18-Bit ADC with I2C Interface and Onboard Reference DS

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DS20005394A-page 1

LR745

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

Applications

• Notebook and laptop computers
• Telecommunication power supplies
• Battery chargers
• Motor controllers

Description

LR745 is a high input voltage SMPS start-up circuit.
LR745 is ideally suited for use with industry standard
low-voltage, Pulse-Width Modulation (PWM) ICs hav-
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.

High-Input Voltage SMPS Start-up

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LR745

DS20005394A-page 2

 2015 Microchip Technology Inc.

Package Type

TO-92

See 

Table 2-1

 for pin information

TO-243AA (SOT-89)

VIN

VOUT

GND

GND

GND

VIN

VOUT

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DS20005394A-page 3

LR745

1.0

 ELECTRICAL CHARACTERISTICS

ABSOLUTE MAXIMUM RATINGS 

Input Voltage .................................................................................................................................................................................. 450V
Output voltage.................................................................................................................................................................................. 25V
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

TABLE 1-1:

ELECTRICAL CHARACTERISTICS

1

1

Test Conditions unless otherwise specified: T

A

 = 25°C, V

IN

 = 450V

Symbol

Parameter

Min

Typ

Max

Units Conditions

V

OUT

Output voltage

18.0

24

V

I

OUT

= 0

V

OUT

 over temperature

17.7

24.3

V

I

OUT

= 0, T

A

= -40°C to +85°C

I

OUT

Output current limiting

2.0

3.0

4.0

mA

V

IN

Operating input voltage range

35

450

V

I

INQ

Input quiescent current

500

µA

V

IN

= 400V, I

OUT

= 0

V

OFF

Output turn off voltage

12.6

13.25

13.9

V

V

OFF

 over temperature

12.3

13.25

14.2

V

T

A

= -40°C to +85°C

V

RESET

Output reset voltage

6.3

7.0

7.7

V

V

RESET

 over temperature

6.0

7.0

8.0

V

T

A

= -40°C to +85°C

I

OFF

V

IN

 off-state leakage current

75

µA

V

IN

= 400V

V

AUX

External voltage applied to 
V

OUT

22

V

I

AUX

Input current applied to V

OUT

500

µA

V

AUX

= 22V

TABLE 1-2:

THERMAL CHARACTERISTICS

Package

θja

TO-92

132°C/W

TO-243AA (SOT-89)

133°C/W

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LR745

DS20005394A-page 4

 2015 Microchip Technology Inc.

2.0

 

PIN DESCRIPTION

The locations of the pins are listed in 

Package Type

.

TABLE 2-1:

PIN DESCRIPTION

Function

Description

VIN

Regulator input. 8 - 450V.

GND

Ground return for all internal circuitry. This pin must be electrically connected to circuit common.

VOUT

Regulator output.

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DS20005394A-page 5

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.

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.

3.2

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

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.

3.3

Stage III

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
LR745 (22V).

FIGURE 3-1:

SIMPLIFIED SMPS USING LR745 

High Voltage

V

IN

VOUT

V

CC

I

AUX

V

AUX

C

1

C

2

D

2

I

IN

GND

LR7

PWM IC

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LR745

DS20005394A-page 6

 2015 Microchip Technology Inc.

FIGURE 3-2:

START-UP WAVEFORMS 

12.0

8.0

4.0

0.0

30.0

20.0

10.0

0.0

PWM IC Start Threshold Voltage

LR7 V

OFF  

Trip Point

Auxiliary Supply Powers PWM IC

t

t

t

t

16.0

13.5
12.0

8.0

4.0

0.0

V

OUT

(V)

I

IN

(mA)

V

AUX

(V)

I

AUX

(mA)

V

AUX

 = 12V

I

AUX 

= 20mA

I

IN 

≈ 0mA

3.0

2.0

1.0

0.0

Stage

I

Stage

II

Stage

III

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DS20005394A-page 7

LR745

3.4

Block Diagram

FIGURE 3-3:

 BLOCK DIAGRAM 

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 - 450V DC can be
applied 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 R1, R2, and R3set 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 out-

put of comp1 will be at 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 M

2

 off. The data

input for the D flip-flop, D, is internally connected to a
logic high. As V

OUT

 becomes greater than 7.0V, comp1

will change to a logic low state. V

OUT

 will continue to

increase, and the constant current source, typically
3.0mA output, will charge an external storage capaci-
tor. 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 ris-
ing 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 tran-
sistor 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

external 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 VOUT 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.

CLK

Clock

Q

GND

R   D

V

OUT

VOUT

VIN

M2

M

1

R

3

2.0 - 4.0mA 

+

23V

-

V

Z

V

REF

Reset

comp1 

comp1 

R

2

R

1

R

4

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LR745

DS20005394A-page 8

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4.0

DESIGN CONSIDERATIONS

To ensure the best design using LR745, evaluate the
value of C

1

 and the SMPS requirements.

4.1

Calculating the value for C

1

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 auxil-
iary supply is available. The value of C

can be approx-

imated by the following equation:

Definitions:

- 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 volt-

age

Consider for example, a PWM IC with a switching fre-
quency 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 aux-
iliary voltage to 10V, the minimum value of C

1

 is calcu-

lated as follows:

4.2

SMPS with wide minimum to 
maximum load

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 4-1

 is

recommended.

In 

Figure 4-1

, 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

minus 5.0V) because the ground of the LR745 is now
at 5.0V. The LR745 will immediately turn off its output,
VOUT, without 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.

FIGURE 4-1:

USING V

REF

 FOR GROUND VOLTAGE 

C

1

1

f

---

N 1

V

START

V

MIN

------------------------------------

=

C

1

1

100kHz

---------------------

100 20mA

16V 10V

-----------------------------------------------------------------

=

C

1

3.3

F

=

GND

VIN

VOUT

VCC

VREF

C

1

PWM IC

LR7

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DS20005394A-page 9

LR745

5.0

PACKAGING INFORMATION

5.1

Package Marking Information

Legend: XX...X

Product Code or Customer-specific information

Y

Year code (last digit of calendar year)

YY

Year code (last 2 digits of calendar year)

WW

Week code (week of January 1 is week ‘01’)

NNN

Alphanumeric traceability code

  

Pb-free JEDEC

®

 designator for Matte Tin (Sn)

*

This package is Pb-free. The Pb-free JEDEC designator (     )
can be found on the outer packaging for this package.

Note:

In the event the full Microchip part number cannot be marked on one line, it will
be carried over to the next line, thus limiting the number of available
characters for product code or customer-specific information. Package may or
not include the corporate logo.

3

e

3

e

3-lead TO-243AA * 

(SOT-89)

Example

XXXYYWW

NNN

LR7513

343

3-lead TO-92

YWWNNN

XXXXXX
XXXX

e3

Example

513343

LR745
N3

e3

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LR745

DS20005394A-page 10

 2015 Microchip Technology Inc.

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

2.00

1.50

BSC

3.00

BSC

3.94

0.73

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.
 This dimension differs from the JEDEC drawing
Drawings not to scale.

b

b1

D

D1

E H

E1

C

A

1

2

3

e

e1

Top View

Side View

L

Note: For the most current package drawings, see the Microchip Packaging Specification at www.microchip.com/packaging.

Note: For the most current package drawings, see the Microchip Packaging Specification at www.microchip.com/packaging.

Maker
Microchip Technology Inc.