3

LR645

LR645: SMPS Start-Up Circuit

Symbol

Parameter

Min

Typ

Max

Units

Conditions

I

AUX

Input current to V

OUT

-

-

200

µA

V

ΔV

ΔV

IN

50

60

-

dB

120Hz, No Load

e

n

-

25

-

µV

0.01 to 100KHz

I

PEAK

-

30

-

mA

C

V

AUX

External voltage applied to V

OUT

-

-

13.2

V

---

8-lead, adjustable output voltage version only:

Test conditions unless otherwise specified: T

V

OUT

8

-

12

V

No load

ΔV

OUT

-

200

400

mV

V

-

100

400

mV

V

Notes:

1. Guaranteed by design, not tested in production.

2. Pulse test duration < 1.0msec, Duty cycle < 2%

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

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

shown in Figure1. A minimum output capacitance of 0.01µF

is recommended for stability. The wide operating input volt-

age range of the LR645 allows the SMPS to operate and

start-up from rectified AC or a DC voltage of 15V to 450V

without adjustment.

During start-up, the LR645 powers the V

IC with a nominal output voltage of 10V. The auxiliary voltage

connected through a diode to the V

start to increase. When the auxiliary voltage becomes larger

than the output voltage the LR645 turns OFF its internal high

voltage input line and output voltage, allowing the auxiliary

voltage to power the V

rent drawn by the LR645 from the high voltage line after

start-up will therefore only be leakage current of the internal

MOSFET switch, which is typically 0.1µA.

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

tion 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.

Figure 1: SMPS Start-Up Circuit

+

15V

to

450V

–

+

5.0V

–

V

C

OUT

C

IN

V

CC

PWM IC

LR6

GND

V

IN