Analog Devices Inc. LT1425CS#PBF

Overview

LT1425CS#PBF from Analog Devices is a monolithic isolated flyback switching regulator IC with integrated 1.6A/35V power switch, 285kHz fixed-frequency current-mode control, and primary-side output voltage sensing-eliminating need for optoisolators or third-winding feedback. It operates from 3V to 20V input, delivers up to 6W output power, and maintains ±5% regulation across 20–200mA load in isolated –9V applications such as Ethernet power supplies.

For engineers reviewing the LT1425CS#PBF datasheet, LT1425CS#PBF pinout, LT1425CS#PBF application, or LT1425CS#PBF equivalent, key selection criteria include primary-side regulation architecture, 16-pin SOIC package compatibility, shutdown current of 15µA, load compensation capability via ROCOMP/RCCOMP, and discontinuous-mode stability without external trim components.

Technical Context

The LT1425CS#PBF implements a proprietary flyback error amplifier that samples the VSW flyback pulse waveform to derive isolated output voltage information directly on the primary side. Its control loop uses RREF-referred transconductance (400–1600 µmho) and VC node integration for frequency compensation, enabling stable operation without secondary-side feedback components.

It features synchronized timing with external clock input (SYNC pin), minimum ON time enforcement (170–260 ns), enable delay (150 ns), and collapse-detect-based feedback windowing to support both continuous and discontinuous conduction modes. Load compensation is implemented via ROCOMP resistor and RCCOMP capacitor to correct for secondary-side ESR and winding impedance effects.

Key Specifications

Pinout & Package

LT1425CS#PBF is housed in a 16-lead narrow plastic SOIC (S package) with θJA = 75°C/W and TJMAX = 145°C. Pin 1 is GND (substrate ground); pins 1, 8, 9, and 16 are dedicated substrate grounds requiring direct connection to PCB ground plane.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar isolated flyback regulator applications.

Compared with UCC28740DR and LT3748EMSE#PBF, the LT1425CS#PBF uniquely delivers primary-side regulation in a 16-pin SOIC without optoisolators-offering lowest component count and smallest PCB area for ≤6W isolated –9V outputs, while maintaining full specification compliance from 0°C to 125°C.

Availability

LT1425CS#PBF is available at Aetrix Electronics and suitable for isolated Ethernet power supplies, medical instrument bias rails, telecom DC/DC converters, and industrial sensor interfaces requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for LT1425CS#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, Inc. is a global semiconductor company specializing in high-performance analog, mixed-signal, and digital signal processing technologies for precision measurement and power conversion.

The LT1425CS#PBF belongs to Analog Devices' isolated power controller product line, designed specifically for compact, low-component-count flyback converters where galvanic isolation and primary-side regulation are critical-targeting medical, telecom, and industrial applications.

FAQ

What is the maximum output power achievable with the LT1425CS#PBF in an isolated flyback design?

The LT1425CS#PBF can deliver up to 6W of output power without external power devices. This assumes proper thermal management on a standard PCB with 2oz copper, use of recommended transformer specifications (e.g., Dale LPE 4841-330MB), and operation within its 3V–20V input range. Exceeding 6W requires external MOSFETs and changes to the basic topology, which voids the integrated switch benefit of the LT1425CS#PBF.

Does the LT1425CS#PBF require an optoisolator or third-winding feedback for regulation?

No, the LT1425CS#PBF does not require an optoisolator or third-winding feedback. It achieves isolated output voltage regulation by directly sensing the flyback pulse amplitude and timing at the VSW pin on the primary side. This eliminates two major failure-prone and space-consuming components, simplifying design and improving long-term reliability in the LT1425CS#PBF-based power supply.

How is output voltage programmed on the LT1425CS#PBF?

Output voltage on the LT1425CS#PBF is resistor-programmed using the RFB and RREF pins. The ratio RFB/RREF-combined with transformer turns ratio and diode forward voltage-determines final output. For example, with RREF = 3.01kΩ and RFB = 22.6kΩ, the LT1425CS#PBF achieves ±5% accurate –9V output without user trims. The internal reference current (~400µA) ensures predictable scaling across temperature.

What is the purpose of the ROCOMP and RCCOMP pins on the LT1425CS#PBF?

The ROCOMP and RCCOMP pins implement optional load compensation on the LT1425CS#PBF. ROCOMP connects to an external resistor that injects load-dependent current into the RFB node, while RCCOMP connects to a 0.1µF capacitor that filters this current. Together, they correct for voltage droop caused by secondary winding resistance, Schottky diode VF, and output capacitor ESR-improving load regulation by up to 60% in high-current applications.

Can the LT1425CS#PBF be synchronized to an external clock, and what is the acceptable frequency range?

Yes, the LT1425CS#PBF can be synchronized to an external clock via the SYNC pin. It accepts logic-level square waves with 10–90% duty cycle and amplitude ≥1.5V (typical). The synchronization range is 320kHz to 450kHz, allowing EMI reduction through frequency dithering or alignment with system clocks. When unused, the SYNC pin should be grounded for best noise immunity-not left floating.