LR745 Datasheet, PDF (1/15 Pages) Supertex, Inc – High Input Voltage SMPS Start-up Circuit

LR745
High-Input Voltage SMPS Start-up
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
to operate at a V_CC voltage below their start-threshold
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.
 2015 Microchip Technology Inc.
DS20005394A-page 1