Analog Devices Inc. LT1725IGN#TRPBF

Overview

LT1725IGN#TRPBF from Analog Devices (formerly Linear Technology) is a current-mode isolated flyback controller IC designed for telecom and industrial offline power supplies. It drives an external MOSFET, senses output voltage directly from the primary-side third winding (eliminating optoisolators), supports 36V–72V input, delivers up to 2A at 5V output, and operates at 50kHz–250kHz with external timing capacitor.

For engineers reviewing the LT1725IGN#TRPBF datasheet, LT1725IGN#TRPBF pinout, LT1725IGN#TRPBF application, or LT1725IGN#TRPBF equivalent, key selection considerations include its primary-side regulation architecture, UVLO programmability via external resistor divider, soft-start implementation on SFST pin, load compensation capability via ROCMP/RCMPC, and 16-pin SSOP package thermal performance (θ_JA = 100°C/W).

Technical Context

The LT1725IGN#TRPBF implements a specialized flyback error amplifier that samples the reflected flyback pulse during transformer reset-not a continuous DC feedback signal-enabling accurate regulation without secondary-side sensing. Its control loop integrates VC pin current from a transconductance amplifier (400–1800 µmho) and uses collapse-detect logic to terminate feedback window based on 80% V_FB threshold.

Timing functions are fully resistor-programmable: t_ON (min switch-on time), ENDLY (enable delay), MINENAB (min enable time), and OSCAP (switching frequency). Load compensation is implemented via external ROCMP resistor and RCMPC capacitor to counteract output impedance effects from diode VF, winding resistance, and capacitor ESR across load transients.

Key Specifications

Pinout & Package

LT1725IGN#TRPBF is housed in a 16-lead plastic SSOP (GN) package with 0.65mm lead pitch and θ_JA = 100°C/W. Pin 1 is PGND; pin 16 is GATE. The package is RoHS-compliant and lead-free (#PBF suffix).

Key Features

Applications

Equivalent & Alternatives

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

Compared with UCC28740DR and MAX17596ATP+T, the LT1725IGN#TRPBF uniquely combines primary-side regulation without optoisolator, programmable load compensation, and a dedicated 3V auxiliary output-making it optimal for cost-sensitive, space-constrained telecom and industrial flyback designs requiring robust light-load regulation and auxiliary bias capability.

Availability

LT1725IGN#TRPBF is available at Aetrix Electronics and suitable for telecom isolated converters, offline isolated power supplies, and instrumentation power supplies requiring stable component supply, long-term lifecycle support, and guaranteed RoHS-compliant sourcing.

Supply support for LT1725IGN#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 healthcare markets.

The LT1725IGN#TRPBF belongs to Linear's legacy isolated power controller product line, engineered specifically for primary-side regulated flyback topologies in telecom and industrial applications where optoisolator elimination, wide input range, and programmable timing are critical.

FAQ

What is the maximum input voltage range supported by the LT1725IGN#TRPBF?

The LT1725IGN#TRPBF itself operates from a V_CC supply of 14V to 22V, but the application input voltage (VIN) is limited only by external power path components-typically 36V to 72V as confirmed in the datasheet's typical application schematic. This wide range is enabled because V_CC is derived from a third transformer winding, not directly from VIN. The LT1725IGN#TRPBF does not regulate VIN; it regulates the output based on reflected flyback pulses, making it suitable for high-input telecom systems.

Does the LT1725IGN#TRPBF require an optoisolator for feedback?

No, the LT1725IGN#TRPBF does not require an optoisolator. It achieves isolated output regulation by sensing the flyback voltage from a dedicated third winding on the transformer and scaling it via an external resistor divider to the FB pin. This primary-side sensing architecture eliminates the need for secondary-side feedback components-including optoisolators and TL431 references-reducing cost, board space, and certification complexity. The LT1725IGN#TRPBF's internal flyback error amplifier and collapse-detect logic make this approach viable across continuous and discontinuous conduction modes.

How is the switching frequency set on the LT1725IGN#TRPBF?

The switching frequency of the LT1725IGN#TRPBF is set by an external capacitor connected to the OSCAP pin (Pin 6). A 100pF capacitor yields a typical frequency of 100kHz; values from 33pF to 200pF scale frequency from ~80kHz to ~125kHz. Unlike fixed-frequency controllers, the LT1725IGN#TRPBF's oscillator is designed for direct capacitor-based programming-no external resistors or complex timing networks are needed. This simplifies EMI filtering design and allows frequency optimization for transformer size, core loss, and regulatory compliance without changing the LT1725IGN#TRPBF's internal configuration.

What is the purpose of the ROCMP and RCMPC pins on the LT1725IGN#TRPBF?

The ROCMP (Pin 4) and RCMPC (Pin 5) pins implement optional load compensation to correct for output voltage droop caused by real-world secondary-side impedances-including Schottky diode forward voltage, transformer winding resistance, and output capacitor ESR. ROCMP accepts a resistor that develops a current proportional to average switch current; RCMPC connects a 0.1µF capacitor to filter this signal. Together, they inject compensating current into the FB node, raising the effective reference voltage under load to maintain tight regulation. This feature is unique to the LT1725IGN#TRPBF among primary-side controllers and is especially valuable in high-current, low-voltage applications like 5V/2A telecom supplies.

Can the LT1725IGN#TRPBF drive a logic-level MOSFET directly?

Yes, the LT1725IGN#TRPBF can drive logic-level MOSFETs directly. Its GATE pin (Pin 16) delivers 12.1V output high at 100mA sink/source capability and has ≤30ns rise/fall times into 1000pF. This is sufficient to fully enhance logic-level devices such as the IRF620 (V_GS(th) = 2–4V) without external level-shifting. However, a small series gate resistor (≥5Ω) is recommended to dampen ringing in non-ideal PCB layouts. The LT1725IGN#TRPBF's gate driver is optimized for discrete external FETs-not integrated high-side switches-making it ideal for flexible, high-efficiency flyback designs where MOSFET selection is application-specific.