LR745
High Input Voltage SMPS Start-up
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
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 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.
Applications
►
►
►
►
Notebook and laptop computers
Telecommunication power supplies
Battery chargers
Motor controllers
Ordering Information
Device
LR745
Package Options
TO-92
LR745N3-G
TO-243AA (SOT-89)
LR745N8-G
Pin Configurations
GND
VOUT
GND
VIN
VOUT
VIN
GND
-G indicates package is RoHS compliant (‘Green’)
TO-92 (N3)
TO-243AA (SOT-89) (N8)
Package Markings
Absolute Maximum Ratings
Parameter
Input voltage
Output voltage
Operating and storage temperature
Soldering temperature*
Value
450V
25V
-55°C to +150 C
O
Si
L R
7 4 5
YWLL
Y = Last Digit of Year Sealed
W = Code for Week Sealed
L = Lot Number
= “Green” Packaging
TO-92 (N3)
Package may or may not include the following marks: Si or
300
O
C
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.
LR7W
W = Code for Week Sealed
= “Green” Packaging
TO-243AA (SOT-89) (N8)
Package may or may not include the following marks: Si or
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
●
Tel: 408-222-8888
●
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
V
IN
R4
+
23V
–
M1
V
Z
2.0 to 4.0mA
V
OUT
V
REF
M2
Reset
Reset
+
+
comp1
comp1
–
–
R1
R2
V
OUT
Q R D
Clock
CLK
GND
comp2
+
–
R3
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
●
Tel: 408-222-8888
●
www.supertex.com
2
LR745
Block Diagram Detailed Description
Typical Application
The Supertex LR745 is a high voltage, switch-mode pow-
er supply start-up circuit which has 3 terminals: V
IN
, GND,
and V
OUT
. An input voltage range of 35 - 450VDC can be
applied directly at the input V
IN
pin. The output voltage, V
OUT
,
is monitored by the 2 comparators, COMP1 and COMP2.
An internal reference, V
REF
, and resistor divider R1, R2, and
R3 set the nominal V
OUT
trip points of 7.0V for COMP1 and
13.25V for COMP2.
When a voltage is applied on V
IN
, 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 M2 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 cur-
rent source of typically 3.0mA output will charge an external
storage 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
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.
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 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 V
OUT
is greater than 7.0V. Once V
OUT
decreases
below 7.0V, COMP1 will reset the D flip flop, thereby turning
transistor M2 off and transistor M1 back on.
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 V
OUT
terminal is connected to the V
CC
line of a
PWM IC, Unitrode part #UC3844. An auxiliary winding 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.
Stage I
Once a voltage is applied on V
IN
, the LR745 will start to
charge the V
CC
capacitor, C1. 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.
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 C1 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 C1. 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, C1, 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 C1 as described in Stage I.
Stage II
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
●
Tel: 408-222-8888
●
www.supertex.com
3
LR745
Figure 1: Simplified SMPS Using LR745
V
IN
I
IN
High Voltage
I
AUX
V
AUX
C2
D2
LR7
GND
V
OUT
V
CC
PWM IC
UC3844
C1
Figure 1: LR745 Start-up Waveforms
Stage
I
16.0
V
OUT
13.5
(Volts) 12.0
8.0
4.0
0.0
t
3.0
I
IN
(mA)
2.0
1.0
0.0
12.0
V
AUX
(Volts)
8.0
4.0
0.0
30.0
20.0
10.0
t
0.0
I
AUX
= 20mA
V
AUX
= 12V
t
I
IN
≈ 0mA
t
Stage
II
Stage
III
PWM IC Start Threshold Voltage
LR7 V
OFF
Trip Point
Auxiliary Supply Powers PWM IC
I
AUX
(mA)
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
●
Tel: 408-222-8888
●
www.supertex.com
4
LR745
Stage III
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:
1
x (100) x (20mA)
100KHz
(16V -10V)
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).
Design Considerations
Sizing the V
CC
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:
1
x (N) x (l)
f
(V
START
- V
MIN
)
C1 =
I. Calculating the value for C1
C1 = 3.3µF
C1 =
where,
f = switching frequency
N = number of clock cycles required to chargeV
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
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 V
REF
pin of the UC3844 is used to bias the
ground pin of the LR745. The V
REF
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
Figure 3: Using V
REF
for the LR745 Ground Voltage
V
IN
V
OUT
V
CC
LR7
GND
PWM IC
UC3844
C1
V
REF
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
●
Tel: 408-222-8888
●
www.supertex.com
5
