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LR645

High Input Voltage SMPS Start-up / Linear Regulator

Supertex Inc 

LR645

LR645: High Current SMPS Start-Up Circuit

The 8 lead version of the LR645 has connections for an ex-

ternal 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 the LR645.

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

ternal resistors, R1 and R2. The ratio of R2/R1 determines

the output voltage. R2 is connected between the VOUT and

TRIM pins. 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 200KΩ to

300KΩ. This minimizes loading and optimizes accuracy of

the output voltage. Figure 5 uses an R1 + R2 of 250KΩ.

Figure 2: High Current SMPS Start-Up Circuit

+

DN2540

15V

to

400V

CIN

–

GATE

LR645

VOUT

R2

TRIM

GND

R1

COUT

Note: When used with the DN25, +VIN is not connected on the LR6.

VCC

PWM IC

+

5.0V

–

VAUX = 12V

LR645: Off Line Linear Regulator

Circuits requiring low voltages to operate logic and analog

circuits benefit from the LR645. The conventional use of

step down transformers can be eliminated, thereby saving

space and cost. Some examples of these 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 15V 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: Cascading for Precision

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

regulator for high precision regulation. Higher output cur-

rent is also possible by using an external depletion-mode

MOSFET DN2540N5 as shown in Figure 4.

Power Dissipation Considerations

The LR645 is a true linear regulator. Its power dissipation is-

therefore a function of input voltage and output load current.

Forexample, if the LR645 is providing a continuous load cur-

rent of 3mAat 10V while its input voltage is 400V, total dis-

sipation in the LR645 will be:

PDISS= (VIN - VOUT) x (IOUT + IMAX ) QUIESCENT

= (400V - 10V) x (3.0mA + 150µA)

= 1.23 Watts

AC Line

24V

to

277V

LR6

CIN

1.0µF

Max

875

ACSA

COUT

0.1µF

5.000V

± 0.002V

@

0 to 3mA

The 1.23 watts is for continuous operation. This is within the

dissipation capabilities of the TO-220 and SOT-89 packag-

es. See the thermal characteristics chart on page 2 for derat-

ings. 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 10µF, and an AC line of 277V.

4

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