9

LR8

Constant Current Operation

The LR8 may be configured to provide a constant current output. The current is independent of both supply voltage and load impedance.

Constant current operation finds application in driving LEDs and trickle-charging NiCad batteries, as shown below. The trickle charger is

for applications that require battery backup (i.e. no cycling), such as emergency lights.

Start-up Circuit

output voltage, the LR8 goes into standby mode, consuming very little current. All current is then supplied from the bootstrap circuit rather

than from the high voltage source, increasing overall efficiency.

The output voltage of the LR8 should be set high enough above the minimum operating voltage of the PWM controller, yet low enough

to ensure the bootstrap circuit takes over after start-up.

With 240VAC input, instantaneous power dissipation can reach 3.4W (340VDC * 10mA). This level exceeds the LR8’s rating, but exists

for only as long as it takes for the supply to bootstrap. Thermal mass will prevent die temperature from rising quickly. If boot time is short,

die temperatures will not reach the overtemperature protection trip point. It is advisable to mount the LR8 on 2 oz. copper with an area

of at least 2.5 square centimeters.

Constant Current LED Driver

NiCad Battery Trickle Charger

Startup Current for Off-line Switching Power Supply

LR8

PWM

Controller

bootstrap

winding

AC

Mains

LR8

15V

to

450V

R

LR8

15V

to

450V

R