Analog Devices Inc. LT1241CS8#PBF

Overview

LT1241CS8#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 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 LT1241CS8#PBF datasheet, LT1241CS8#PBF pinout, LT1241CS8#PBF application, or LT1241CS8#PBF equivalent, key selection criteria include undervoltage lockout threshold (9.6V start-up), oscillator trim accuracy (±5% freq), current sense blanking (100ns), 5V reference tolerance (±1%), and SO-8 package thermal resistance (150°C/W).

Technical Context

The LT1241CS8#PBF implements a fixed-frequency current-mode control architecture with a trimmed 50kHz–500kHz oscillator, where RT/CT sets both frequency and deadtime via a 8.2mA sink current. Its error amplifier delivers 90dB DC gain and 1MHz unity-gain bandwidth, with internal 2.5V reference tied to FB pin.

Current sensing uses a comparator with leading-edge blanking (100ns at VFB = 2.5V), 1V clamp on ISENSE input, and trip threshold proportional to COMP voltage. Output stage provides ±1A drive with 40ns rise/30ns fall times into 1nF, clamped to 18V high-level and actively pulled low (<1V) during UVLO.

Key Specifications

Pinout & Package

S8 package: 8-lead plastic SOIC, 150°C/W θJA, operating temperature range 0°C to 100°C.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UC1842N8#PBF and UCC3803D, the LT1241CS8#PBF delivers superior high-frequency capability (500kHz), lower start-up loss, and built-in noise immunity-making it optimal for compact, high-efficiency flyback and forward converters where layout simplicity and transient response matter.

Availability

LT1241CS8#PBF is available at Aetrix Electronics and suitable for AC/DC flyback converters, isolated DC/DC forward supplies, and industrial off-line SMPS requiring stable component supply and long-term manufacturability.

Supply support for LT1241CS8#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 power management semiconductors.

The LT1241 series belongs to Linear's legacy PWM controller product line, designed specifically for high-speed, current-mode switching power supplies targeting off-line and DC/DC conversion with minimal external components.

FAQ

What is the maximum switching frequency supported by the LT1241CS8#PBF?

The LT1241CS8#PBF supports up to 500kHz output switching frequency, enabled by its trimmed oscillator and elimination of cross-conduction current spikes. Its oscillator runs at up to 1MHz, allowing 500kHz output for LT1241, LT1244, and LT1245 variants. This is confirmed in the Applications Information section (Rev B, p.9), which states "The oscillator of LT1241 series devices will run at frequencies up to 1MHz, allowing 500kHz output switching frequencies for all devices." The LT1241CS8#PBF achieves this with 40ns rise and 30ns fall times into 1nF load.

Does the LT1241CS8#PBF include built-in current sense blanking, and how does it function?

Yes, the LT1241CS8#PBF includes a built-in leading-edge blanking circuit at the current sense comparator output. Blanking duration is 100ns under normal operation (VFB = 2.5V) and scales down to zero when FB is pulled to 0V-minimizing blanking during start-up or short-circuit faults. This feature prevents false triggering from transformer interwinding capacitance and diode reverse recovery spikes, eliminating the need for external RC filters at the ISENSE pin in most applications. The blanking circuit is explicitly detailed in the Applications Information section (Rev B, p.11).

What is the undervoltage lockout behavior of the LT1241CS8#PBF, and how does it affect VREF and OUTPUT pins?

During undervoltage lockout, the LT1241CS8#PBF actively pulls both VREF (Pin 8) and OUTPUT (Pin 6) low using Darlington-connected PNP transistors, clamping them to approximately 1V. This eliminates the need for external pull-down resistors on either pin. The start-up threshold is 9.6V and minimum operating voltage is 7.6V, with 2.0V hysteresis. This behavior is documented in the Absolute Maximum Ratings table (p.2), Electrical Characteristics (p.4), and Applications Information (p.11), confirming active low assertion rather than high-impedance tri-state.

Can the LT1241CS8#PBF drive a 600V MOSFET directly, and what output protection features exist?

Yes, the LT1241CS8#PBF can directly drive medium-to-high voltage MOSFETs, including 600V types, thanks to its ±1A totem-pole output stage and 18V high-level output clamp. The clamp prevents gate oxide damage by limiting V_GS to ≤18V, while the 40ns rise/30ns fall times ensure fast switching. Additionally, the output is actively pulled low (~1V) during UVLO, preventing unintended turn-on. These specifications are verified in the Absolute Maximum Ratings (p.2), Electrical Characteristics (p.4), and Applications Information (p.11).

How is the oscillator frequency set on the LT1241CS8#PBF, and what parameters are trimmed for accuracy?

The oscillator frequency of the LT1241CS8#PBF is set externally using resistor RT (connected from VREF to RT/CT) and capacitor CT (from RT/CT to GND). Both oscillator frequency and sink current are trimmed: initial frequency accuracy is ±5% (e.g., 50kHz ±2.5kHz), and oscillator sink current is trimmed to 8.2mA ±0.3mA. This tight specification minimizes variation in frequency and deadtime across units and temperature. Details appear in the Electrical Characteristics table (p.3) and Applications Information (p.9–10).