SUPERTEX LR8

LR8
LR8
High Input Voltage
Adjustable 3-Terminal Linear Regulator
Features
General Description
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The Supertex LR8 is a high voltage, low output current, adjustable
linear regulator. It has a wide operating input voltage range of
12V to 450V. The output voltage can be adjusted from 1.20V to
440V provided that the input voltage is at least 10V greater than
the output voltage. The output voltage can be adjusted by means
of two external resistors R1 and R2 as shown in the typical
application circuits. The LR8 regulates the voltage difference
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
times R2.
12V to 450V input voltage range
Adjustable 1.20V to 440V output regulation
5% output voltage tolerance
Output current limiting
10µA typical ADJ current
Internal junction temperature limiting
Applications
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The LR8 has current limiting and temperature limiting. The
output current limit is typically 15mA and the minimum temperature
limit is 125°C. An output short circuit current will therefore be
limited to 15mA. When the junction temperature reaches its
temperature limit, the output current and/or output voltage will
decrease to keep the junction temperature from exceeding its
temperature limit. For SMPS start-up circuit applications, the
LR8 turns off when an external voltage greater than the output
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.
Off-line SMPS startup circuits
Adjustable high voltage constant current source
Industrial Controls
Motor controls
Battery chargers
Power supplies
The device is available in TO-92, TO-243AA (SOT-89), and
TO-252 (D-PAK) packages.
LR8 Block Diagram and Typical Application
*
VIN
VIN
VOUT
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
Supertex website: http://www.supertex.com. For complete liability information on all Supertex products,
1 refer to the most current databook or to the Legal/Disclaimer page on the Supertex website.
LR8
Ordering Information
Product marking for TO-243AA:
Package Options
*
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.
Pin Configurations
Absolute Maximum Ratings
VIN Input Voltage
-0.5V to +480V*
Output Voltage Range
-0.5V to +470V
Operating Ambient Temperature Range
-40°C to +85°C
Operating Junction Temperature Range
-40°C to +125°C
Storage Temperature Range
-65°C to +150°C
2 (TAB)
TAB
1
2
1
3
3
TO-243AA
(SOT-89)
*Voltages referenced to ADJ.
TO-252
(D-PAK)
123
TO-92
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
VIN - VOUT
Input to Output Voltage Difference
12
Typ
VOUT
Overall Output Voltage Regulation
1.14
VOUT
Overall Output Voltage Regulation
375
400
425
V
∆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
%
VIN=15V, VOUT=5V, IOUT=10mA,
1.20
-1
Max
Units
450
V
1.26
V
Test Conditions
12V<VIN<400V, R1=2.4KΩ, R2=0
R1=2.4KΩ, R2=782KΩ
-40°C<TA<85°C
IOUT
Output Current Limit
10
IOUT
Output Current Limit
IOUT
Minimum Output Current
IADJ
Adjust Output Current
5
C2
Minimum Output Load Capacitance
1
DVOUT/DVIN Ripple Rejection Ratio
50
TLIMIT
125
Junction Temperature Limit
20
mA
TJ < 85°C, VIN - VOUT = 10V
0.5
mA
TJ > 125°C, VIN - VOUT = 450V
0.3
0.5
mA
Includes R1 and load current
10
15
µA
µF
60
dB
°C
2
120Hz, VOUT = 5V
LR8
Thermal Characteristics
Power Dissipation
@ TA=25°C
θJC
θJA
°C/W
°C/W
TO-92
0.74W
125
170
TO-243AA
1.6W
15
78†
TO-252
2.5W
6.25
50†
Package
†
Mounted on FR4 board, 25mm x 25mm x 1.57mm.
Significant PD increase possible on ceramic substrate.
Functional Block Diagram
VIN
LR8
Pass
Element
Overtemp &
Overcurrent
1.2V
3
VOUT
10µA
ADJ
LR8
Typical Application Circuits
*
V
VIN=15V to 450V
VOUT=5.0V
V
IN
OUT
ADJ
R1
6.04KΩ
±1%
C1
C2
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
+
VOUT
IN
ADJ
Vout2
-
Vcc
FB
PWM IC
Figure 2: SMPS Start-Up Circuit
1µF
IOUT =
LR8
+
VIN
1.20V
R
VOUT
R
ADJ
VIN = 15V
to 450V
Load
-
Figure 3: High Voltage Adjustable Constant Current Source
4
LR8
Typical Performance Curves
Temperature Variation
1.30
VIN
1.25
LR8
ADJ
12V
2.4KΩ
1.0µF
VOUT (V)
VOUT=1.2V
1.20
1.15
1.10
1.05
1.00
-50
-25
0
25
50
75
100
125
100
125
T(junction) (°C)
Adjustment Current
12
11
IADJ (µA)
VOUT=1.2V
VIN
LR8
12V,
200V,
400V
ADJ
2.4KΩ
10
VIN = 400V
VIN = 12V
9
1.0µF
I
8
ADJ
VIN = 200V
7
6
-50
-25
0
25
50
75
T(junction) (°C)
Load Regulation
5.2
VIN
VOUT=5.0V
IOUT
LR8
6.04KΩ
±1%
1.0µF
25V
RLOAD
VOUT (V)
ADJ
5.1
5.0
18.2KΩ
±1%
4.9
4.8
0
2
4
6
IOUT (mA)
5
8
10
LR8
Typical Performance Curves
VOUT vs. VIN
6
VOUT=5.0V
VIN
5
ADJ
6.04KΩ
±1%
1.0µF
0V to 50V
1KΩ
VOUT (V)
LR8
18.2KΩ
±1%
4
3
2
1
0
0
10
20
30
40
50
8
10
VIN (V)
Ripple Rejection
VOUT =5.0V
VIN
Ripple Rejection Ratio (dB)
-65
IOUT
LR8
20VP-P
@ 60Hz
ADJ
6.04KΩ
±1%
1.0µF
RLOAD
65V
18.2KΩ
±1%
-64
-63
-62
-61
-60
0
2
4
6
IOUT (mA)
6
LR8
Typical Performance Curves
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Ω
±
&$!
Ω
Ω
±
Ω
'%
Load Transient Response
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Ω
±
#!
Ω
Ω
±
'%
Line Transient Response
#!
#!
'%
'%
Line Power Up Transient
Line Power Down Transient
12/03/01rev.2
©2001 Supertex Inc. All rights reserved. Unauthorized use or reproduction prohibited.
7
1235 Bordeaux Drive, Sunnyvale, CA 94089
TEL: (408) 744-0100 • FAX: (408) 222-4895
www.supertex.com
LR8LR8
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.
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.
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 CIN is:
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.
CIN > I LOAD
LR8 Specifications
Input Voltage Range
where
1.2V to (VIN - 10V)
Output Voltage Accuracy
Power Dissipation
ILOAD = Load current
t
= Time between peaks of input
waveform
VIN(pk) = Peak input voltage
VOUT = Output voltage
(VOUT +10V) to 450V
Output Voltage Range
±5%
TO-92:
0.74W
TO-243AA (SOT-89): 1.6W
TO-252 (D-PAK):
2.5W
Output Current
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.
0.5 to 10mA
Load Regulation
t
VIN ( pk ) − VOUT − 10V
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
VIN
Supply

R 
VOUT = 1.2 v1 + 2  + 10µA • R2
R1 

LR8
Pass
Element
VOUT
Load
R1
CIN
Overtemp &
Overcurrent
1.2V
10µA
ADJ
COUT
≥ 1µF
R2
8
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
LR8
AC
Mains
PWM
Controller
R1
CIN
COUT
R2
9
LR8
Comparison with Discrete Startup Implementations
The LR8 provides several advantages when compared with discretely implemented start-up circuits.
Zener Implementation
VIN
Transistor Implementation
VOUT
VIN
LR8 Implementation
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
high current drain at high input voltages
• Poor regulation
• LR8 goes into standby mode after supply has
bootstrapped, drawing no current from high voltage
input
• No current limit
• Good regulation
• No overtemperature protection
• Built-in current limiting
• In the Zener implementation, requires large power resistor
and Zener
• Overtemperature protection
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 turnoff threshold.
VIN
1
Input voltage is applied. The LR8 begins operating (in current
limiting mode since COUT appears as a short). VOUT begins to
rise as COUT charges. The PWM controller draws a small
amount of current.
2
The output voltage of the LR8 reaches the PWM controller’s
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.
3
Bootstrap voltage reaches the level of the LR8’s output and
takes over. LR8 current drops to zero.
4
Power supply reaches steady-state operation.
1
VOUT
4
VHYS
tBOOT
VBOOT
2
3
IPWM
ILR8
10mA
0mA
10
LR8
The minimum capacitance required for given boot-up time is
given by the following equation:
COUT > t BOOT
where
COUT
tBOOT
IPWM
ILIM
VHYS
=
=
=
=
=
I PWM − I LIM
VHYS
Capacitor at LR8 output
Time required for supply to bootstrap
Current used by PWM controller
LR8 current limit (10mA min)
PWM controller UVL hysteresis
Remember that this equation is valid only when PWM current
exceeds the LR8’s current limit.
11/30/01rev.1
11
1235 Bordeaux Drive, Sunnyvale, CA 94089
TEL: (408) 744-0100 • FAX: (408) 222-4895
www.supertex.com