 2015 Microchip Technology Inc.

DS20005394A-page 1

LR745

Features

• Accepts inputs from 35 to 450V

• Output current limiting

• For PWM ICs with start-up threshold voltage of

13.9 - 18.0V

• Very low power consumption after start-up

Applications

• Notebook and laptop computers

• Telecommunication power supplies

• Battery chargers

• Motor controllers

Description

LR745 is a high input voltage SMPS start-up circuit.

LR745 is ideally suited for use with industry standard

low-voltage, Pulse-Width Modulation (PWM) ICs hav-

ing start thresholds of 13.9 to 18.0V. It allows the PWM

ICs to be operated from rectified 120 or 240VAC lines,

and eliminates the use of power resistors often used for

this purpose.

The internal circuitry of the LR745 allows the PWM ICs

voltage after start-up. The auxiliary voltage can be less

than the start-threshold voltage, which allows for

improved efficiency. Current from the high voltage line

is drawn only during the start-up period. After start-up,

the internal, high-voltage line is disconnected from the

IC, thereby reducing the continuous power dissipation

to a minimum.

High-Input Voltage SMPS Start-up