LR8
LR8
Application Note
AN-H40
High Voltage Linear Regulators and
Constant Current Sources Using LR8
by Scott Lynch, Senior Applications Engineer
Introduction
The LR8 is a high voltage 3-terminal adjustable linear regulator.
Intended for operation directly off rectified AC mains, the LR8
operates at input voltages up to 450 volts, making it compatible
with line voltages up to 240 VAC. It’s output voltage adjustability
assures that it can be used in most any application. The LR8 is
ideally suited for low power off-line DC power supplies and
SMPS start-up circuits.
Available in a leaded TO-92 package and a surface mount
SOT-89 and D-PAK package, it is ideal for applications where
space is at a premium.
Relevant specifications are shown in the table below.
Operation
Except for it’s higher voltage rating, the LR8 operates like any
other 3-terminal adjustable linear regulator.
A simple resistive divider sets the output voltage while a capacitor
at the output improves transient response and ensures regulator
stability. When applicable, an input capacitor is required to
provide energy storage for rectified AC.
Keeping in mind that the LR8 requires at least a 10V difference
between input and output for proper operation, the minimum
value for C
IN
is:
C
IN
>
I
LOAD
t
V
IN
(
pk
)
−
V
OUT
−
10V
LR8 Specifications
Input Voltage Range
Output Voltage Range
Output Voltage Accuracy
Power Dissipation
(V
OUT
+10V) to 450V
1.2V to (V
IN
- 10V)
±5%
TO-92:
0.74W
TO-243AA (SOT-89): 1.6W
TO-252 (D-PAK):
2.5W
0.5 to 10mA
3%
0.01%/V
60dB typ @120Hz
where
I
LOAD
= Load current
t
= Time between peaks of input
waveform
V
IN(pk)
= Peak input voltage
V
OUT
= Output voltage
Output Current
Load Regulation
Line Regulation
Supply Rejection
Note that the LR8 requires a minimum of 0.5mA load current for
proper operation. The current through the resistive divider may
be included as part of the minimum load.
LR8 Block Diagram and Typical Application
Needed for protection should V
OUT
exceed V
IN
Supply
V
IN
Pass
Element
LR8
R
V
OUT
=
1
.
2
v
1
+
2
+
10
µ
A •
R
2
R
1
V
OUT
Load
R
1
C
IN
Overtemp &
Overcurrent
1.2V
10µA
ADJ
C
OUT
≥
1µF
R
2
8
SUPERTEX [ Supertex, Inc ]
