LR645 datasheet(6/24 Pages) MICROCHIP

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DS20005384B-page 6

LR645

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 50 dB 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 0.01 µF 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

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

voltage is 400V, total dissipation in the LR645 can be

calculated using Equation 3-1.

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

deratings. For SMPS start-up applications, the output

current is usually required only during start-up. This

duration depends upon the auxiliary supply output

capacitor and COUT, but is typically a few hundred

milliseconds. All package types of the LR645 have

been characterized for use with a COUT of at least

10 µF, and an AC line of 277V.

EQUATION 3-1:

FIGURE 3-5:

High-Current Regulation.

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

OUT

0.1μF

1.0μF

LR6

Max

875

ACSA

PDISS

VIN VOUT

–



IOUT IMAXQuiescent





400V 10V

–



3.0 mA 150

A





1.23 Watts

AC Line

24 - 277V

5.0V

0 - 150mA

5.0V

REG

OUT

GATE

DN2540N5

GND

LR645

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