Analog Devices Inc. LT1241IS8#PBF

Overview

LT1241IS8#PBF from Analog Devices is an 8-pin, fixed-frequency, current-mode PWM controller IC optimized for off-line and DC/DC converters. It features <250µA start-up current, 50ns current sense delay, 500kHz max output switching frequency, 1A totem-pole output, and active pull-down during undervoltage lockout. It drives power MOSFETs in isolated flyback and forward converter topologies.

For engineers reviewing the LT1241IS8#PBF datasheet, LT1241IS8#PBF pinout, LT1241IS8#PBF application, or LT1241IS8#PBF equivalent, key selection criteria include its –40°C to 100°C operating range, S8 plastic SO package, 9.6V UVLO start threshold, 50% max duty cycle, and compatibility with UC1842-series designs requiring improved speed and lower quiescent current.

Technical Context

The LT1241IS8#PBF implements a fixed-frequency current-mode control architecture with a trimmed oscillator (±5% initial accuracy), leading-edge blanking on the current sense comparator, and internal 1V clamp limiting peak sense voltage. Its error amplifier provides 90dB DC gain and 1MHz unity-gain bandwidth, referenced to a 2.5V internal node derived from the 5V bandgap reference.

Undervoltage lockout includes hysteresis (2.0V) and actively pulls down both VREF (Pin 8) and OUTPUT (Pin 6) via Darlington PNP transistors (~1V saturation). The 1A totem-pole output stage eliminates cross-conduction spikes and clamps high-level output to 18V, enabling direct MOSFET gate drive without external level-shifting.

Key Specifications

Pinout & Package

S8 package: 8-lead plastic SOIC, 150°C/W junction-to-ambient thermal resistance, surface-mount compatible, RoHS-compliant lead-free finish.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UC3842N and TL494CN, the LT1241IS8#PBF delivers superior high-frequency stability via built-in blanking and tighter oscillator tolerance, enables higher power density through 500kHz operation and 1A drive, and reduces BOM count by integrating UVLO pull-down and output clamping - all within the same 8-pin SO footprint.

Availability

LT1241IS8#PBF is available at Aetrix Electronics and suitable for industrial off-line power supplies, telecom DC/DC converters, universal AC/DC adapters, and high-density SMPS designs requiring stable component supply, extended temperature operation, and long-term lifecycle support.

Supply support for LT1241IS8#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 (now part of Analog Devices, Inc. following merger with Linear Technology) is a global leader in high-performance analog, mixed-signal, and digital signal processing semiconductors.

The LT1241 series was designed specifically for high-efficiency, high-reliability current-mode switch-mode power supplies - emphasizing low start-up current, noise-immune current sensing, and robust gate drive for industrial and telecom power conversion.

FAQ

What is the maximum operating frequency of the LT1241IS8#PBF?

The LT1241IS8#PBF supports oscillator frequencies up to 1MHz, enabling 500kHz output switching frequency (due to divide-by-2 architecture in LT1241 variant). This is confirmed in the Applications Information section (Rev B, p.9), where it states "The oscillator of LT1241 series devices will run at frequencies up to 1MHz, allowing 500kHz output switching frequencies for all devices." The LT1241IS8#PBF is rated for 500kHz operation under full temperature and load conditions.

Does the LT1241IS8#PBF require external components for current sense blanking?

No, the LT1241IS8#PBF integrates a fixed 100ns leading-edge blanking circuit at the current sense comparator output, eliminating the need for external RC filters. As stated in the Applications Information (Rev B, p.11), "The LT1241 series devices blank (lock out) the signal at the output of the current sense comparator for a fixed amount of time after the switch is turned on," and "This blanking circuit eliminates the need for an input filter at the current sense input except in extreme cases."

What is the purpose of the active pull-down on VREF and OUTPUT during undervoltage lockout?

During undervoltage lockout, the LT1241IS8#PBF actively pulls both VREF (Pin 8) and OUTPUT (Pin 6) low via internal Darlington PNP transistors (~1V saturation). This ensures the external MOSFET gate remains safely discharged and prevents unintended switching. Per the Applications Information (Rev B, p.11), this "eliminates the need for an external pull-down resistor which was required in older designs" and allows the VREF pull-down to reset external soft-start capacitors.

Can the LT1241IS8#PBF drive GaN FETs directly?

The LT1241IS8#PBF can drive enhancement-mode GaN FETs directly if their gate threshold and drive voltage requirements fall within its specifications: 18V output clamp, ±1A drive strength, and 30ns/50ns typical fall/rise times into 1nF. However, GaN FETs often require negative turn-off voltage or faster edge rates; the LT1241IS8#PBF's 18V clamp and lack of negative drive mean it is best suited for standard Si MOSFETs unless supplemental gate driving circuitry is added.

How does the LT1241IS8#PBF differ from the UC1842 in terms of start-up current?

The LT1241IS8#PBF reduces start-up current to <250µA (170µA typical), compared to ~1mA for the UC1842. This is explicitly stated in the Features list ("Low Start-Up Current: < 250µA") and Electrical Characteristics table (Start-Up Current: 170–250µA). Lower start-up current minimizes power loss in the high-value start-up resistor used in off-line converters, improving no-load efficiency and thermal performance.