Supertex inc.
High Input Voltage
SMPS Start-up
Features
►
Accepts inputs from 35 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
►
►
►
►
Notebook and laptop computers
Telecommunication power supplies
Battery chargers
Motor controllers
LR745
General Description
Applications
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 to 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 V
CC
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.
Ordering Information
Part Number
LR745N3-G
LR745N3-G P002
LR745N3-G P003
LR745N3-G P005
LR745N3-G P013
LR745N3-G P014
LR745N8-G
Package Options
TO-92
TO-92
TO-92
TO-92
TO-92
TO-92
TO-243AA (SOT-89)
Packing
1000/Bag
2000/Reel
2000/Reel
2000/Reel
2000/Reel
2000/Reel
2000/Reel
Pin Configuration
GND
VOUT
VIN
GND
VOUT
GND
VIN
TO-92
TO-243AA (SOT-89)
-G denotes a lead (Pb)-free / RoHS compliant package
Package Marking
SiLR
7 4 5
YWLL
Typical Thermal Resistance
Package
TO-92
TO-243AA (SOT-89)
θ
ja
(
O
C/W)
132
O
C/W
133
O
C/W
Y = Last Digit of Year Sealed
W = Code for Week Sealed
L = Lot Number
= “Green” Packaging
TO-92
Package may or may not include the following marks: Si or
Absolute Maximum Ratings
Parameter
Input voltage
Output voltage
Operating and storage temperature
Value
450V
25V
-55°C to +150
O
C
LR7W
W = Code for Week Sealed
= “Green” Packaging
TO-243AA (SOT-89)
Package may or may not include the following marks: Si or
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.
Doc.# DSFP-LR745
C080113
Supertex inc.
www.supertex.com
LR745
Electrical Characteristics
Sym
V
OUT
I
OUT
V
IN
I
INQ
V
OFF
V
RESET
I
OFF
V
AUX
I
AUX
Parameter
Output voltage
V
OUT
over temperature
Output current limiting
Operating input voltage range
Input quiescent current
Output turn off voltage
V
OFF
over temperature
Output reset voltage
V
RESET
over temperature
V
IN
off-state leakage current
External voltage applied to V
OUT
Input current applied to V
OUT
(Test conditions unless otherwise specified: T
A
= 25°C; V
IN
= 450V)
Min
18.0
17.7
2.0
35
-
12.6
12.3
6.3
6.0
-
-
-
Typ
-
-
3.0
-
-
13.25
13.25
7.0
7.0
-
-
-
Max
24
24.3
4.0
450
500
13.9
14.2
7.7
8.0
75
22
500
Units
V
V
mA
V
µA
V
V
V
V
µA
V
µA
Conditions
I
OUT
= 0
I
OUT
= 0, T
A
= -40°C to +85
O
C
---
---
V
IN
= 400V, I
OUT
= 0
---
T
A
= -40°C to +85
O
C
---
T
A
= -40°C to +85
O
C
V
IN
= 400V
---
V
AUX
= 22V
Block Diagram
VIN
R
4
+
23V
-
M
1
V
Z
M2
2.0 - 4.0mA
VOUT
V
REF
Reset
-
comp1
+
R
1
V
OUT
Q
GND
R
2
R D
CLK
Clock
-
comp1
+
R
3
Doc.# DSFP-LR745
C080113
2
Supertex inc.
www.supertex.com
LR745
The Supertex LR745 is a high voltage, switch-mode power
supply start-up circuit which has 3 terminals: VIN, GND, and
VOUT. An input voltage range of 35 - 450VDC can be ap-
plied directly at the input VIN pin. The output voltage, V
OUT
,
is monitored by the 2 comparators, COMP1 and COMP2.
An internal reference, V
REF
, and resistor divider R
1
, R
2
, and
R
3
set the nominal V
OUT
trip points of 7.0V for COMP1 and
13.25V for COMP2.
When a voltage is applied on VIN, V
OUT
will start to ramp up
from 0V. When V
OUT
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 M
2
off. The data input for the D flip
flop, D, is internally connected to a logic high. As V
OUT
be-
comes greater than 7.0V, COMP1 will change to a logic low
state. V
OUT
will continue to increase, and the constant current
source of typically 3.0mA output will charge an external stor-
age capacitor. As V
OUT
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 thresh-
old voltage of transistor M
1
. The zener voltage is typically
23V, and the threshold voltage of M
1
is typically 1.5V. The
zener diode is biased by resistor R
4
.
V
OUT
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 V
OUT
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 M
2
will
then be turned on pulling the gate of transistor M
1
to ground,
thereby turning transistor M
1
off. Transistor M
1
will remain off
as long as V
OUT
is greater than 7.0V. Once V
OUT
decreases
below 7.0V, COMP1 will reset the D flip flop, thereby turning
transistor M
2
off and transistor M
1
back on.
Block Diagram Detailed Description
on the transformer is used to generate a V
CC
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.
Once a voltage is applied on VIN, the LR745 will start to
charge the V
CC
capacitor, C
1
. The V
CC
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 V
CC
line. The V
CC
voltage will con-
tinue to increase until it reaches the PWM IC’s start thresh-
old voltage of typically 16V.
Stage I
Once V
CC
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, V
OUT
, is internally current limited to 3.0mA. The
remaining 17mA will be supplied by C
1
causing the V
CC
volt-
age decrease. When V
CC
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 C
1
. The PWM IC can now operate to a minimum
V
CC
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 V
AUX
. It is nec-
essary to size the V
CC
storage capacitor, C
1
, such that V
AUX
increases to a voltage greater than 10V before V
CC
decreas-
es to 10V. This allows V
AUX
to supply the required operating
current for the PWM IC.
If for some reason the auxiliary voltage does not reach 10V,
V
CC
will continue to decrease. Once V
CC
goes below 10V,
the PWM IC will return to its start-up condition. The PWM
IC will now only draw 0.5mA. V
CC
will continue to decrease
but at a much slower rate. Once V
CC
decreases below 7.0V,
the LR745 will turn the output, V
OUT
, back on. V
OUT
will start
charging C
1
as described in Stage I.
At this stage the LR745’s output is turned off and the PWM
IC is operating from the V
AUX
supply. The auxiliary voltage,
V
AUX
, can be designed to vary anywhere between the mini-
mum operating V
CC
voltage of the PWM IC (10V) to the maxi-
mum auxiliary voltage rating of the LR745 (22V).
Stage II
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
Stage III
Doc.# DSFP-LR745
C080113
3
Supertex inc.
www.supertex.com
LR745
Figure 1: Simplified SMPS Using LR745
V
IN
I
IN
High Voltage
V
AUX
I
AUX
C
2
D
2
LR7
GND
VOUT
V
CC
C
1
PWM IC
UC3844
Figure 1: LR745 Start-up Waveforms
Stage
I
16.0
V
OUT
(V)
13.5
12.0
8.0
4.0
0.0
3.0
I
IN
(mA)
2.0
1.0
0.0
12.0
Stage
II
Stage
III
PWM IC Start Threshold Voltage
LR7 V
OFF
Trip Point
Auxiliary Supply Powers PWM IC
t
I
IN
≈ 0mA
t
V
AUX
(V)
8.0
4.0
0.0
V
AUX
= 12V
t
30.0
20.0
10.0
0.0
t
I
AUX
= 20mA
I
AUX
(mA)
Doc.# DSFP-LR745
C080113
4
Supertex inc.
www.supertex.com
LR745
Sizing the V
CC
capacitor, C
1
, is an important factor. Making
C
1
too large will cause the SMPS to power up too slowly.
However, if too small, C
1
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 C
1
can be approximated by the following equation:
I. Calculating the value for C
1
Design Considerations
C
1
=
1
f
•
N
•
I
An important point is that the LR745’s output voltage, V
OUT
,
must discharge to below the nominal V
OFF
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 V
CC
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 V
CC
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 V
CC
voltage
reaches the start threshold voltage. Once V
CC
reaches the
start threshold voltage, V
REF
will switch digitally from 0V to
5.0V. During start-up, the LR745 will be on, and V
CC
will start
to increase up to 16V. Once V
CC
reaches16V, the UC3844
will start to operate and V
REF
will increase from 0V to 5.0V.
The LR745 will see an effective V
OUT
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, V
OUT
, with-
out having to wait for the V
CC
voltage to decrease. The V
REF
switching from 0 to 5.0V during start is a common feature in
most PWM ICs.
II. SMPS with wide minimum to maximum load
V
START
- V
MIN
where,
f = switching frequency
N = number of clock cycles required to charge V
AUX
to
V
MIN
value
I = PWM operating current
V
START
= PWM IC start threshold rating
V
MIN
= PWM IC minimum V
CC
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 C
1
is calculated as follows:
C
1
=
1
100kHz
•
100
•
20mA
16V - 10V
C
1
= 3.3µF
Figure 3: Using V
REF
for the LR745 Ground Voltage
VIN
LR7
GND
VOUT
C
1
VCC
VREF
PWM IC
UC3844
Doc.# DSFP-LR745
C080113
5
Supertex inc.
www.supertex.com
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