Analog Devices Inc. LT1245CN8#PBF

Overview

LT1245CN8#PBF from Analog Devices is an 8-pin PDIP-packaged, fixed-frequency current-mode PWM controller optimized for off-line and DC/DC converters. It delivers 1A totem-pole output drive, operates up to 500kHz switching frequency, features 50ns current sense delay with leading-edge blanking, and incorporates undervoltage lockout with 8.4V start-up threshold-enabling reliable gate driving of power MOSFETs in isolated flyback supplies.

For engineers reviewing the LT1245CN8#PBF datasheet, LT1245CN8#PBF pinout, LT1245CN8#PBF application, or LT1245CN8#PBF equivalent, key selection criteria include its 7.6V minimum operating voltage, ±1A output sink/source capability, 18V high-level output clamp, and pin compatibility with UC1845 for legacy design migration.

Technical Context

The LT1245CN8#PBF implements a fixed-frequency current-mode control architecture where oscillator timing is set by external RT/CT components, and peak current sensing is referenced to the error amplifier output via a 3× attenuation network. Its 50ns current sense comparator delay and 100ns blanking time suppress noise-induced false triggering during MOSFET turn-on transients.

Undervoltage lockout actively pulls down both VREF (Pin 8) and OUTPUT (Pin 6) to ~1V using Darlington PNP transistors, eliminating need for external pull-down resistors. The 5V bandgap reference is trimmed to ±1% initial tolerance and supplies bias to internal circuitry including the oscillator charging current source.

Key Specifications

Pinout & Package

N8 package: 8-lead plastic dual in-line package (PDIP), 0°C to 100°C operating temperature range, 130°C/W thermal resistance.

Key Features

Applications

Equivalent & Alternatives

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

Compared with UC1845N8 and LT1243CN8#PBF, the LT1245CN8#PBF offers superior noise immunity, lower quiescent loss, and enhanced gate drive capability-making it optimal for high-reliability, high-density off-line power supplies where startup efficiency and transient robustness are critical.

Availability

LT1245CN8#PBF is available at Aetrix Electronics and suitable for isolated flyback converters, universal-input AC-DC adapters, and industrial DC/DC buck-boost regulators requiring stable component supply and long-term lifecycle support.

Supply support for LT1245CN8#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 LT1245CN8#PBF belongs to the LT1241 series of current-mode PWM controllers, designed specifically for high-efficiency, noise-immune off-line and DC/DC power conversion with emphasis on MOSFET gate drive integrity and startup reliability.

FAQ

What is the maximum switching frequency supported by the LT1245CN8#PBF?

The LT1245CN8#PBF supports up to 500kHz output switching frequency, achieved by running its internal oscillator at 1MHz. This is confirmed in the Applications Information section (page 9), which states "The output switching frequency will be equal to one-half the oscillator frequency for LT1241, LT1244 and LT1245" and notes "The oscillator of LT1241 series devices will run at frequencies up to 1MHz." The 500kHz limit is a hard architectural constraint for this variant.

Does the LT1245CN8#PBF require external components for current sense filtering?

No, the LT1245CN8#PBF does not require external RC filtering at the ISENSE pin due to its built-in 100ns leading-edge blanking circuit. As stated in the Applications Information (page 11), "This blanking circuit eliminates the need for an input filter at the current sense input except in extreme cases." The blanking time is dynamically adjusted based on FB pin voltage and is minimized during start-up or short-circuit faults for faster fault response.

How does the LT1245CN8#PBF handle undervoltage lockout conditions?

During undervoltage lockout, the LT1245CN8#PBF actively pulls both OUTPUT (Pin 6) and VREF (Pin 8) low using internal Darlington PNP transistors, sinking several milliamps and pulling pins to ~1V. This eliminates the need for external pull-down resistors on either node. The UVLO has a start-up threshold of 8.4V and hysteresis of 0.4V, ensuring clean, chatter-free enablement above the minimum operating voltage of 7.6V.

Can the LT1245CN8#PBF drive a high-side N-channel MOSFET directly?

Yes, the LT1245CN8#PBF can drive a high-side N-channel MOSFET directly via its ±1A totem-pole OUTPUT pin (Pin 6), provided the MOSFET's gate charge and Miller plateau requirements fall within the driver's capability. Its 18V output clamp protects against gate overvoltage, and the 40ns typical rise time (with 1nF load) supports efficient switching. However, bootstrap or isolated gate drive remains necessary for floating high-side configurations like half-bridge topologies.

What is the purpose of the COMP pin on the LT1245CN8#PBF?

The COMP pin (Pin 1) is the output of the internal error amplifier and serves two key functions: (1) it provides the node for external loop compensation (e.g., Type II or III networks), and (2) it allows clamping of the current sense threshold voltage-by limiting COMP voltage between 1.4V and 4.4V, the effective ISENSE trip point can be reduced below 1V to limit peak current or sense resistor power dissipation. The pin sources ≥0.5mA and sinks ≥2mA.