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.
Constant Current LED Driver
NiCad Battery Trickle Charger
R
R
LR8
LR8
15V
to
450V
15V
to
450V
Start-up Circuit
The schematic below depicts a simplified off-line switching power supply using the LR8 for start-up. When VBOOT rises above the LR8’s
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.
Startup Current for Off-line Switching Power Supply
bootstrap
winding
VBOOT
D1
VIN
VOUT
PWM
Controller
LR8
AC
Mains
R1
R2
COUT
CIN
9
SUPERTEX [ Supertex, Inc ]
