 2015 Microchip Technology Inc.

DS20005384A-page 5

LR645

3.0

FUNCTIONAL DESCRIPTION

3.1

SMPS Start-Up Circuit

One of the main applications for LR645 is a start-up cir-

cuit for off-line, switch-mode power supplies (SMPS),

as shown in Figure 3-1. A minimum output capacitance

of 10 nF is recommended for stability. The wide operat-

ing, input voltage range of LR645 allows the SMPS to

operate and start-up from rectified AC, or a DC voltage

of 15 to 450V, without adjustment.

Pulse-Width Modulation (PWM) IC with a nominal out-

put voltage of 10V. The auxiliary voltage connected

increase. When the auxiliary voltage becomes larger

than the output voltage LR645 turns OFF both its inter-

nal high voltage input line and output voltage, allowing

IC. After startup, LR645 doesn’t draw any input current

from the high-voltage line other than the leakage cur-

rent of the internal MOSFET switch, which is typically

0.1µA.

The 3-terminal version shown in Figure 3-1 has load

regulation guaranteed from 0 to 3.0mA at a fixed nom-

inal output voltage of 10V. Applications requiring higher

output current and/or a different output voltage can use

the 8 pin adjustable version.

FIGURE 3-1:

SMPS START-UP CIRCUIT

3.2

High-Current SMPS Start-Up

Circuit

The 8-lead version of LR645 has connections for an

external depletion-mode MOSFET for higher-output

current and external resistors for adjustable-output

voltage. As shown in Figure 3-2, the output current is

increased to 150mA by using the DN2540, a 400V

depletion-mode MOSFET. The maximum operating

input voltage will be limited by the drain-to-source,

breakdown voltage of the external MOSFET, but can-

not exceed the 450V rating of LR645.

The output voltage can be adjusted from 8 to 12V with

two external resistors: R1 and R2. The ratio of R2/R1

determines the output voltage. R2 is connected

between TRIM and GND pins. Figure 3-3 is a curve

showing output voltage versus resistor ratio R2/R1.

The optimum range for R1 + R2 is 200KΩ to 300KΩ.

This minimizes loading and optimizes accuracy of the

output voltage. Figure 3-3 uses an R1 + R2 of 250KΩ.

FIGURE 3-2:

HIGH-CURRENT SMPS START-UP CIRCUIT

+

15 - 450V

-

+

5.0V

-

VCC

PWM IC

LR6

GND

VIN

R2

R1

+

15V

to

400V

–

+

5.0V

–

DN2540

VCC

V

Note:

When used with the DN25, +V

PWM IC

LR645

VOUT

GND

GATE

TRIM