Microchip Technology LR745N3-G

Overview

LR745N3-G from Microchip Technology is a high-input-voltage SMPS start-up circuit designed to power PWM ICs during initial startup only. It accepts 35–450V DC input, delivers regulated 18.0–24.0V output with 3.0mA current limiting, and operates in TO-92 package for notebook, telecom, and battery charger power supplies.

For engineers reviewing the LR745N3-G datasheet, LR745N3-G pinout, LR745N3-G application, or LR745N3-G equivalent, key selection criteria include its 13.25V turn-off threshold, 7.0V reset voltage, ultra-low 500µA quiescent current at 400V, and compatibility with PWM ICs having 13.9–18.0V start thresholds.

Technical Context

The LR745N3-G implements a self-contained start-up controller with dual comparators (7.0V reset and 13.25V turn-off thresholds), internal 3.0mA constant-current source, and high-voltage MOSFET switch (M1) controlled by D-flip-flop logic. It draws current only during startup and disconnects VIN after VOUT exceeds VOFF.

Its operation is defined by three stages: Stage I charges the VCC capacitor until PWM IC start threshold (~16V); Stage II turns off when VOUT drops below 13.25V; Stage III sustains PWM IC operation via auxiliary winding while LR745N3-G remains inactive with <10µA input leakage.

Key Specifications

Pinout & Package

LR745N3-G is housed in a 3-lead TO-92 package with axial lead configuration and Pb-free matte tin plating (ROHS-compliant). Thermal resistance θ_JA is 132°C/W.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar SMPS start-up circuit applications.

Compared with LR745N3-G, LR745N8-G offers identical electrical performance in an SMT package but demands layout revision, while ICE2QS03G integrates start-up and PWM functions at the cost of design flexibility and component-level troubleshooting access.

Availability

LR745N3-G is available at Aetrix Electronics and suitable for notebook adapters, telecom power supplies, and battery charger designs requiring stable component supply with ROHS-compliant TO-92 packaging and long-term industrial availability.

Supply support for LR745N3-G 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

Microchip Technology is a U.S.-based semiconductor company specializing in microcontrollers, analog devices, and power management ICs, with ISO/TS-16949 certified manufacturing facilities.

The LR745 product line was developed specifically for high-voltage SMPS start-up applications where efficiency, reliability, and compatibility with standard PWM controllers are critical.

FAQ

What is the primary function of the LR745N3-G in a switch-mode power supply?

The LR745N3-G serves exclusively as a high-voltage start-up circuit that supplies regulated power to the VCC pin of a PWM IC during initial power-on. Once the PWM IC begins switching and its auxiliary winding generates sufficient voltage, the LR745N3-G automatically disconnects its output to eliminate continuous power loss. This behavior is confirmed in DS20005394A Sections 1.1 and 3.0, where the device is explicitly described as "a high input voltage SMPS start-up circuit" with no operational role beyond startup.

Can the LR745N3-G be used with PWM ICs having a start threshold outside the 13.9–18.0V range?

No - the LR745N3-G is specifically characterized and guaranteed only for PWM ICs with start thresholds between 13.9V and 18.0V, as stated in the official Microchip datasheet (DS20005394A, page 1). Its VOUT turn-off voltage is fixed at 13.25V (typ), and reset occurs at 7.0V; using it with ICs requiring lower or higher startup voltages may cause unreliable handoff or failure to initiate operation.

How does the LR745N3-G achieve low power consumption after startup?

The LR745N3-G achieves ultra-low post-startup power consumption by completely disconnecting its high-voltage input (VIN) from internal circuitry once VOUT falls below 13.25V. As documented in Section 3.2, this reduces input current from 3.0mA to just 75µA (max) at 400V, effectively eliminating resistive startup losses. The internal MOSFET (M1) is turned off, leaving only leakage paths active.

What is the maximum allowable input voltage for the LR745N3-G, and what happens beyond that limit?

The absolute maximum input voltage for the LR745N3-G is 450V DC, per Table 1-1 in DS20005394A. Exceeding this rating risks permanent damage, as noted in the Absolute Maximum Ratings section: "Stresses above those listed… may cause permanent damage." The device contains no overvoltage protection; system-level clamping (e.g., MOVs or TVS diodes) must be implemented externally to prevent transient excursions above 450V.

Is the LR745N3-G pin-compatible with other members of the LR745 family, such as LR745N8-G?

No - although LR745N3-G (TO-92) and LR745N8-G (SOT-89) share identical electrical specifications and functional behavior, they are not pin-compatible due to differing package geometries and terminal arrangements. The TO-92 has axial leads with VIN–GND–VOUT order, while SOT-89 uses gull-wing leads with VIN–GND–VOUT in a surface-mount footprint. Board redesign is required to substitute one for the other.