Page 1
High Input Voltage
SMPS Start-up Circuit
Ordering Information
Maximum Input
Voltage
450V LR745N3 LR745N8
*Same as SOT-89. Product supplied on 2000 piece carrier tape reels.
Order Number / Package
TO-92 TO-243AA*
LR745
Product marking for TO-243AA:
LR7❋
where ❋ = 2-week alpha date code
Features
❏ 25V to 450V operating input voltage range
❏ Compatible with industry standard PWM ICs.
See application notes AN-H28 and AN-H29.
❏ Output current limiting
❏ For PWM ICs with start-up threshold voltage
of 13.9V to 18.8V
❏ Very low power consumption after start-up
Applications
❏ Notebook and Laptop computers
❏ Telecommunication power supplies
❏ Battery chargers
❏ Motor controller
Absolute Maximum Ratings
Input Voltage 450V
Output Voltage 25V
Operating and Storage Temperature –55°C to 150°C
Soldering Temperature* 300°C
*Distance of 1.6mm from case for 10 seconds
General Description
The Supertex LR7 is a high input voltage SMPS start-up circuit.
The LR7 is ideally suited for use with industry standard low
voltage PWM ICs having start thresholds of 13.9V to 18.8V. It
allows the PWM ICs to be operated from rectified 120V or
240VAC lines, and eliminates the use of power resistors often
used for this purpose. The internal circuitry of the LR7 allows the
PWM ICs to operate at a V
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 startup period. After start-up, the internal high voltage line is disconnected from the IC thereby reducing the continuous power
dissipation to a minimum.
voltage below their start threshold
CC
Pin Configuration
1
2
3
TO-243AA
(SOT-89)
TAB
1 2 3
TO-92
V
IN
TO-92 1 2 3
TO-243AA 1 2, TAB 3
For detailed circuit and application information, please refer
to application notes AN-H28 and AN-H29.
11/12/01
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, refer to the most current databook or to the Legal/Disclaimer page on the Supertex website.
1
GND V
OUT
Page 2
Electrical Characteristics
Test conditions unless otherwise specified: TA = 25°C; VIN = 450V
Symbol Parameter Min Typ Max Unit Conditions
V
OUT
I
OUT
V
IN
I
INQ
V
OFF
V
RESET
I
OFF
V
AUX
I
AUX
Output Voltage 18.8 24 V I
V
over Temperature 18.5 24.3 V I
OUT
Output Current Limiting 2 3 4 mA
Operating Input Voltage Range 25 450 V
Input Quiescent Current 500 µAVIN = 400V, I
Output Turn OFF Voltage 12.6 13.25 13.9 V
Over Temperature 12.3 13.25 14.2 V TA = -40°C to +85°C
V
OFF
Output Reset Voltage 6.3 7 7.7 V
V
Over Temperature 6 7 8 V TA = -40°C to +85°C
RESET
VIN Off-State Leakage Current 75 µAVIN = 400V
External Voltage Applied to V
Input Current to V
OUT
OUT
22 V
500 µAV
= 0
OUT
= 0, TA = -40°C to +85°C
OUT
OUT
= 22V
AUX
LR745
= 0
Block Diagram
V
IN
M2
Q
RD
V
Clk
REF
Reset
Clock
R4
M1
+
23V
comp1
V
OUT
comp2
Vz
–
+
–
+
–
R1
R2
R3
2 to 4 mA
V
OUT
GND
2
Page 3
LR745
Block Diagram Detailed
Description
The Supertex LR7 is a high voltage switch mode power supply
start-up circuit, which has 3 terminals: V
input voltage range of 25VDC to 450VDC can be applied directly
at the input V
pin. The output voltage, V
IN
, GND, and V
IN
, is monitored by
OUT
the 2 comparators, comp1 and comp2. An internal reference,
V
, and resistor divider R1, R2, and R3 set the nominal V
REF
trip points of 7.0V for comp1 and 13.25V for comp2.
When a voltage is applied on V
0V. When V
is less than 7.0V, the output of comp1 will be at
OUT
, V
IN
will start to ramp up from
OUT
a logic high state keeping the D flip flop in a reset 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
change to a logic low state. V
becomes greater than 7.0V, comp1 will
OUT
will continue to increase, and
OUT
the constant current source of typically 3mA output will charge an
external storage capacitor. As V
output of comp2, will then switch from a logic high to a logic low
reaches above 13.25V, the
OUT
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 typically 23V and
the threshold voltage of M1 is typically 1.5V. The zener diode is
biased by resistor R4.
will start to decrease when it is connected to an external
V
OUT
load greater than the internal constant current source, which is
the case when the PWM IC starts up. When V
OUT
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
Once V
decreases below 7.0V, comp1 will reset the D flip flop,
OUT
is greater than 7.0V.
OUT
thereby turning transistor M2 off and transistor M1 back on.
. An
OUT
OUT
falls below
Typical Application
Figure 1 shows a simplified typical configuration of a switch
mode power supply, SMPS, using the Supertex LR7 in the startup circuit.
The LR7’s V
IC, Unitrode part #UC3844. An auxiliary winding on the transformer is used to generate a V
after start-up. The LR7 is used to supply power for the PWM IC
only during start-up. After start-up, the LR7 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
capacitor, C1. The VCC voltage will start to increase at a rate
V
CC
limited by the internal current limiter of 3.0mA. The PWM IC is
in its start-up condition and will typically draw 0.5mA from the V
line. The VCC voltage will continue to increase until it reaches the
PWM IC’s start threshold voltage of typically 16V.
Stage II
Once V
CC
and will draw typically 20mA depending on the operating frequency and size of the switching MOSFET. The output of LR7,
, is internally current limited to 3.0mA. The remaining 17mA
V
OUT
will be supplied by C1 causing the V
decreases to 13.25V, the LR7 will turn off its output thereby
V
CC
reducing its input current from 3.0mA to 10’s of microamperes.
At this point, all 20mA will be supplied by C1. The PWM IC can
now operate to a minimum V
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
size the V
a voltage greater than 10V before V
allows V
AUX
terminal is connected to the VCC line of a PWM
OUT
voltage to power the PWM IC
CC
, the LR7 will start to charge the
IN
CC
reaches 16V, the PWM IC is in its operating condition
voltage decrease. When
CC
voltage of typically 10V.
CC
. It is necessary to
storage capacitor, C1, such that V
CC
AUX
decreases to 10V. This
CC
increases to
AUX
to supply the required operating current for the PWM IC.
(Continued on page 14-9)
V
IN
I
IN
High Voltage
I
AUX
V
AUX
D2
C2
LR7
V
OUT
V
CC
PWM IC
UC3844
GND
C1
Figure 1: Simplified SMPS using LR7
3
Page 4
LR7 Start-up Waveforms
Stage Stage Stage
I
IIIII
LR745
V
OUT
(Volts)
I
IN
(mA)
16.0
13.5
12.0
8.0
4.0
0.0
3.0
2.0
1.0
0.0
PWM IC Start Threshold Voltage
LR7 V
Trip Point
OFF
Auxiliary Supply Powers PMW IC
≈ 0mA
I
IN
t
t
V
AUX
(Volts)
I
AUX
(mA)
Figure 2
12.0
8.0
4.0
0.0
30.0
20.0
10.0
0.0
V
AUX
I
AUX
= 12V
t
= 20mA
t
4
Page 5
LR745
(Continued from page 14-7)
If for some reason the auxiliary voltage does not reach 10V, V
will continue to decrease. Once VCC goes below 10V, the PWM
CC
IC will return to its start-up condition. The PWM IC will now only
draw 0.5mA. V
rate. Once V
output, V
OUT
will continue to decrease but at a much slower
CC
decrease below 7.0V, the LR7 will turn the
CC
, back on. V
will start charging C1 as described in
OUT
Stage I.
Stage III
At this stage the LR7’s output is turned off and the PWM IC is
operating from the V
be designed to vary anywhere between the minimum operating
supply. The auxiliary voltage, V
AUX
AUX
, can
VCC voltage of the PWM IC (10V) to the maximum auxiliary
voltage rating of the LR7 (22V).
Design Considerations
I. Calculating the value for C1
Sizing the V
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
approximately by the following equation:
where, f = switching frequency
capacitor, C1, is an important factor. Making C1
CC
1
Nl
×
×
(
(
)
C1
f
=
VV
(
START
)
−
)
MIN
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
= PWM IC minimum VCC operating voltage
MIN
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
C1=
100KHZ
100 20mA
×
(
16V - 10V
(
×
)
(
)
)
C1= 3.3µF
II. SMPS with wide minimum to maximum load
An important point is that the LR7’s output voltage, V
discharge to below the nominal V
trip point of 13.25V in order
OFF
OUT
, must
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
will fall below 13.25V under minimum load conditions.
CC
Consider an SMPS that is required to power small as well as
large loads and is also required to power up quickly. Such as
SMPS may power up too fast with a small load, not allowing the
voltage to fall below 13.25V. For such conditions, the circuit
V
CC
in Figure 3 is recommended.
In Figure 3, the V
pin of the LR7. The V
which stays at 0V until the V
old voltage. Once V
will switch digitally from 0V to 5.0V. During start-up, the LR7 will
be on and V
CC
16V, the UC3844 will start to operate and V
0V to 5.0V. The LR7 will see an effective V
pin of the UC3844 is used to bias the ground
REF
pin on the UC3844 is a 5.0V reference,
REF
reaches the start threshold voltage, V
CC
voltage reaches the start thresh-
CC
REF
will start to increase up to 16V. Once VCC reaches
will increase from
REF
voltage of 11V
OUT
(16V minus 5.0V) because the ground of the LR7 is now at 5.0V.
The LR7 will immediately turn off its output V
to wait for the V
voltage to decrease. The V
CC
without having
OUT
switching from
REF
0 to 5V during start is a common feature in most PWM ICs.
V
IN
V
LR7
OUT
GND
Figure 3: Using V
©2001 Supertex Inc. All rights reserved. Unauthorized use or reproduction prohibited.
V
CC
PWM IC
UC3844
V
REF
C1
for the LR7 Ground Voltage
REF
1235 Bordeaux Drive, Sunnyvale, CA 94089
5
11/12/01
TEL: (408) 744-0100 • FAX: (408) 222-4895
www.supertex.com
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