3

PWM IC

LR645LG

GND

R2

R1

Gate

+

15V

to

400V

–

+

5.0V

–

Trim

DN2540

PWM IC

LR6

GND

+

15V

to

450V

–

+

5.0V

–

LR6: High Current SMPS

Start-Up Circuit

The 8 pin version of the LR6 has connections for an external

depletion-mode MOSFET for higher output current and external

resistors for adjustable output voltage. As shown in Figure 2, the

output current is increased to 150mA by using the Supertex 400V

depletion-mode MOSFET DN2540. The maximum operating

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

voltage of the external MOSFET, but cannot exceed the 450V

rating of LR6.

The output voltage can be adjusted from 8V to 12V with 2

external resistors, R1 and R2.The ratio of R2/R1 determines the

R1 is connected between Trim and GND pins. Figure 5 is a curve

showing output voltage versus resistor ratio R2/R1.The optimum

range for R1 + R2 is 200K

Ω to 300KΩ. This minimizes loading

and optimizes accuracy of the output voltage. Figure 5 uses an

R1 + R2 of 250K

Ω.

Figure 2: High Current SMPS Start-Up

Figure 1: SMPS Start-Up Circuit

LR6: SMPS Start-Up Circuit

One of the main applications for the LR6 is a start-up circuit for

off-line switch-mode power supplies (SMPS) as shown in Figure

1. A minimum output capacitance of 0.01

µF is recommended for

stability. The wide operating input voltage range of the LR6

allows the SMPS to operate and start-up from rectified AC or a

DC voltage 15V to 450V without adjustment.

a nominal output voltage of 10V.The auxiliary voltage connected

through a diode to the V

When the auxiliary voltage becomes larger than the output

voltage the LR6 turns OFF its internal high voltage input line and

output voltage allowing the auxiliary voltage to power the V

of the PWM IC. The input current drawn by the LR6 from the high

voltage line after start-up will therefore only be leakage current

of the internal MOSFET switch, which is typically 0.1

µA.

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

guaranteed from 0 to 3.0mA at a fixed nominal output voltage of

10V.Applications requiring higher output current and/or a different

output voltage can use the 8 pin adjustable version.

LR645

Note: When used with the DN25, +V