Datasheet SG1844Y, SG1844Y-883B, SG1844Y-DESC, SG1845F-DESC, SG1845J Datasheet (Microsemi Corporation)

LIN DOC #: 1844

SG1844/SG1845 Series

C

URRENT MODE

PWM C

ONTROLLER

T HE I NFINITE P OWER OF I NNOVATION

DESCRIPTION KEY FEATURES

The SG1844/45 family of control ICs
provides all the necessary features to
implement off-line fixed frequency,
current-mode switching power supplies
with a minimum number of external
components. Current-mode architecture
demonstrates improved line regulation,
improved load regulation, pulse-by­pulse current limiting and inherent
protection of the power supply output
switch.

The bandgap reference is trimmed to
±1% over temperature. Oscillator
discharge current is trimmed to less than
±10%. The SG1844/45 has under­voltage lockout, current-limiting circuitry

and start-up current of less than 1mA.

The totem-pole output is optimized to
drive the gate of a power MOSFET. The
output is low in the off state to provide
direct interface to an N-channel device.

Both operate up to a maximum duty
cycle range of zero to <50% due to an
internal toggle flip-flop which blanks the
output off every other clock cycle.

The SG1844/45 is specified for
operation over the full military ambient
temperature range of -55°C to 125°C.
The SG2844/45 is specified for the
industrial range of -25°C to 85°C, and
the SG3844/45 is designed for the
commercial range of 0°C to 70°C.

PRODUCT HIGHLIGHT

TYPICAL APPLICATION OF SG3844 IN A FLYBACK CONVERTER

P RODUCTION DATA SHEET

■ OPTIMIZED FOR OFF-LINE CONTROL

■ LOW START-UP CURRENT (<1mA)

■ AUTOMATIC FEED FORWARD

COMPENSATION

■ TRIMMED OSCILLATOR DISCHARGE

CURRENT

■ PULSE-BY-PULSE CURRENT LIMITING

■ ENHANCED LOAD RESPONSE

CHARACTERISTICS

■ UNDER-VOLTAGE LOCKOUT WITH 6V

HYSTERESIS (SG1844 only)

■ DOUBLE PULSE SUPPRESSION

■ HIGH-CURRENT TOTEM-POLE OUTPUT

■ INTERNALLY TRIMMED BANDGAP

REFERENCE

■ 500kHz OPERATION

■ UNDERVOLTAGE LOCKOUT

SG1844 - 16 volts
SG1845 - 8.4 volts

■ LOW SHOOT-THROUGH CURRENT <75mA

OVER TEMPERATURE

(°C)

T

A

0 to 70

-25 to 85

-55 to 125

MIL-STD/883

DESC

R

AC

INPUT

I

V

CC

SG3844

ST

ST

■ AVAILABLE TO MIL-STD-883B

■

LINFINITY LEVEL "S" PROCESSING AVAILABLE

PACKAGE ORDER INFORMATION

HIGH RELIABILITY FEATURES

Plastic DIP

M

8-pin

SG3844M SG3844N SG3844DM SG3844D SG3844Y SG3844J — —

SG3845M SG3845N SG3845DM SG3845D SG3845Y SG3845J — —

SG2844M SG2844N SG2844DM SG2844D SG2844Y SG2844J — —

SG2845M SG2845N SG2845DM SG2845D SG2845Y SG2845J — —

— — — — SG1844Y SG1844J — SG1844L

— — — — SG1845Y SG1845J — SG1845L

— — — — SG1844Y/883B SG1844J/883B — SG1844L/883B

— — — — SG1845Y/883B SG1845J/883B — SG1845L/883B

— — — — SG1844Y/DESC SG1844J/DESC SG1844F/DESC SG1844L/DESC

Plastic DIP

N

14-pin

Plastic SOIC

DM

8-pin

— — SG1845Y/DESC SG1845J/DESC SG1845F/DESC SG1845L/DESC

Note: All surface-mount packages are available in Tape & Reel.

Plastic SOIC

D

14-pin

Ceramic DIP

Y

8-pin

Ceramic DIP

J

14-pin

Cer. Flatpack

F

10-pin

L

Ceramic LCC
20-pin

Copyright © 2000
Rev. 1.4 4/00

FOR FURTHER INFORMATION CALL [phone]

11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841

1

PRODUCT DATABOOK 1996/1997

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

PWM C

ONTROLLER

ABSOLUTE MAXIMUM RATINGS (Notes 1 & 2)

Supply Voltage (ICC < 30mA) ...............................................................Self Limiting

Supply Voltage (Low Impedance Source) ........................................................ 30V

Output Current (Peak) .......................................................................................±1A

Output Current (Continuous) .......................................................................350mA

Output Energy (Capacitive Load) ....................................................................... 5µJ

Analog Inputs (Pins 2, 3) ................................................................. -0.3V to +6.3V

Error Amp Output Sink Current .....................................................................10mA

Operating Junction Temperature

Hermetic (J, Y, F, L Packages) ................................................................... 150°C

Plastic (N, M, D, DM Packages) ................................................................ 150°C

Storage Temperature Range .......................................................... -65°C to +150°C

Lead Temperature (Soldering, 10 Seconds) .................................................. 300°C

Note 1. Exceeding these ratings could cause damage to the device.
Note 2. All voltages are with respect to Pin 5. All currents are positive into the specified

terminal.

THERMAL DATA

M PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

N PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

DM PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

D PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

Y PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

J PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

F PACKAGE:

THERMAL RESISTANCE-JUNCTION TO CASE,

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

L PACKAGE:

THERMAL RESISTANCE-JUNCTION TO CASE,

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

Junction Temperature Calculation: TJ = TA + (PD x θJA).
The θJA numbers are guidelines for the thermal performance of the device/pc-board
system. All of the above assume no ambient airflow.

θθ

θ

θθ

JA

θθ

θ

θθ

JA

θθ

θ

θθ

JA

θθ

θ

θθ

JA

θθ

θ

θθ

JA

θθ

θ

θθ

JA

θθ

θ

θθ

JC

θθ

θ

θθ

JA

θθ

θ

θθ

JC

θθ

θ

θθ

JA

95°C/W

65°C/W

165°C/W

120°C/W

130°C/W

80°C/W

80°C/W

145°C/W

35°C/W

120°C/W

1. COM

2. V

FB

3. I

SENSE

4. RT/C

T

5. OWER GND

32

4

5

6

7

8

91110

PACKAGE PIN OUTS

1 8

COM

27

V

FB

36

I

SENSE

45

RT/C

T

M & Y PACKAGE

(Top View)

SENSE

V

FB

T

1 8

27

36

45

COM

I

RT/C

DM PACKAGE

(Top View)

N.C.

V
N.C.
I

SENSE

N.C.

FB

T

1 14

213

3 12

411

510

69

78

COM

RT/C

D PACKAGE

(Top View)

1 14

COM

213

N.C.

3 12

V

FB

411

N.C.

510

I

SENSE

69

N.C.

78

RT/C

T

J & N PACKAGE

(Top View)

1 10
29
38
47
56

F PACKAGE

(Top View)

12019

12 13

L PACKAGE

(Top View)

V

REF

V

CC

OUT UT
GND

V

REF

V

CC

OUT UT
GND

V

REF

N.C.
V

CC

V

C

OUT UT
GND

WR GND

V

REF

N.C.
V

CC

V

C

OUT UT
GROUND

OWER GND

1. N.C.

2. N.C.

3. COM .

18

4. N.C.

17

5. V

16

15

14

FB

6. N.C.

7. I

SENSE

8. RT/C

9. N.C.

10. N.C.

T

10.V

REF

9. V

CC

8. V

C

7. OUT UT

6. GND

11. N.C.

12. WR GND

13. GND

14. N.C.

15. OUT UT

16. N.C.

17. V

C

18. V

CC

19. N.C.

20. V

REF

2

Copyright © 2000

Rev. 1.4 4/00

PRODUCT DATABOOK 1996/1997

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

RECOMMENDED OPERATING CONDITIONS (Note 3)

PWM C

ONTROLLER

Parameter

Symbol

Supply Voltage Range
Output Current (Peak)
Output Current (Continuous)
Analog Inputs (Pin 2, Pin 3)
Error Amp Output Sink Current
Oscillator Frequency Range
Oscillator Timing Resistor R
Oscillator Timing Capacitor C

T

T

Recommended Operating Conditions

Min. Typ. Max.

Units

30 V
±1 A

200 mA

02.6V
5mA

0.1 500 kHz

0.52 150 kΩ

0.1 1.0 µF

Operating Ambient Temperature Range:

SG1844/45
SG2844/45
SG3844/45

-55 125 °C

-25 85 °C
070°C

Note 3. Range over which the device is functional.

ELECTRICAL CHARACTERISTICS

(Unless otherwise specified, these specifications apply over the operating ambient temperatures for SG1844/SG1845 with -55°C ≤ TA ≤ 125°C, SG2844/
SG2845 with -25°C ≤ TA ≤ 85°C, SG3844/SG3845 with 0°C ≤ TA ≤ 70°C, VCC = 15V (Note 7), RT = 10kΩ, and CT = 3.3nF. Low duty cycle pulse testing
techniques are used which maintains junction and case temperatures equal to the ambient temperature.)

Parameter

Symbol

Test Conditions

SG1844/45

SG2844/45

Min. Typ. Max. Min. Typ. Max. Min. Typ. Max.

SG3844/45

Units

Reference Section

Output Voltage TJ = 25°C, IO = 1mA
Line Regulation 12 ≤ VIN ≤ 25V
Load Regulation 1 ≤ IO ≤ 20mA
Temperature Stability (Note 4)
Total Output Variation (Note 4) Line, Load, Temp.
Output Noise Voltage (Note 4) VN10Hz ≤ f ≤ 10kHz, TJ = 25°C
Long Term Stability (Note 4) TA = 125°C, 1000hrs
Output Short Circuit

4.95 5.00 5.05 4.95 5.00 5.05 4.90 5.00 5.10 V
620 620 620 mV
625 625 625 mV

0.2 0.4 0.2 0.4 0.2 0.4 mV/°C

4.90 5.10 4.90 5.10 4.82 5.18 V

50 50 50 µV

525 525 525 mV

-30 -100 -180 -30 -100 -180 -30 -100 -180 mA

Oscillator Section

Initial Accuracy (Note 8) TJ = 25°C
Voltage Stability 12V ≤ VCC ≤ 25V
Temperature Stability (Note 4) T
Amplitude V

≤ TA ≤ T

MIN

(Peak to Peak)

RT/CT

MAX

Discharge Current TJ = 25°C

T

≤ TA ≤ T

MIN

MAX

47 52 57 47 52 57 47 52 57 kHz

.021 0.21 0.21 %

555%

1.7 1.7 1.7 V

7.8 8.3 9.1 7.5 8.4 9.3 7.5 8.4 9.3 mA

6.8 9.3 7.2 9.5 7.2 9.5 mA

Error Amp Section

Input Voltage V
Input Bias Current
Open Loop Gain A

VOL

Unity Gain Bandwidth (Note 4) TJ = 25°C
Power Supply Rejection Ratio PSRR 12 ≤ VCC ≤ 25V
Output Sink Current V
Output Source Current V
V

High V

OUT

V

Low V

OUT

= 2.5V

COMP

2 ≤ VO ≤ 4V

= 2.7V, V

VFB

= 2.3V, V

VFB

= 2.3V, RL = 15K to gnd

VFB

= 2.7V, RL = 15K to V

VFB

COMP

COMP

= 1.1V
= 5V

2.45 2.50 2.55 2.45 2.50 2.55 2.42 2.50 2.58 V

-0.3 -1 -0.3 1 -0.3 -2 µA

65 90 65 90 65 90 dB

0.7 1 0.7 1 0.7 1 MHz
60 70 60 70 60 70 dB

26 26 26 mA

-0.5 -0.8 -0.5 -0.8 -0.5 -0.8 mA
56 56 56 V

REF

0.7 1.1 0.7 1.1 0.7 1.1 V

(Electrical Characteristics continue next page.)

Copyright © 2000
Rev. 1.4 4/00

3

PRODUCT DATABOOK 1996/1997

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

ELECTRICAL CHARACTERISTICS (Cont'd.)

PWM C

ONTROLLER

Parameter

Symbol

Test Conditions

Current Sense Section

Gain (Notes 5 & 6)
Maximum Input Signal (Note 5) V
Power Supply Rejection Ratio (Note 5) PSRR 12 ≤ V
Input Bias Current
Delay to Output (Note 4)

COMP

= 5V

CC

≤ 25V

Output Section

Output Low Level I

Output High Level I

Rise Time (Note 4) T J = 25°C, CL = 1nF
Fall Time (Note 4) TJ = 25°C, CL = 1nF

= 20mA

SINK

I

= 200mA

SINK

SOURCE

I

SOURCE

= 20mA
= 200mA

Under-Voltage Lockout Section

Start Threshold 1844

1845

Min. Operation Voltage After Turn-On 1844

1845

PWM Section

Maximum Duty Cycle
Minimum Duty Cycle

Power Consumption Section

Start-Up Current
Operating Supply Current VFB = V
VCC Zener Voltage ICC = 25mA

Notes: 4. These parameters, although guaranteed, are not 100% tested in

production.

5. Parameter measured at trip point of latch with V

ISENSE

VFB

= 0V

= 0.

SG1844/45

SG2844/45

Min. Typ. Max. Min. Typ. Max. Min. Typ. Max.

2.85 3 3.15 2.85 3 3.15 2.85 3 3.15 V/V

0.9 1 1.1 0.9 1 1.1 0.9 1 1.1 V
70 70 70 dB

-2 -10 -2 -10 -2 -10 µA

150 300 150 300 150 300 ns

0.1 0.4 0.1 0.4 0.1 0.4 V

1.5 2.2 1.5 2.2 1.5 2.2 V
13 13.5 13 13.5 13 13.5 V
12 13.5 12 13.5 12 13.5 V

50 150 50 150 50 150 ns
50 150 50 150 50 150 ns

15 16 17 15 16 17 14.5 16 17.5 V

7.8 8.4 9.0 7.8 8.4 9.0 7.8 8.4 9.0 V
9 10 11 9 10 11 8.5 10 11.5 V

7.0 7.6 8.3 7.0 7.6 8.2 7.0 7.6 8.2 V

46 48 50 46 48 50 46 48 50 %

000%

0.5 1 0.5 1 0.5 1 mA
11 17 11 17 11 17 mA
34 34 34 V

∆ V

6. Gain defined as: A = ; 0 ≤ V

7. Adjust VCC above the start threshold before setting at 15V.

8. Output frequency equals one half of oscillator frequency.

∆ V

COMP

ISENSE

SG3844/45

≤ 0.8V.

ISENSE

Units

4

Copyright © 2000

Rev. 1.4 4/00

V

GROUND**

R

T/CT

PRODUCT DATABOOK 1996/1997

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

BLOCK DIAGRAM

*

CC

34V

16V (1844)

8.4 (1845)

2.5V

OSCILLATOR

UVLO

6V (1844)

0.8V (1845)

PWM C

S / R

V

REF

GOOD LOGIC

5V

REF

ONTROLLER

T

INTERNAL

BIAS

V

REF

5.0V
50mA

V

*

C

OUTPUT

ERROR AMP

V

FB

COMP

CURRENT SENSE

- V

and VC are internally connected for 8-pin packages.

*

CC

- POWER GROUND and GROUND are internally connected for 8-pin packages.

**

2R

R

1V

S

R

CURRENT SENSE
COMPARATOR

PWM
LATCH

POWER GROUND**

Copyright © 2000
Rev. 1.4 4/00

5

PRODUCT DATABOOK 1996/1997

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

PWM C

ONTROLLER

GRAPH / CURVE INDEX

Characteristic Curves

FIGURE #

1. DROPOUT VOLTAGE vs. TEMPERATURE

2. OSCILLATOR TEMPERATURE STABILITY

3. CURRENT SENSE TO OUTPUT DELAY vs. TEMPERATURE

4. START-UP CURRENT vs. TEMPERATURE

5. REFERENCE VOLTAGE vs. TEMPERATURE

6. START-UP VOLTAGE THRESHOLD vs. TEMPERATURE

7. START-UP VOLTAGE THRESHOLD vs. TEMPERATURE

8. OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE

9. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT AND

TEMPERATURE (SINK TRANSISTOR)

10. CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT

11. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT AND

TEMPERATURE (SOURCE TRANSISTOR)

FIGURE INDEX

Application Information

FIGURE #

12. OSCILLATOR TIMING CIRCUIT

13. OSCILLATOR FREQUENCY vs. R

FOR VARIOUS C

T

T

Typical Applications Section

FIGURE #

14. CURRENT SENSE SPIKE SUPPRESSION

15. MOSFET PARASITIC OSCILLATIONS

16. BIPOLAR TRANSISTOR DRIVE

17. ISOLATED MOSFET DRIVE

18. ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL WITH

SOFTSTART

19. EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT

SYNCHRONIZATION

20. OSCILLATOR CONNECTION

21. ERROR AMPLIFIER CONNECTION

6

Copyright © 2000

Rev. 1.4 4/00

PRODUCT DATABOOK 1996/1997

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

CHARACTERISTIC CURVES

PWM C

ONTROLLER

FIGURE 1. — DROPOUT VOLTAGE vs. TEMPERATURE

10.0

SG1844

9.6

9.2

8.8

8.4

Minimum Operating Voltage - (V)

8.0

-50-25 0255075100125

-75

SG1845

Junction Temperature - (°C)

FIGURE 3. — CURRENT SENSE TO OUTPUT DELAY vs.

TEMPERATURE

FIGURE 2. — OSCILLATOR TEMPERATURE STABILITY

2

0

-2

-4

-6

-8

Frequency Drift - (%)

-10

-50-25 0255075100125

-75

VIN = 15V
Duty Cycle = 50%

Junction Temperature - (°C)

FIGURE 4. — START-UP CURRENT vs. TEMPERATURE

Current Sense Delay - (nS)

Copyright © 2000
Rev. 1.4 4/00

220

200

180

160

140

120

V

PIN3

-50-25 0255075100125

-75

Junction Temperature - (°C)

= 1.1V

0.7

0.6

0.5

0.4

0.3

Start-Up Current - (mA)

0.2

-50-25 0255075100125

-75

Junction Temperature - (°C)

SG1844

SG1845

7

PRODUCT DATABOOK 1996/1997

Junction Temperature - (°C)

Oscillator Discharge Current - (mA)

7.4

-75

-50-25 0255075100125

7.6

7.8

8.0

8.2

7.2

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

CHARACTERISTIC CURVES

PWM C

ONTROLLER

FIGURE 5. — REFERENCE VOLTAGE vs. TEMPERATURE

5.02

5.01

VCC = 15V

5.00

4.99

Reference Voltage - (V)

4.98

-75

-50-25 0255075100125

Junction Temperature - (°C)

FIGURE 7. — START-UP VOLTAGE THRESHOLD vs.

TEMPERATURE

16.08

FIGURE 6. — START-UP VOLTAGE THRESHOLD vs.

TEMPERATURE

8.32

8.30

8.28

8.26

8.24

Reference Voltage - (V)

8.22

8.20

8.18

-75

-50-25 0255075100125

SG1845

Junction Temperature - (°C)

FIGURE 8. — OSCILLATOR DISCHARGE CURRENT vs.

TEMPERATURE

16.06

16.04

16.02

Start Up Voltage - (V)

16.00

15.98

-75

-50-25 0255075100125

Junction Temperature - (°C)

8

SG1844

Copyright © 2000

Rev. 1.4 4/00

PRODUCT DATABOOK 1996/1997

Error Amp Output Voltage - (V)

Current Sense Threshold - (V)

0.2

1.0 2.0 3.0 4.0 5.0

0.4

0.6

0.8

1.0

0

0.1

0.3

0.5

0.7

0.9

125°C

25°C

-55°C

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

CHARACTERISTIC CURVES

PWM C

ONTROLLER

FIGURE 9. — OUTPUT SATURATION VOLTAGE vs.

OUTPUT CURRENT & TEMPERATURE

2.5

2.0

1.5

1.0

Saturation Voltage - (V)

0.5

0

100 200 300 400 500

-55°C

+25°C

+125°C

VIN = 15V

Duty Cycle < 5%

Output Current - (mA)

FIGURE 11. — OUTPUT SATURATION VOLTAGE vs.

OUTPUT CURRENT & TEMPERATURE

FIGURE 10. — CURRENT SENSE THRESHOLD vs.

ERROR AMPLIFIER OUTPUT

4.0

3.0

Copyright © 2000
Rev. 1.4 4/00

2.0

Saturation Voltage - (V)

1.0

0

VIN = 15V

Duty Cycle < 5%

+125°C

+25°C

-55°C

+25°C

+125°C

100 200 300 400 500

Output Current - (mA)

9

PRODUCT DATABOOK 1996/1997

T

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

APPLICATION INFORMATION

OSCILLATOR

The oscillator of the 1844/45 family of PWM's is programmed by
the external timing components (RT, CT) as shown in Figure 13.

PWM C

ONTROLLER

V

REF

RT

RT/C

T

C

GND

1.86

F ≈ where RT ≥ 5kΩ

RTC

T

FIGURE 12 — OSCILLATOR TIMING CIRCUIT

- ( )
R

100K

T

10K

1K

100

100nF

47nF

22nF

1K

10nF

4.7nF

2.2nF

10K

1nF

100K

Oscillator Frequency - (Hz)

FIGURE 13 — OSCILLATOR FREQUENCY vs. RT FOR VARIOUS C

1M

T

10

Copyright © 2000

Rev. 1.4 4/00

PRODUCT DATABOOK 1996/1997

VCCV

IN

7 (12)

7 (11)

6 (10)

5 (8)

R

S

Q1

SG1844/45

3 (5)

R

1

»

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

TYPICAL APPLICATION CIRCUITS

Pin numbers referenced are for 8-pin package and pin numbers in parenthesis are for 14-pin package.

VCCV

IN

7 (12)

PWM C

ONTROLLER

SG1844/45

7 (11)

6 (10)

5 (8)

3 (5)

Q1

I

PK

R

1.0V

I

=

PK(MAX)

R

R

C

S

S

FIGURE 14. — CURRENT SENSE SPIKE SUPPRESSION

The RC low-pass filter will eliminate the leading edge current spike
caused by parasitics of Power MOSFET.

I

B

V

C

+

»

R

2

V

C1

_

R1|| R

2

SG1844/45

V

C

7 (11)

6 (10)

V

C

R

2

R

5 (8)

V

IN

C1

1

Q1

1

FIGURE 15. — MOSFET PARASITIC OSCILLATIONS

A resistor (R1) in series with the MOSFET gate reduce overshoot and
ringing caused by the MOSFET input capacitance and any inductance
in series with the gate drive. (Note: It is very important to have a
low inductance ground path to insure correct operation of the I.C.
This can be done by making the ground paths as short and as wide
as possible.)

SG1844/45

7 (12)

7 (11)

6 (10)

5 (8)

3 (5)

V

CC

Isolation

Boundary

R

R

C

S

V

IN

Q1

VGS Waveforms
+
0

_

50% DC

+
0

_

N

S

25% DC

N

P

The 1844/45 output stage can provide negative base current to
remove base charge of power transistor (Q1) for faster turn off. This
is accomplished by adding a capacitor (C1) in parallel with a resistor
(R1). The resistor (R1) is to limit the base current during turn on.

Copyright © 2000
Rev. 1.4 4/00

3 (5)



R

S

V (PIN 1) - 1.4

IPK =

3R

S



N

P





N

S





FIGURE 17. — ISOLATED MOSFET DRIVEFIGURE 16. — BIPOLAR TRANSISTOR DRIVE

Current transformers can be used where isolation is required
between PWM and Primary ground. A drive transformer is then
necessary to interface the PWM output with the MOSFET.

11

PRODUCT DATABOOK 1996/1997

A

SG1844/SG1845 Series

C

URRENT-MODE

P RODUCTION DATA SHEET

TYPICAL APPLICATION CIRCUITS (continued)

PWM C

ONTROLLER

7 (12)

8 (14)

4 (7)

2 (3)

1 (1)

R

2

1N4148

C

IPK = Where: 0 ≤ V1 ≤ 1.0V t

2N2907

R

1

V

1

R

S

1.43 - 0.23

and V1 = where: V2 =

1+

SG1844/45

R

1

R

2

R

1

R

2

5 (9)

SOFTSTART

V

7 (11)

6 (10)

5 (8)

3 (5)

= -ln1 - C

V

CC

IN

Q1

R

S

R1 R

V





C





R1+R

V

2





0.5

R

1

1+

R

2

FIGURE 18. — ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL

WITH SOFTSTART

Softstart and adjustable peak current can be done with the external
circuitry shown above.

8 (14)

R

A

84

6

R

B

C

2

2

f =

f =

2

1.44

(RA + 2RB)C

R

B

RA + 2R

555

TIMER

1

B

4 (7)

3

To other

SG1844/45

FIGURE 19. — EXTERNAL DUTY CYCLE CLAMP AND

MULTI-UNIT SYNCHRONIZATION

Precision duty cycle limiting for a duty cycle of <50%, as well as
synchronizing several 1844/45's is possible with the above circuitry.

SG1844/45

5 (9)

5V

2.8V

1.1V

7 (11)

R

T

6 (10)

C

SG1844/45

Discharge

Current

I

= 8.2mA

d

The oscillator is programmed by the values selected for the timing
components RT and CT. Refer to application information for
calculation of the component values.

12

2.5V

SG1844/45

0.5m

2 (3)

T

R

i

³ 10K

R

F

1 (1)

R

F

FIGURE 21. — ERROR AMPLIFIER CONNECTIONFIGURE 20. — OSCILLATOR CONNECTION

Error amplifier is capable of sourcing and sinking current up to 0.5mA.

Copyright © 2000

Rev. 1.4 4/00