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LR8K4

High Input Voltage Adjustable 3-Terminal Linear Regulator

Supertex Inc 

LR8LR8

Application Note

AN-H40

High Voltage Linear Regulators and

Constant Current Sources Using LR8

by Scott Lynch, Senior Applications Engineer

Introduction

The LR8 is a high voltage 3-terminal adjustable linear regulator.

Intended for operation directly off rectified AC mains, the LR8

operates at input voltages up to 450 volts, making it compatible

with line voltages up to 240 VAC. It’s output voltage adjustability

assures that it can be used in most any application. The LR8 is

ideally suited for low power off-line DC power supplies and

SMPS start-up circuits.

Available in a leaded TO-92 package and a surface mount

SOT-89 and D-PAK package, it is ideal for applications where

space is at a premium.

Relevant specifications are shown in the table below.

LR8 Specifications

Input Voltage Range

(VOUT +10V) to 450V

Output Voltage Range

1.2V to (VIN - 10V)

Output Voltage Accuracy

±5%

Power Dissipation

TO-92:

0.74W

TO-243AA (SOT-89): 1.6W

TO-252 (D-PAK):

2.5W

Output Current

0.5 to 10mA

Load Regulation

3%

Line Regulation

0.01%/V

Supply Rejection

60dB typ @120Hz

Operation

Except for it’s higher voltage rating, the LR8 operates like any

other 3-terminal adjustable linear regulator.

A simple resistive divider sets the output voltage while a capacitor

at the output improves transient response and ensures regulator

stability. When applicable, an input capacitor is required to

provide energy storage for rectified AC.

Keeping in mind that the LR8 requires at least a 10V difference

between input and output for proper operation, the minimum

value for CIN is:

CIN

>

I LOAD

VIN( pk)

t

− VOUT

− 10V

where

I = Load current

LOAD

t = Time between peaks of input

waveform

V = Peak input voltage

IN(pk)

VOUT = Output voltage

Note that the LR8 requires a minimum of 0.5mA load current for

proper operation. The current through the resistive divider may

be included as part of the minimum load.

Supply

LR8 Block Diagram and Typical Application

Needed for protection should VOUT exceed VIN

VIN

Pass

Element

LR8

VOUT

VOUT

= 1.2v1 +

R2

R1





+ 10µA •

R2

Load

R1

CIN

Overtemp &

Overcurrent 1.2V

10µA

ADJ

COUT

≥ 1µF

R2

8

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