Analog Devices Inc. LT1737CS#PBF

Overview

LT1737CS#PBF from Analog Devices (formerly Linear Technology) is a current-mode isolated flyback controller IC that drives an external N-channel MOSFET to regulate isolated DC/DC outputs up to ~30W. It senses output voltage directly from the primary-side transformer winding without optocouplers, operates from 4.5V–20V VCC, supports 50kHz–250kHz switching via external OSCAP capacitor, and maintains regulation into discontinuous conduction mode - used in medical instrument power supplies and industrial isolated converters.

For engineers reviewing the LT1737CS#PBF datasheet, LT1737CS#PBF pinout, LT1737CS#PBF application, or LT1737CS#PBF equivalent, key selection considerations include primary-side feedback accuracy (±1.2% FB reference), UVLO threshold (1.25V typ), gate drive capability (500mA sink/source), soft-start control via SFST pin, and load compensation support via ROCMP/RCMPC pins for improved line/load regulation in high-precision isolated supplies.

Technical Context

The LT1737CS#PBF implements a proprietary flyback error amplifier that samples the reflected flyback pulse during a precisely timed enable window-governed by programmable tON, ENDLY, and MINENAB resistors-to reject leakage spikes and maintain stability across continuous/discontinuous modes. Its VC pin serves as both feedback amplifier output and current comparator input, enabling frequency compensation via external capacitor.

It integrates a 3V reference (3VOUT pin, ±3% tolerance, 15mA limit), shutdown logic tied to UVLO pin (50µA shutdown current), and dual-function ISENSE input with 250mV nominal current-limit threshold. The controller's timing architecture includes minimum on-time enforcement, collapse-detect-based amplifier disable, and dynamic enable period adjustment to accommodate variable duty cycles.

Key Specifications

Pinout & Package

LT1737CS#PBF is housed in a 16-lead plastic SO (Small Outline) package with standard JEDEC MS-012AC footprint (5.3mm × 10.2mm, 1.27mm pitch). Thermal resistance θ_JA = 110°C/W enables operation up to 100°C ambient in properly laid-out PCBs with thermal relief.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UCC28740DR and MAX17596ATP+, the LT1737CS#PBF delivers superior primary-side regulation accuracy and programmable flyback sampling timing - making it optimal for medical and test equipment where optocoupler elimination and tight load regulation are non-negotiable, despite its narrower 4.5V–20V VCC range.

Availability

LT1737CS#PBF is available at Aetrix Electronics and suitable for medical instrumentation power supplies, industrial sensor interfaces, laboratory-grade isolated converters, and test equipment auxiliary rails requiring stable component supply with full traceability and long-term lifecycle support.

Supply support for LT1737CS#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. (ADI) is a global leader in high-performance analog, mixed-signal, and digital signal processing semiconductors, formed through the acquisition of Linear Technology in 2017.

The LT1737CS#PBF belongs to ADI's legacy Linear Technology isolated power controller product line, engineered specifically for high-accuracy, optocoupler-free flyback topologies in safety-critical and precision measurement applications.

FAQ

What is the maximum achievable output power using LT1737CS#PBF in a typical design?

The LT1737CS#PBF supports up to ~30W of isolated output power in well-designed flyback converters using appropriate external MOSFETs (e.g., IRFL014), transformers (e.g., Coiltronics CTX150-4), and layout practices. Actual power is constrained by thermal limits of the SO-16 package (θ_JA = 110°C/W), MOSFET SOA, and transformer core saturation - not by the controller itself. Designs delivering 15V/2A (30W) have been validated in Linear Technology Application Note 19.

Does LT1737CS#PBF require an optocoupler for isolated feedback?

No, the LT1737CS#PBF does not require an optocoupler. It implements primary-side sensing by sampling the reflected flyback pulse from a third transformer winding (or primary winding) at the FB pin. This eliminates optocoupler CTR variation, aging, and temperature drift - a key advantage for long-life medical and industrial applications where certification and reliability are critical.

How is the switching frequency set on LT1737CS#PBF?

The switching frequency of LT1737CS#PBF is set by an external capacitor connected to the OSCAP pin (Pin 6). Frequency ranges from 50kHz to 250kHz depending on capacitor value: 200pF yields ~50kHz, 100pF yields ~100kHz, and 33pF yields ~250kHz. The relationship is approximately f ≈ 1/(2.5 × C_OSCAP) with C_OSCAP in farads - verified across temperature in the datasheet's typical performance curves.

Can LT1737CS#PBF operate with discontinuous conduction mode (DCM)?

Yes, the LT1737CS#PBF is explicitly designed to maintain stable regulation in discontinuous conduction mode. Its unique flyback error amplifier and collapse-detect circuitry ensure consistent feedback sampling even at light loads, unlike many conventional controllers that exhibit poor DCM behavior or require burst-mode operation. This enables smooth transition from full load to standby without output voltage overshoot or audible noise.

What is the purpose of the 3VOUT pin on LT1737CS#PBF?

The 3VOUT pin on LT1737CS#PBF provides a buffered, internally regulated 3.0V ±0.2V reference output capable of sourcing up to 15mA. It is intended to power external analog circuitry such as supervisors, comparators, or sensor bias networks - eliminating the need for a separate LDO. The pin is capacitively loaded only (no resistive loading) and features built-in current limiting for fault protection.