● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com
LR745
Features
Accepts inputs from 35V to 450V
Output current limiting
For PWM ICs with start-up threshold voltage of
13.9 - 18.0V
Very low power consumption after start-up
Applications
Notebook and laptop computers
Telecommunication power supplies
Battery chargers
Motor controllers
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General Description
The Supertex LR745 is a high input voltage SMPS start-
up circuit. The LR745 is ideally suited for use with industry
standard low voltage PWM ICs having start thresholds of 13.9
- 18.0V. It allows the PWM ICs to be operated from rectified
120 or 240VAC lines, and eliminates the use of power resistors
often used for this purpose.
The internal circuitry of the LR745 allows the PWM ICs to
operate at a VCC voltage below their start threshold voltage
after start-up. The auxiliary voltage can be less than the
start threshold voltage, which allows for improved efficiency.
Current from the high voltage line is drawn only during the
start-up period. After start-up, the internal high voltage line is
disconnected from the IC, thereby reducing the continuous
power dissipation to a minimum.
High Input Voltage SMPS Start-up
Ordering Information
Device Package Options
TO-92 TO-243AA (SOT-89)
LR745 LR745N3-G LR745N8-G
-G indicates package is RoHS compliant (‘Green’)
Absolute Maximum Ratings
Parameter Value
Input voltage 450V
Output voltage 25V
Operating and storage temperature -55°C to +150OC
Soldering temperature* 300OC
Absolute Maximum Ratings are those values beyond which damage to the device
may occur. Functional operation under these conditions is not implied. Continuous
operation of the device at the absolute rating level may affect device reliability. All
voltages are referenced to device ground.
* Distance of 1.6mm from case for 10 seconds.
Pin Configurations
GND
VIN
VOUT
VIN
VOUT
GND
GND
TO-92 (N3) TO-243AA (SOT-89) (N8)
TO-243AA (SOT-89) (N8)
Y = Last Digit of Year Sealed
W = Code for Week Sealed
L = Lot Number
= “Green” Packaging
S i L R
745
YWLL
TO-92 (N3)
LR7W W = Code for Week Sealed
= “Green” Packaging
Package Markings
Package may or may not include the following marks: Si or
Package may or may not include the following marks: Si or
2
LR745
● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com
Sym Parameter Min Typ Max Units Conditions
VOUT
Output voltage 18.0 - 24 V IOUT = 0
VOUT over temperature 17.7 - 24.3 V IOUT = 0, TA = -40°C to +85OC
IOUT Output current limiting 2.0 3.0 4.0 mA ---
VIN Operating input voltage range 35 - 450 V ---
IINQ Input quiescent current - - 500 µA VIN = 400V, IOUT = 0
VOFF
Output turn off voltage 12.6 13.25 13.9 V ---
VOFF over temperature 12.3 13.25 14.2 V TA = -40°C to +85OC
VRESET
Output reset voltage 6.3 7.0 7.7 V ---
VRESET over temperature 6.0 7.0 8.0 V TA = -40°C to +85OC
IOFF VIN off-state leakage current - - 75 µA VIN = 400V
VAUX External voltage applied to VOUT - - 22 V ---
IAUX Input current applied to VOUT - - 500 µA VAUX = 22V
Electrical Characteristics
(Test conditions unless otherwise specified: TA = 25°C; VIN = 450V)
Block Diagram
+
+
–
–
CLK Clock
Reset
Q
GND
R D
VOUT
VOUT
VIN
M2
M1
R1
R2
R3
2.0 to 4.0mA
R4
23V VZ
comp1
VREF
Reset
+
–
comp2
+
–
comp1
3
LR745
● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com
Block Diagram Detailed Description
The Supertex LR745 is a high voltage, switch-mode pow-
er supply start-up circuit which has 3 terminals: VIN, GND,
and VOUT. An input voltage range of 35 - 450VDC can be
applied directly at the input VIN pin. The output voltage, VOUT,
is monitored by the 2 comparators, COMP1 and COMP2.
An internal reference, VREF, and resistor divider R1, R2, and
R3 set the nominal VOUT trip points of 7.0V for COMP1 and
13.25V for COMP2.
When a voltage is applied on VIN, VOUT will start to ramp up
from 0V. When VOUT is less than 7.0V, the output of COMP1
will be at a logic high state, keeping the D flip flop in a re-
set state. The output of the D flip flop, Q, will be at logic
low keeping transistor M2 off. The data input for the D flip
flop, D, is internally connected to a logic high. As VOUT be-
comes greater than 7.0V, COMP1 will change to a logic low
state. VOUT will continue to increase, and the constant cur-
rent source of typically 3.0mA output will charge an external
storage capacitor. As VOUT reaches above 13.25V, the output
of COMP2 will then switch from a logic high to a logic low
state. The D flip flop’s output does not change state since its
clock input is designed to trigger only on a rising edge, logic
low to logic high transition. When there is no load connected
to the output, the output voltage will continue to increase
until it reaches 21.5V, which is the zener voltage minus the
threshold voltage of transistor M1. The zener voltage is typi-
cally 23V, and the threshold voltage of M1 is typically 1.5V.
The zener diode is biased by resistor R4.
VOUT will start to decrease when it is connected to an exter-
nal load greater than the internal constant current source,
which is the case when the PWM IC starts up. When VOUT
falls below 13.25V, the output of COMP2 will switch from a
logic low to a logic high. The output of COMP2 will clock in a
logic 1 into the D flip flop, causing the D flip flop’s output, Q,
to switch from a logic low to a logic high. Transistor M2 will
then be turned on pulling the gate of transistor M1 to ground,
thereby turning transistor M1 off. Transistor M1 will remain
off as long as VOUT is greater than 7.0V. Once VOUT decreases
below 7.0V, COMP1 will reset the D flip flop, thereby turning
transistor M2 off and transistor M1 back on.
Typical Application
Figure 1 shows a simplified typical configuration of a switch-
mode power supply, SMPS, using the Supertex LR745 in the
start-up circuit.
The LR745’s VOUT terminal is connected to the VCC line of a
PWM IC, Unitrode part #UC3844. An auxiliary winding on
the transformer is used to generate a VCC voltage to pow-
er the PWM IC after start-up. The LR745 is used to supply
power for the PWM IC only during start-up. After start-up, the
LR745 turns off and the auxiliary winding is used to supply
power for the PWM IC. Figure 2 shows the typical current
and voltage waveforms at various stages from power up to
operation powered by the auxiliary winding.
Stage I
Once a voltage is applied on VIN, the LR745 will start to
charge the VCC capacitor, C1. The VCC voltage will start to
increase at a rate limited by the internal current limiter of
3.0mA. The PWM IC is in its start-up condition and will typi-
cally draw 0.5mA from the VCC line. The VCC voltage will con-
tinue to increase until it reaches the PWM IC’s start thresh-
old voltage of typically 16V.
Stage II
Once VCC reaches 16V, the PWM IC is in its operating condi-
tion and will draw typically 20mA, depending on the operat-
ing frequency and size of the switching MOSFET. The output
of the LR745, VOUT, is internally current limited to 3.0mA. The
remaining 17mA will be supplied by C1 causing the VCC volt-
age decrease. When VCC decreases to 13.25V, the LR745
will turn off its output, thereby reducing its input current from
3.0mA to 10s of microamperes. At this point, all 20mA will be
supplied by C1. The PWM IC can now operate to a minimum
VCC voltage of typically 10V.
Once the switching MOSFET starts operating, the energy in
the primary winding is transferred to the secondary outputs
and the auxiliary winding, thereby building up VAUX. It is nec-
essary to size the VCC storage capacitor, C1, such that VAUX
increases to a voltage greater than 10V before VCC decreas-
es to 10V. This allows VAUX to supply the required operating
current for the PWM IC.
If for some reason the auxiliary voltage does not reach 10V,
VCC will continue to decrease. Once VCC goes below 10V,
the PWM IC will return to its start-up condition. The PWM
IC will now only draw 0.5mA. VCC will continue to decrease
but at a much slower rate. Once VCC decreases below 7.0V,
the LR745 will turn the output, VOUT, back on. VOUT will start
charging C1 as described in Stage I.
4
LR745
● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com
0.0
3.0
2.0
1.0
0.0
12.0
8.0
4.0
0.0
30.0
20.0
10.0
0.0
4.0
PWM IC Start Threshold Voltage
LR7 VOFF Trip Point
Auxiliary Supply Powers PWM IC
t
t
t
t
8.0
12.0
13.5
16.0
VOUT
(Volts)
I
Stage Stage Stage
IIN
(mA)
VAUX
(Volts)
IAUX
(mA)
VAUX
= 12V
IAUX
= 20mA
IIN ≈ 0mA
IIIII
Figure 1: Simplified SMPS Using LR745
High Voltage
VIN
VOUT VCC
IAUX VAUX
C1
C2
D2
IIN
GND
LR7 PWM IC
UC3844
Figure 1: LR745 Start-up Waveforms
5
LR745
● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com
Figure 3: Using VREF for the LR745 Ground Voltage
Stage III
At this stage the LR745’s output is turned off and the PWM
IC is operating from the VAUX supply. The auxiliary voltage,
VAUX, can be designed to vary anywhere between the mini-
mum operating VCC voltage of the PWM IC (10V) to the maxi-
mum auxiliary voltage rating of the LR745 (22V).
Design Considerations
I. Calculating the value for C1
Sizing the VCC capacitor, C1, is an important factor. Making
C1 too large will cause the SMPS to power up too slowly.
However, if too small, C1 will not allow the SMPS to power
up due to insufficient charge in the capacitor to power the
IC and MOSFET until the auxiliary supply is available. The
value of C1 can be approximated by the following equation:
where,
f = switching frequency
N = number of clock cycles required to chargeVAUX to
VMIN value
I = PWM operating current
VSTART = PWM IC start threshold rating
VMIN = PWM IC minimum VCC operating voltage
Consider for example, a PWM IC with a switching frequency
of 100KHz, operating current of 20mA, start threshold of
16V, and a minimum operating voltage of 10V. If 100 clock
cycles are required to charge the auxiliary voltage to 10V,
the minimum value of C1 is calculated as follows:
C1 = 3.3µF
II. SMPS with wide minimum to maximum load
An important point is that the LR745’s output voltage, VOUT,
must discharge to below the nominal VOFF trip point of 13.25V
in order for its output to turn off. If the SMPS requires a wide
minimum to maximum output load variation, it will be difficult
to guarantee that VCC will fall below 13.25V under minimum
load conditions. Consider an SMPS that is required to power
small as well as large loads and is also required to power up
quickly. Such a SMPS may power up too fast with a small
load, not allowing the VCC voltage to fall below 13.25V. For
such conditions, the circuit in Figure 3 is recommended.
In Figure 3, the VREF pin of the UC3844 is used to bias the
ground pin of the LR745. The VREF pin on the UC3844 is
a 5.0V reference, which stays at 0V until the VCC voltage
reaches the start threshold voltage. Once VCC reaches the
start threshold voltage, VREF will switch digitally from 0V to
5.0V. During start-up, the LR745 will be on, and VCC will start
to increase up to 16V. Once VCC reaches16V, the UC3844
will start to operate and VREF will increase from 0V to 5.0V.
The LR745 will see an effective VOUT voltage of 11V (16V mi-
nus 5.0V) because the ground of the LR745 is now at 5.0V.
The LR745 will immediately turn off its output, VOUT, with-
out having to wait for the VCC voltage to decrease. The VREF
switching from 0 to 5.0V during start is a common feature in
most PWM ICs.
GND
VIN
VOUT VCC
VREF
C1
PWM IC
UC3844
LR7
C1 =
1
fx (N) x (l)
(VSTART - VMIN)
C1 =
1
100KHz x (100) x (20mA)
(16V -10V)
6
LR745
● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com
3-Lead TO-92 Package Outline (N3)
Symbol A b c D E E1 e e1 L
Dimensions
(inches)
MIN .170 .014†.014†.175 .125 .080 .095 .045 .500
NOM - - - - - - - - -
MAX .210 .022†.022†.205 .165 .105 .105 .055 .610*
JEDEC Registration TO-92.
* This dimension is not specified in the original JEDEC drawing. The value listed is for reference only.
† This dimension is a non-JEDEC dimension.
Drawings not to scale.
Supertex Doc.#: DSPD-3TO92N3, Version D080408.
Seating Plane
1
2
3
Front View Side View
Bottom View
E1 E
D
e1
L
e
c
1 2 3
b
A
Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives an
adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability to the
replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and specifications
are subject to change without notice. For the latest product specifications refer to the Supertex inc. website: http//www.supertex.com.
©2008 All rights reserved. Unauthorized use or reproduction is prohibited.
1235 Bordeaux Drive, Sunnyvale, CA 94089
Tel: 408-222-8888
www.supertex.com
7
LR745
(The package drawings in this data sheet may not reflect the most current specifications. For the latest package outline
information go to http://www.supertex.com/packaging.html.)
Doc.# DSFP-LR745
B123008
3-Lead TO-243AA (SOT-89) Package Outline (N8)
Symbol A b b1 C D D1 E E1 e e1 H L
Dimensions
(mm)
MIN 1.40 0.44 0.36 0.35 4.40 1.62 2.29 2.13
1.50
BSC
3.00
BSC
3.94 0.89
NOM - - - - - - - - - -
MAX 1.60 0.56 0.48 0.44 4.60 1.83 2.60 2.29 4.25 1.20
JEDEC Registration TO-243, Variation AA, Issue C, July 1986.
Drawings not to scale.
Supertex Doc. #: DSPD-3TO243AAN8, Version D070908.
bb1
