LR645

DS20005384A-page 6

 2015 Microchip Technology Inc.

FIGURE 3-3:

TYPICAL OUTPUT

VOLTAGE VS RESISTOR

RATIO

3.3

Off Line Linear Regulator

Circuits that require low voltages to operate logic and

analog circuits benefit from LR645. The conventional

use of step-down transformers can be eliminated,

thereby saving space and cost. Some examples of low-

voltage applications are: proximity controlled light

switches, street lamp controls, and low-voltage power

supplies for appliances such as washing machines,

dishwashers, and refrigerators.

The wide operating-input voltage range of 15 to 450V,

as well as the ripple rejection ratio of 50dB minimum,

allows the use of a small, high-voltage input capacitor.

The input AC line can be either full-wave or half-wave

rectified. A minimum output capacitance of 0.01µF is

recommended for output stability.

Figure 3-4 shows the LR645 as a pre-regulator to a

precision regulator for high precision regulation. Higher

output current is also possible by using an external

depletion-mode MOSFET DN2540N5 as shown in

Figure 3-5.

FIGURE 3-4:

CASCADING FOR

PRECISION

3.4

Power Dissipation Considerations

LR645 is a true linear regulator. Its power dissipation is

therefore a function of input voltage and output load

current. For example, if the LR645 provides a continu-

ous load current of 3mA at 10V, while its input voltage

is 400V, total dissipation in the LR645 will be:

The 1.23 watts is for continuous operation. This is

within the dissipation capabilities of the TO-220 and

SOT-89 packages. See Table 1-2 on Page 3 for derat-

ings. For SMPS start-up applications, the output cur-

rent is usually required only during start-up. This

duration depends upon the auxiliary supply output

liseconds. All package types of the LR645 have been

an AC line of 277V.

FIGURE 3-5:

HIGH-CURRENT REGULATION

12

10

8

2.5

3.0

3.5

4.0

Resistor Ratio (R2/R1)

R1 + R2 = 250kΩ

5.000V

± 0.002V

@

0 to 3mA

AC Line

24V - 277V

C

OUT

0.1μF

C

IN

1.0μF

LR6

Max

875

ACSA

–



+





400V 10V

–



3.0mA 150

A

+





1.23Watts

=

=

=

AC Line

24 - 277V

5.0V

+

0 - 150mA

-

5.0V

REG

C

OUT

C

IN

V

OUT

GATE

DN2540N5

GND

LR645