The LR8 is a high voltage, adjustable linear regulator that can regulate output voltages from 1.2V to 440V provided the input voltage is at least 10V greater than the output voltage. It has a wide input voltage range of 12V to 450V and can source output currents from 0.5mA to 10mA with 5% output voltage tolerance. The LR8 uses an external resistor divider to set the output voltage and includes current limiting, temperature limiting, and short circuit protection. It is well suited for applications requiring high input voltages such as off-line power supplies, battery chargers, and motor controls.

1. LR8
LR8
High Input Voltage
Adjustable 3-Terminal Linear Regulator
Features General Description
❏ 12V to 450V input voltage range The Supertex LR8 is a high voltage, low output current, adjustable
linear regulator. It has a wide operating input voltage range of
❏ Adjustable 1.20V to 440V output regulation
12V to 450V. The output voltage can be adjusted from 1.20V to
❏ 5% output voltage tolerance 440V provided that the input voltage is at least 10V greater than
the output voltage. The output voltage can be adjusted by means
❏ Output current limiting
of two external resistors R1 and R2 as shown in the typical
❏ 10µA typical ADJ current application circuits. The LR8 regulates the voltage difference
❏ Internal junction temperature limiting between VOUT and ADJ pins to a nominal value of 1.20V. The
1.20V is amplified by the external resistor ratio R1 and R2. An
internal constant bias current of typically 10µA is connected to
the ADJ pin. This increases VOUT by a constant voltage of 10µA
Applications times R2.
❏ Off-line SMPS startup circuits The LR8 has current limiting and temperature limiting. The
output current limit is typically 15mA and the minimum temperature
❏ Adjustable high voltage constant current source limit is 125°C. An output short circuit current will therefore be
❏ Industrial Controls limited to 15mA. When the junction temperature reaches its
temperature limit, the output current and/or output voltage will
❏ Motor controls
decrease to keep the junction temperature from exceeding its
❏ Battery chargers temperature limit. For SMPS start-up circuit applications, the
LR8 turns off when an external voltage greater than the output
❏ Power supplies
voltage of the LR8 is applied to VOUT of the LR8. To maintain
stability, a bypass capacitor of 1.0µF or larger and a minimum DC
output current of 500µA are required.
The device is available in TO-92, TO-243AA (SOT-89), and
TO-252 (D-PAK) packages.
LR8 Block Diagram and Typical Application
*
VOUT
VIN VIN VOUT
ADJ
R1
LR8
C2
C1 RLOAD
R2
*Required for conditions where VIN is less than VOUT.
12/03/01
Supertex Inc. does not recommend the use of its products in life support applications and will not knowingly sell its products for use in such applications unless it receives an adequate "products liability
indemnification insurance agreement." Supertex does not assume responsibility for use of devices described and limits its liability to the replacement of devices determined to be defective due to
workmanship. No responsibility is assumed for possible omissions or inaccuracies. Circuitry and specifications are subject to change without notice. For the latest product specifications, refer to the
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1

2. LR8
Ordering Information
Package Options Product marking for TO-243AA:
TO-92 TO-243AA* TO-252 LR8❋
LR8N3 LR8N8 LR8K4 where ❋ = 2-week alpha date code
* Same as SOT-89. Product supplied on 2000 piece carrier tape reels.
Absolute Maximum Ratings Pin Configurations
VIN Input Voltage -0.5V to +480V*
2 (TAB)
Output Voltage Range -0.5V to +470V TAB
1
2
Operating Ambient Temperature Range -40°C to +85°C 3 1
3
Operating Junction Temperature Range -40°C to +125°C TO-243AA
(SOT-89) 123 TO-252
Storage Temperature Range -65°C to +150°C TO-92 (D-PAK)
*Voltages referenced to ADJ.
VIN VOUT ADJ
TO-92 1 2 3
TO-243AA 1 2, TAB 3
TO-252 1 2 (TAB) 3
Electrical Characteristics
Test conditions unless otherwise specified: -40°C < TA < 85°C.
Symbol Parameter Min Typ Max Units Test Conditions
VIN - VOUT Input to Output Voltage Difference 12 450 V
VOUT Overall Output Voltage Regulation 1.14 1.20 1.26 V 12V<VIN<400V, R1=2.4KΩ, R2=0
VOUT Overall Output Voltage Regulation 375 400 425 V R1=2.4KΩ, R2=782KΩ
∆VOUT Line Regulation 0.003 0.01 %/V 15V<VIN<400V, VOUT=5V, IOUT=0.5mA
∆VOUT Load Regulation 1.4 3.0 % VIN=15V, VOUT=5V, 0.5mA<IOUT<10mA
∆VOUT Temperature Regulation -1 +1 % VIN=15V, VOUT=5V, IOUT=10mA,
-40°C<TA<85°C
IOUT Output Current Limit 10 20 mA TJ < 85°C, VIN - VOUT = 10V
IOUT Output Current Limit 0.5 mA TJ > 125°C, VIN - VOUT = 450V
IOUT Minimum Output Current 0.3 0.5 mA Includes R1 and load current
IADJ Adjust Output Current 5 10 15 µA
C2 Minimum Output Load Capacitance 1 µF
DVOUT/DVIN Ripple Rejection Ratio 50 60 dB 120Hz, VOUT = 5V
TLIMIT Junction Temperature Limit 125 °C
2

3. LR8
Thermal Characteristics
Package Power Dissipation θJC θJA
@ TA=25°C °C/W °C/W
TO-92 0.74W 125 170
TO-243AA 1.6W 15 78†
TO-252 2.5W 6.25 50†
†
Mounted on FR4 board, 25mm x 25mm x 1.57mm.
Significant PD increase possible on ceramic substrate.
Functional Block Diagram
VIN Pass LR8 VOUT
Element
Overtemp &
Overcurrent 1.2V 10µA ADJ
3

4. LR8
Typical Application Circuits
*
VOUT=5.0V
VIN=15V to 450V V V
IN OUT
ADJ
LR8 R1
6.04KΩ
±1% C2
C1 1.0µF RLOAD ≤16.5KΩ
R2
18.2KΩ
±1%
VOUT = 1.20V 1+ R2 + IADJ R2
R1
Figure 1: High Input Voltage, 5.0V Output Linear Regulator
* Required for conditions where VIN is less than VOUT.
VIN=15V to 450V
+
VAuxiliary
Vout1
LR8 -
V
IN
VOUT +
ADJ
Vout2
Vcc
-
FB PWM IC
Figure 2: SMPS Start-Up Circuit
1µF
1.20V
LR8 IOUT =
R
+ VIN VOUT
ADJ R
VIN = 15V
to 450V Load
-
Figure 3: High Voltage Adjustable Constant Current Source
4

5. LR8
Typical Performance Curves
Temperature Variation
1.30
1.25
VIN VOUT=1.2V
LR8
VOUT (V)
1.20
ADJ
1.15
12V 2.4KΩ 1.0µF
1.10
1.05
1.00
-50 -25 0 25 50 75 100 125
T(junction) (°C)
Adjustment Current
12
11
VIN VOUT=1.2V
IADJ (µA)
10
LR8
VIN = 400V
12V, ADJ 9 VIN = 12V
200V, 2.4KΩ 1.0µF
400V
I 8
ADJ
VIN = 200V
7
6
-50 -25 0 25 50 75 100 125
T(junction) (°C)
Load Regulation
5.2
V V =5.0V IOUT
IN OUT
LR8
5.1
VOUT (V)
ADJ 6.04KΩ
±1%
1.0µF 5.0
25V
RLOAD
18.2KΩ
±1%
4.9
4.8
0 2 4 6 8 10
IOUT (mA)
5

6. LR8
Typical Performance Curves
VOUT vs. VIN
6
VIN VOUT=5.0V 5
LR8
4
VOUT (V)
ADJ 6.04KΩ
±1%
0V to 50V 1.0µF 1KΩ 3
18.2KΩ
±1% 2
1
0
0 10 20 30 40 50
VIN (V)
Ripple Rejection
-65
Ripple Rejection Ratio (dB)
VIN VOUT =5.0V IOUT -64
LR8
ADJ 6.04KΩ -63
20VP-P
±1%
@ 60Hz
1.0µF
RLOAD -62
65V 18.2KΩ
±1%
-61
-60
0 2 4 6 8 10
IOUT (mA)
6

7. LR8
Typical Performance Curves
Load Transient Response
VIN VOUT=5.0V
LR8 Closed
SW
ADJ 6.04KΩ SW Open
±1%
25V 1.0µF 10KΩ
18.2KΩ 523 Ω Vout
±1%
Load Transient Response
Line Transient Response
400V 400V
VIN VOUT
VIN LR8
0V
ADJ 6.04KΩ Vin
±1%
1.0µF 10KΩ
5.0V
18.2KΩ 0V
±1%
Vout
0V
Line Transient Response
400V 400V
Vin Vin
0V 0V
5.0V 5.0V
Vout Vout
0V 0V
Line Power Up Transient Line Power Down Transient
12/03/01rev.2
1235 Bordeaux Drive, Sunnyvale, CA 94089
TEL: (408) 744-0100 • FAX: (408) 222-4895
©2001 Supertex Inc. All rights reserved. Unauthorized use or reproduction prohibited.
7 www.supertex.com

8. LR8 LR8
Application Note
AN-H40
High Voltage Linear Regulators and
Constant Current Sources Using LR8
by Scott Lynch, Senior Applications Engineer
Introduction Operation
The LR8 is a high voltage 3-terminal adjustable linear regulator. Except for it’s higher voltage rating, the LR8 operates like any
Intended for operation directly off rectified AC mains, the LR8 other 3-terminal adjustable linear regulator.
operates at input voltages up to 450 volts, making it compatible
A simple resistive divider sets the output voltage while a capacitor
with line voltages up to 240 VAC. It’s output voltage adjustability
at the output improves transient response and ensures regulator
assures that it can be used in most any application. The LR8 is
stability. When applicable, an input capacitor is required to
ideally suited for low power off-line DC power supplies and
provide energy storage for rectified AC.
SMPS start-up circuits.
Keeping in mind that the LR8 requires at least a 10V difference
Available in a leaded TO-92 package and a surface mount
between input and output for proper operation, the minimum
SOT-89 and D-PAK package, it is ideal for applications where
value for CIN is:
space is at a premium.
Relevant specifications are shown in the table below. t
CIN > I LOAD
VIN ( pk ) − VOUT − 10V
LR8 Specifications where ILOAD = Load current
Input Voltage Range (VOUT +10V) to 450V t = Time between peaks of input
Output Voltage Range 1.2V to (VIN - 10V) waveform
VIN(pk) = Peak input voltage
Output Voltage Accuracy ±5% VOUT = Output voltage
Power Dissipation TO-92: 0.74W
TO-243AA (SOT-89): 1.6W
TO-252 (D-PAK): 2.5W Note that the LR8 requires a minimum of 0.5mA load current for
proper operation. The current through the resistive divider may
Output Current 0.5 to 10mA
be included as part of the minimum load.
Load Regulation 3%
Line Regulation 0.01%/V
Supply Rejection 60dB typ @120Hz
LR8 Block Diagram and Typical Application
Needed for protection should V OUT exceed V IN
 R 
VOUT = 1.2 v1 + 2  + 10µA • R2
 R1 
VIN Pass LR8 VOUT
Supply Element Load
R1
Overtemp &
Overcurrent 1.2V 10µA ADJ
CIN COUT
≥ 1µF
R2
8

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.
Constant Current LED Driver NiCad Battery Trickle Charger
R R
LR8 LR8
15V
to 15V
450V 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
LR8
Controller
AC R1
Mains
CIN COUT
R2
9

10. LR8
Comparison with Discrete Startup Implementations
The LR8 provides several advantages when compared with discretely implemented start-up circuits.
Zener Implementation Transistor Implementation LR8 Implementation
VIN VOUT VIN VOUT
VIN LR8 VOUT
Disadvantages
• Continues to draw current from high voltage source after
supply has bootstrapped, resulting in inefficiencies Advantages
• Bias must be set for minimum input voltage, resulting in • LR8 goes into standby mode after supply has
high current drain at high input voltages bootstrapped, drawing no current from high voltage
• Poor regulation input
• No current limit • Good regulation
• No overtemperature protection • Built-in current limiting
• In the Zener implementation, requires large power resistor • Overtemperature protection
and Zener
Exceeding LR8’s Current Limit for Startup Applications
The LR8 has a built-in current limit of 10mA minimum. If the current drawn by the PWM controller exceeds this limit, the LR8 may still be
used. To do this, the LR8’s output capacitor supplies a portion of the current until the power supply can bootstrap itself and the LR8 is no
longer needed. The following figure graphically illustrates how this is accomplished.
Most PWM controllers have an undervoltage lockout (UVL) circuit or programmable start/stop voltages. When the voltage supplied to the
PWM controller reaches the turn-on threshold, the controller begins operating and consuming current. If current exceeds the current limit
for the LR8, the voltage at VOUT begins to decay. With a large enough capacitor, the supply will bootstrap before voltage decays to the turn-
off threshold.
1 Input voltage is applied. The LR8 begins operating (in current
VIN 1 limiting mode since COUT appears as a short). VOUT begins to
rise as COUT charges. The PWM controller draws a small
amount of current.
VOUT 4
2 The output voltage of the LR8 reaches the PWM controller’s
VHYS turn-on threshold. Controller begins operating, drawing current.
Bootstrap voltage begins climbing while VOUT decays since
current drawn by the controller exceeds the LR8’s current limit.
VBOOT tBOOT
2 3
3 Bootstrap voltage reaches the level of the LR8’s output and
IPWM takes over. LR8 current drops to zero.
10mA
ILR8
4 Power supply reaches steady-state operation.
0mA
10

11. LR8
The minimum capacitance required for given boot-up time is
given by the following equation:
I PWM − I LIM
COUT > t BOOT
VHYS
where COUT = Capacitor at LR8 output
tBOOT = Time required for supply to bootstrap
IPWM = Current used by PWM controller
ILIM = LR8 current limit (10mA min)
VHYS = PWM controller UVL hysteresis
Remember that this equation is valid only when PWM current
exceeds the LR8’s current limit.
11/30/01rev.1
1235 Bordeaux Drive, Sunnyvale, CA 94089
TEL: (408) 744-0100 • FAX: (408) 222-4895
11 www.supertex.com