Analog Devices Inc. LT1243CS8#PBF

Overview

LT1243CS8#PBF from Analog Devices is a fixed-frequency, current-mode PWM controller IC optimized for off-line and DC/DC converters. It features 500 kHz max switching frequency, <250 µA start-up current, 1 A totem-pole output, 8.4 V undervoltage lockout start threshold, and active pull-down on VREF and OUTPUT during UVLO - enabling reliable MOSFET gate drive in isolated flyback and forward power supplies.

For engineers reviewing the LT1243CS8#PBF datasheet, LT1243CS8#PBF pinout, LT1243CS8#PBF application, or LT1243CS8#PBF equivalent, key selection criteria include its UC1843-compatible pinout, trimmed oscillator sink current (8.2 mA typ), 100% max duty cycle capability, leading-edge blanking (100 ns), and 18 V high-level output clamp - all critical for stable high-frequency current-mode control in industrial and telecom power designs.

Technical Context

The LT1243CS8#PBF implements a current-mode control architecture with cycle-by-cycle peak current limiting, where the error amplifier output (Pin 1) sets the current sense comparator threshold via a 3× attenuation and diode offset. Its oscillator uses a trimmed 8.2 mA sink current and external RT/CT network to set both frequency and deadtime, supporting up to 1 MHz oscillator operation for 500 kHz output switching.

Undervoltage lockout includes hysteresis (0.4 V) and actively pulls down both OUTPUT (Pin 6) and VREF (Pin 8) to ~1 V during fault conditions, eliminating need for external pull-down resistors. The built-in 100 ns blanking window at ISENSE (Pin 3) suppresses noise from transformer leakage and diode recovery without compromising loop response.

Key Specifications

Pinout & Package

S8 package: 8-lead plastic SOIC, 1.27 mm pitch, JEDEC MS-012AC compliant, operating junction temperature range –40°C to 100°C.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UC1843N and UCC2803D, the LT1243CS8#PBF delivers superior integration - combining UC1843 pin compatibility with advanced features like blanking, active UVLO pull-down, and trimmed oscillator parameters - reducing component count and improving reliability in high-frequency, high-noise power systems.

Availability

LT1243CS8#PBF is available at Aetrix Electronics and suitable for industrial AC/DC power supplies, telecom DC/DC converters, and automotive on-board chargers requiring stable component supply and long-term production continuity.

Supply support for LT1243CS8#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 LT1243CS8#PBF belongs to the LT124x series of current-mode PWM controllers, designed specifically for rugged, high-frequency switched-mode power supplies in off-line and DC/DC topologies where reliability, precision timing, and noise immunity are critical.

FAQ

What is the maximum operating frequency of the LT1243CS8#PBF?

The LT1243CS8#PBF supports up to 500 kHz output switching frequency, enabled by its 1 MHz oscillator capability and 100% maximum duty cycle mode. This is confirmed in the Applications Information section (Rev B, p.9), where LT1243 is explicitly listed with 100% duty cycle and 500 kHz practical operation. The oscillator can run up to 1 MHz, but output frequency equals oscillator frequency only for LT1243 - making 500 kHz the validated, widely implemented upper limit for stable operation.

Does the LT1243CS8#PBF require external components for undervoltage lockout handling?

No, the LT1243CS8#PBF does not require external components for undervoltage lockout handling. As stated in the Applications Information (Rev B, p.11), both VREF (Pin 8) and OUTPUT (Pin 6) are actively pulled low by internal Darlington PNP transistors during UVLO, sinking several milliamps and pulling down to ~1 V. This eliminates the external pull-down resistors needed with earlier controllers like UC1843, simplifying design and improving startup reliability.

How does the leading-edge blanking function in the LT1243CS8#PBF improve system stability?

The LT1243CS8#PBF incorporates a fixed 100 ns leading-edge blanking window at the ISENSE (Pin 3) comparator output, triggered at each OUTPUT turn-on edge. As documented in the Applications Information (Rev B, p.11), this blanking suppresses false triggering caused by transformer interwinding capacitance and diode reverse recovery spikes - eliminating the need for external RC filters that would slow loop response. This preserves fast current-loop bandwidth while preventing instability during transient events.

Is the LT1243CS8#PBF pin-compatible with the UC1843 series?

Yes, the LT1243CS8#PBF is pin-compatible with the UC1843 series. The Features section (Rev B, p.1) explicitly states "Pin Compatible with UC1842 Series", and the Applications Information table (p.9) confirms LT1243 replaces UC1843. Pin functions match exactly: Pin 1 = COMP, Pin 2 = FB, Pin 3 = ISENSE, Pin 4 = RT/CT, Pin 5 = GND, Pin 6 = OUTPUT, Pin 7 = VCC, Pin 8 = VREF - enabling direct substitution without PCB modification.

What is the purpose of the 18 V output clamp in the LT1243CS8#PBF?

The 18 V output clamp in the LT1243CS8#PBF limits the high-level voltage at the OUTPUT (Pin 6) to prevent damage to the gate oxide of external power MOSFETs. Per the Absolute Maximum Ratings (Rev B, p.2) and Electrical Characteristics (p.4), the clamp activates at 18 V (min) and is specified up to 19.5 V. This protection is especially critical in high-voltage gate-drive applications where ringing or overshoot could exceed MOSFET V_GS(max), ensuring robust operation without external Zener clamping.