ing 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.
Applications
•
•
•
•
Notebook and laptop computers
Telecommunication power supplies
Battery chargers
Motor controllers
2015 Microchip Technology Inc.
DS20005394A-page 1
LR745
Package Type
VOUT
VIN
GND
GND
VOUT
GND
VIN
TO-92
See
Table 2-1
for pin information
TO-243AA (SOT-89)
DS20005394A-page 2
2015 Microchip Technology Inc.
LR745
1.0
ELECTRICAL CHARACTERISTICS
ABSOLUTE MAXIMUM RATINGS
Input Voltage .................................................................................................................................................................................. 450V
Operating and storage temperature ............................................................................................................................. -55°C to +150°C
Note:
Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress
rating only and functional operation of the device at those or any other conditions, above those indicated in the operational listings of
this specification, is not implied. Exposure to maximum rating conditions for extended periods may affect device reliability.
1.1
ELECTRICAL SPECIFICATIONS
ELECTRICAL CHARACTERISTICS
1
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
12.6
12.3
6.3
6.0
13.25
13.25
7.0
7.0
Min
18.0
17.7
2.0
35
3.0
Typ
Max
24
24.3
4.0
450
500
13.9
14.2
7.7
8.0
75
22
500
Units Conditions
V
V
mA
V
µA
V
V
V
V
µA
V
µA
V
AUX
= 22V
T
A
= -40°C to +85°C
V
IN
= 400V
T
A
= -40°C to +85°C
V
IN
= 400V, I
OUT
= 0
I
OUT
= 0
I
OUT
= 0, T
A
= -40°C to +85°C
TABLE 1-1:
Symbol
V
OUT
I
OUT
V
IN
I
INQ
V
OFF
V
RESET
I
OFF
V
AUX
I
AUX
1
Test Conditions unless otherwise specified: T
A
= 25°C, V
IN
= 450V
TABLE 1-2:
THERMAL CHARACTERISTICS
Package
TO-92
TO-243AA (SOT-89)
θja
132°C/W
133°C/W
2015 Microchip Technology Inc.
DS20005394A-page 3
LR745
2.0
PIN DESCRIPTION
PIN DESCRIPTION
Description
Regulator input. 8 - 450V.
Ground return for all internal circuitry. This pin must be electrically connected to circuit common.
Regulator output.
The locations of the pins are listed in
Package Type.
TABLE 2-1:
Function
VIN
GND
VOUT
DS20005394A-page 4
2015 Microchip Technology Inc.
LR745
3.0
APPLICATION INFORMATION
Figure 3-1
shows a simplified typical configuration of a
switch mode power supply, SMPS, using LR745 in the
start-up circuit.
LR745’s VOUT terminal is connected to the VCC line of
a PWM IC. An auxiliary winding on the transformer
generates a V
CC
voltage to power the PWM IC after
start-up. LR745 supplies power for the PWM IC only
during start-up. After start-up, LR745 turns off and the
auxiliary winding supplies power for the PWM IC.
Figure 3-2
shows the typical current and voltage wave-
forms at various stages from power-up to operation
powered by the auxiliary winding.
supplied by C
1
, causing the V
CC
voltage decrease.
When V
CC
decreases to 13.25V, 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 min-
imum V
CC
voltage, typically 10V.
Once the switching MOSFET starts operating, the
energy in the primary winding is transferred to the sec-
ondary outputs and the auxiliary winding, thereby build-
ing up V
AUX
. It is necessary to size the V
CC
storage
capacitor, C
1
, such that V
AUX
increases to a voltage
greater than 10V before V
CC
decreases 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 condi-
tion. 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, LR745 will turn the output,
V
OUT
, back on. V
OUT
will start charging C
1
as described
in Stage I.
3.1
Stage I
Once a voltage is applied on VIN, LR745 starts to
charge the V
CC
capacitor, C
1
. The V
CC
voltage starts 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 typically draw 0.5mA from the V
CC
line. The V
CC
voltage will continue to increase until it reaches the
PWM IC’s start threshold voltage, typically 16V.
3.3
Stage III
3.2
Stage II
Once V
CC
reaches 16V, the PWM IC is in its operating
condition and will typically draw 20mA, depending on
the operating frequency and size of the switching
metal–oxide–semiconductor field-effect transistor
(MOSFET). The output of LR745, V
OUT
, is internally
current limited to 3.0mA. The remaining 17mA will be
At this stage, LR745 output is turned off and the PWM
IC is operating from the V
AUX
supply. The auxiliary volt-
age, V
AUX
, can be designed to vary anywhere between
the minimum operating V
CC
voltage of the PWM IC
(10V) to the maximum auxiliary voltage rating of the
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