. It uses primary‑side sensing to regulate the isolated output, eliminating the need for an optocoupler or third winding.

• What type of converters is the LT3748EMS#PBF intended for?

• It is a controller for isolated flyback converters (primary‑side regulated), commonly used for bias and general‑purpose isolated power supplies.

• How many outputs does the LT3748EMS#PBF support?

• One isolated output, per the provided specifications.

• What package is the LT3748EMS#PBF available in?

• MSOP‑12, surface‑mount.

• Is the LT3748EMS#PBF automotive qualified (AEC‑Q100)?

• Not specified for this ordering code in the provided data. The product note indicates that automotive qualification is not claimed for LT3748EMS#PBF. Verify with Analog Devices if your program requires AEC‑Q100.

• What is the operating frequency range and input voltage rating?

• Not specified in the provided data. Consult the datasheet for detailed electrical characteristics and design limits.

• Are evaluation boards available?

• Not specified here. Check the Analog Devices product page for any available demo or evaluation hardware.

Resources

• Product page: https://www.analog.com/en/products/lt3748.html
• Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/3748fa.pdf

Closing thoughts

The Analog Devices LT3748EMS#PBF offers a clean, modern approach to isolated flyback power by removing the optocoupler from the control loop. Its boundary‑mode operation, MSOP‑12 footprint, and RoHS‑compliant construction make it an attractive choice for bias supplies and many general‑purpose isolated rails across industrial and communications applications. With active lifecycle status and straightforward packaging options (tube or tape‑and‑reel), it’s easy to specify and source. For project‑critical limits and application nuances—such as bias voltage, protection thresholds, and frequency behavior—refer directly to the official Analog Devices datasheet and design notes to finalize component selection and ensure robust performance in your end system.