Analog Devices Inc. LT1245CS8#PBF

Overview

LT1245CS8#PBF from Analog Devices is an 8-pin, fixed-frequency, current-mode PWM controller IC optimized for off-line and DC/DC converters. It delivers <250µA start-up current, 50ns current sense delay, 500kHz max output switching frequency (1MHz oscillator), and 1A totem-pole output-enabling high-efficiency flyback and forward converter designs in industrial power supplies.

For engineers reviewing the LT1245CS8#PBF datasheet, LT1245CS8#PBF pinout, LT1245CS8#PBF application, or LT1245CS8#PBF equivalent, key selection criteria include undervoltage lockout threshold (8.4V start / 7.6V min), 50% maximum duty cycle, active pull-down during UVLO, and pin compatibility with UC1845 for legacy design migration.

Technical Context

The LT1245CS8#PBF implements a fixed-frequency current-mode control architecture with internal 5V bandgap reference, trimmed oscillator (±5% initial accuracy), and leading-edge blanking on the current sense comparator. Its error amplifier features 90dB DC gain and 1MHz unity-gain bandwidth, with feedback referenced to a stable 2.5V internal node.

Oscillator timing is set via external RT/CT network; output switching frequency equals half the oscillator frequency (e.g., 1MHz oscillator → 500kHz output). The 1A totem-pole output drives MOSFET gates directly, while active low pull-down on VREF and OUTPUT during undervoltage lockout eliminates need for external clamping resistors.

Key Specifications

Pinout & Package

S8 package: 8-lead plastic SOIC, RoHS-compliant, 0°C to 100°C operating range, θ_JA = 150°C/W.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UC1845N and UCC38C45D, the LT1245CS8#PBF offers superior noise immunity via built-in blanking, lower start-up loss, tighter oscillator accuracy, and integrated UVLO pull-down - reducing external component count and improving reliability in compact, high-density power designs.

Availability

LT1245CS8#PBF is available at Aetrix Electronics and suitable for industrial off-line power supplies, isolated DC/DC converters, and LED driver power stages requiring stable component supply, long-term lifecycle support, and RoHS-compliant packaging.

Supply support for LT1245CS8#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 - including LT1245CS8#PBF - was designed specifically for high-efficiency, high-reliability current-mode switching power supplies targeting industrial, telecom, and medical equipment where low start-up current, tight frequency control, and robust fault handling are critical.

FAQ

What is the UVLO behavior of the LT1245CS8#PBF?

The LT1245CS8#PBF features undervoltage lockout with 8.4V start-up threshold and 7.6V minimum operating voltage, plus 0.8V hysteresis. During UVLO, both the OUTPUT (Pin 6) and VREF (Pin 8) pins are actively pulled low by internal Darlington PNP transistors - sinking several milliamps and holding each pin near 1V. This eliminates need for external pull-down resistors and enables direct soft-start capacitor reset via VREF.

How does the LT1245CS8#PBF achieve 500kHz output switching?

The LT1245CS8#PBF achieves 500kHz output switching by running its internal oscillator at 1MHz and dividing that clock by two in the output stage logic. Oscillator frequency is set externally using RT (from VREF to RT/CT) and CT (from RT/CT to GND); the device's trimmed 8.2mA oscillator sink current ensures tight frequency tolerance across temperature and supply voltage variations.

Can the LT1245CS8#PBF drive large MOSFETs directly?

Yes, the LT1245CS8#PBF integrates a ±1A totem-pole output stage capable of directly driving power MOSFETs with gate charges up to ~30nC at 500kHz. Its 40ns rise and 30ns fall times into 1nF load, combined with 18V output clamp, prevent gate overvoltage and ensure clean switching - eliminating need for external gate drivers in most medium-power applications.

What is the purpose of the COMP pin on the LT1245CS8#PBF?

The COMP pin (Pin 1) on the LT1245CS8#PBF is the output of the internal error amplifier and serves two key functions: (1) it provides the node for external Type II or Type III loop compensation networks, and (2) it allows clamping of the error amplifier output to limit the maximum current sense threshold voltage below 1V - enabling precise control of peak switch current and short-circuit protection level.

Is the LT1245CS8#PBF pin-compatible with UC1845?

Yes, the LT1245CS8#PBF is explicitly designed as a plug-compatible upgrade to the UC1845. Pin assignments, functional roles, and electrical characteristics (including UVLO thresholds, max duty cycle, and output drive strength) match UC1845, allowing drop-in replacement in existing designs - while delivering improved performance including lower start-up current (<250µA vs 1mA), tighter oscillator accuracy, and integrated leading-edge blanking.