PRODUCTION DATA SHEET
THE INFINITE POWER OF INNOVATION
LINFINITY MICROELECTRONICS INC.
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841, 714-898-8121, FAX: 714-893-2570
1
Copyright © 2000
Rev. 1.6c,2006-02-02
SG1844/SG1845 Series
CURRENT MODE PWM CONTROLLER
DESCRIPTION
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-by pulse 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 co mmercial range of 0°C
to 70°C.
IMPORTANT: For the most current data, consult MICROSEMI’s website: http://www.microsemi.com
PRODUCT HIGHLIGHT
AC
INPUT
SG3844
R
ST
I
ST
V
CC
KEY FEATURES
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
HIGH RELIABILITY
FEATURES
AVAILABLE TO MIL-STD-883B
LINFINITY LEVEL "S"
PROCESSING AVAILABLE
PACKAGE ORDER INFO
M Plastic DIP
8-Pin N Plastic DIP
14-Pin DM Plastic SOIC
8-Pin D Plastic SOIC
14-Pin Y Ceramic Dip
8-Pin J Ceramic DIP
14-Pin F Cer Flatpack
10-Pin L Ceramic LCC
20-Pin
TA (°C) RoHS Compliant / Pb-free
Transition DC: 0503 RoHS Compliant / Pb-free
Transition DC:0440
SG3844M SG3844N SG3844DM SG3844D SG3844Y SG3844J
0 to 70 SG3845M SG3845N SG3845DM SG3845D SG3845Y SG3845J
SG2844M SG28424N SG2844DM SG2842D SG2844Y SG2844J
-25 to 85 SG2845M SG2845N SG2845DM SG2845D SG2845Y SG2845J
SG1844Y SG1844J SG1844L -55 to
125 SG1845Y SG1845J SG1845L
SG1842Y/883B SG1844J/883B SG1844L/883B MIL-
STD/883 SG1845Y/883B SG1845J/883 SG1845L/883B
DESC SG1844Y/DESC SG1844J/DESC SG1844F/DESC SG1844L/DESC
Note: Available in Tape & Reel. Append the letters “TR” to the part number. (i.e. LXSG3844N-TR)
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
Copyright © 2000
Rev. 1.6c
2
PRODUCTION DATA SHEET
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 (VFB, ISENSE) ................................................................ -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
PACKAGE PIN OUTS
VREF
VCC
OUT UT
GND
COM
VFB
ISENSE
RT/CT
1 8
27
36
45
M & Y PACKAGE
(Top View)
DM PACKAGE
(Top View)
VREF
VCC
OUT UT
GND
COM
VFB
ISENSE
RT/CT
1 8
27
36
45
VREF
N.C.
VCC
VC
OUT UT
GND
WR GND
COM
N.C.
VFB
N.C.
ISENSE
N.C.
RT/CT
1 14
213
3 12
411
510
69
78
D PACKAGE
(Top View)
M PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θθ
θθ
θJA 95°C/W
N PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θθ
θθ
θJA 65°C/W
DM PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θθ
θθ
θJA 165°C/W
D PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θθ
θθ
θJA 120°C/W
Y PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θθ
θθ
θJA 130°C/W
J PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θθ
θθ
θJA 80°C/W
F PACKAGE:
THERMAL RESISTANCE-JUNCTION TO CASE, θθ
θθ
θJC 80°C/W
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θθ
θθ
θJA 145°C/W
L PACKAGE:
THERMAL RESISTANCE-JUNCTION TO CASE, θθ
θθ
θJC 35°C/W
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θθ
θθ
θJA 120°C/W
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.
THERMAL DATA
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.
1 14
213
3 12
411
510
69
78
J & N PACKAGE
(Top View)
COM
N.C.
VFB
N.C.
ISENSE
N.C.
RT/CT
VREF
N.C.
VCC
VC
OUT UT
GROUND
OWER GND
F PACKAGE
(Top View)
10.VREF
9. VCC
8. VC
7. OUT UT
6. GND
1. COM
2. VFB
3. ISENSE
4. RT/CT
5. OWER GND
1 10
29
38
47
56
L PACKAGE
(Top View)
32
4
5
6
7
8
91110
1. N.C.
2. N.C.
3. COM .
4. N.C.
5. VFB
6. N.C.
7. ISENSE
8. RT/CT
9. N.C.
10. N.C.
12019
18
17
16
15
14
12 13
11. N.C.
12. WR GND
13. GND
14. N.C.
15. OUT UT
16. N.C.
17. VC
18. VCC
19. N.C.
20. VREF
RoHS / Pb-free Peak Package Solder Reflow Temp. (40 second max. exposure)............................... 260°C (+0, -5)
RoHS / Pb-free 100% Matte Tin Lead Finish
RoHS / Pb-free 100% Matte Tin Lead Finish
M Package: RoHS / Pb-free 100% Matte Tin Lead Finish
N Package: RoHS / Pb-free 100% Matte Tin Lead Finish
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
3
Copyright © 2000
Rev. 1.6c
PRODUCTION DATA SHEET
ELECTRICAL CHARACTERISTICS
(Unless otherwise specified, these specifications apply over the operating ambient temperatures for SG1844/SG1845 with -55°C ≤ T
A
≤ 125°C, SG2844/
SG2845 with -25°C ≤ T
A
≤ 85°C, SG3844/SG3845 with 0°C ≤ T
A
≤ 70°C, V
CC
= 15V (Note 7), R
T
= 10kΩ, and C
T
= 3.3nF. Low duty cycle pulse testing
techniques are used which maintains junction and case temperatures equal to the ambient temperature.)
Reference Section
Parameter
Symbol
Test Conditions
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
SG3844/45
Units
Min. Typ. Max. Min. Typ. Max. Min. Typ. Max.
SG2844/45
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) TMIN ≤ TA ≤ TMAX
Amplitude VRT/CT (Peak to Peak)
Discharge Current TJ = 25°C
TMIN ≤ TA ≤ TMAX
Error Amp Section
Input Voltage VCOMP = 2.5V
Input Bias Current
Open Loop Gain AVOL 2 ≤ VO ≤ 4V
Unity Gain Bandwidth (Note 4) TJ = 25°C
Power Supply Rejection Ratio PSRR 12 ≤ VCC ≤ 25V
Output Sink Current VVFB = 2.7V, VCOMP = 1.1V
Output Source Current VVFB = 2.3V, VCOMP = 5V
VOUT High VVFB = 2.3V, R L = 15K to gnd
VOUT Low VVFB = 2.7V, RL = 15K to VREF
SG1844/45
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
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
0.7 1.1 0.7 1.1 0.7 1.1 V
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 RT
Oscillator Timing Capacitor CT
Operating Ambient Temperature Range:
SG1844/45
SG2844/45
SG3844/45
RECOMMENDED OPERATING CONDITIONS (Note 3)
Parameter
Symbol
Units
Recommended Operating Conditions
Min. Typ. Max.
30 V
±1 A
200 mA
02.6V
5mA
0.1 500 kHz
0.52 150 kΩ
0.001 1.0 µF
-55 125 °C
-25 85 °C
070°C
Note 3. Range over which the device is functional.
(Electrical Characteristics continue next page.)
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
Copyright © 2000
Rev. 1.6c
4
PRODUCTION DATA SHEET
ELECTRICAL CHARACTERISTICS (Cont'd.)
Under-Voltage Lockout Section
Parameter
Symbol
Test Conditions
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 = VISENSE = 0V
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 VVFB = 0.
SG3844/45
Units
Min. Typ. Max. Min. Typ. Max. Min. Typ. Max.
SG2844/45
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
SG1844/45
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
6. Gain defined as: A = ; 0 ≤ VISENSE ≤ 0.8V.
7. Adjust VCC above the start threshold before setting at 15V.
8. Output frequency equals one half of oscillator frequency.
Output Low Level ISINK = 20mA
ISINK = 200mA
Output High Level I SOURCE = 20mA
ISOURCE = 200mA
Rise Time (Note 4) T J = 25°C, CL = 1nF
Fall Time (Note 4) TJ = 25°C, CL = 1nF
Current Sense Section
Gain (Notes 5 & 6)
Maximum Input Signal (Note 5) VCOMP = 5V
Power Supply Rejection Ratio (Note 5) PSRR 12 ≤ VCC ≤ 25V
Input Bias Current
Delay to Output (Note 4)
Output Section
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
∆ VCOMP
∆ VISENSE
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
5
Copyright © 2000
Rev. 1.6c
PRODUCTION DATA SHEET
BLOCK DIAGRAM
*- VCC and VC are internally connected for 8-pin packages.
** - POWER GROUND and GROUND are internally connected for 8-pin packages.
OSCILLATOR
S
R
INTERNAL
BIAS
S / R 5V
REF
PWM
LATCH
CURRENT SENSE
COMPARATOR
1V
R
2R
ERROR AMP
UVLO
34V
GROUND**
V
CC
*
R
T
/C
T
V
FB
T
COMP
CURRENT SENSE
POWER GROUND**
OUTPUT
V
C
*
V
REF
5.0V
50mA
16V (1844)
8.4 (1845)
6V (1844)
0.8V (1845)
2.5V
V
REF
GOOD LOGIC
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
Copyright © 2000
Rev. 1.6c
6
PRODUCTION DATA SHEET
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. RT FOR VARIOUS CT
Typical Applications Section
FIGURE #
14. CURRENT SENSE SPIKE SUPPRESSION
15. MOSFET PA RASITIC 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
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
7
Copyright © 2000
Rev. 1.6c
PRODUCTION DATA SHEET
CHARACTERISTIC CURVES
FIGURE 2. — OSCILLATOR TEMPERATURE STABILITY
FIGURE 3. — CURRENT SENSE TO OUTPUT DELAY vs.
TEMPERATURE FIGURE 4. — START-UP CURRENT vs. TEMPERATURE
FIGURE 1. — DROPOUT VOLTAGE vs. TEMPERATURE
Junction Temperature - (°C)
Frequency Drift - (%)
-10
-75 -50-25 0 255075100125
-8
-6
-4
-2
0
V
IN
= 15V
Duty Cycle = 50%
2
Junction Temperature - (°C)
Minimum Operating Voltage - (V)
8.0
-75 -50-25 0 255075100125
8.4
8.8
9.2
9.6
10.0
SG1844
SG1845
Junction Temperature - (°C)
Current Sense Delay - (nS)
120
-75 -50-25 0 255075100125
140
160
180
200
220
V
PIN3
= 1.1V
Junction Temperature - (°C)
Start-Up Current - (mA)
0.2
-75 -50-25 0 255075100125
0.3
0.4
0.5
0.6
0.7
SG1844
SG1845
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
Copyright © 2000
Rev. 1.6c
8
PRODUCTION DATA SHEET
CHARACTERISTIC CURVES
FIGURE 6. — START-UP VOLTAGE THRESHOLD vs.
TEMPERATURE
FIGURE 7. — START-UP VOLTAGE THRESHOLD vs.
TEMPERATURE FIGURE 8. — OSCILLATOR DISCHARGE CURRENT vs.
TEMPERATURE
FIGURE 5. — REFERENCE VOLTAGE vs. TEMPERATURE
Junction Temperature - (°C)
Reference Voltage - (V)
8.18
-75 -50-25 0 255075100125
SG1845
8.22
8.26
8.30
8.32
8.20
8.24
8.28
Junction Temperature - (°C)
Reference Voltage - (V)
4.98
-75 -50-25 0 255075100125
V
CC
= 15V
4.99
5.00
5.01
5.02
Junction Temperature - (°C)
Start Up Voltage - (V)
16.00
-75 -50-25 0 255075100125
SG1844
16.02
16.04
16.06
16.08
15.98
Junction Temperature - (°C)
Oscillator Discharge Current - (mA)
7.4
-75 -50-25 0 255075100125
7.6
7.8
8.0
8.2
7.2
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
9
Copyright © 2000
Rev. 1.6c
PRODUCTION DATA SHEET
CHARACTERISTIC CURVES
FIGURE 10. — CURRENT SENSE THRESHOLD vs.
ERROR AMPLIFIER OUTPUT
FIGURE 11. — OUTPUT SATURATION VOLTAGE vs.
OUTPUT CURRENT & TEMPERATURE
FIGURE 9. — OUTPUT SATURATION VOLTAGE vs.
OUTPUT CURRENT & TEMPERATURE
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
Output Current - (mA)
Saturation Voltage - (V)
0.5
100 200 300 400 500
1.0
1.5
2.0
2.5
0
-55°C
+25°C
+125°C
V
IN
= 15V
Duty Cycle < 5%
Output Current - (mA)
Saturation Voltage - (V)
1.0
100 200 300 400 500
2.0
3.0
4.0
0
V
IN
= 15V
Duty Cycle < 5%
-55°C
+25°C
+125°C
+125°C
+25°C
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
Copyright © 2000
Rev. 1.6c
10
PRODUCTION 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.
FIGURE 12 — OSCILLATOR TIMING CIRCUIT
V
REF
R
T
/C
T
GND
R
T
C
T
FIGURE 13 — OSCILLATOR FREQUENCY vs. RT FOR VARIOUS C T
100K
1K
100
10K
RT - ( )
Oscillator Frequency - (Hz)
100K
1K
10K 1M
100nF
47nF
22nF
10nF
4.7nF
2.2nF
1nF
F ≈where RT ≥ 5kΩ
1.86
RTCT
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
11
Copyright © 2000
Rev. 1.6c
PRODUCTION DATA SHEET
TYPICAL APPLICATION CIRCUITS
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.
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.
Pin numbers referenced are for 8-pin package and pin numbers in parenthesis are for 14-pin package.
FIGURE 14. — CURRENT SENSE SPIKE SUPPRESSION 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.)
The RC low-pass filter will eliminate the leading edge current spike
caused by parasitics of Power MOSFET.
V
CC
V
IN
7 (12)
7 (11)
6 (10)
5 (8)
3 (5)
R
CR
S
Q1
I
PK
I
PK(MAX)
= 1.0V
R
S
SG1844/45
V
CC
V
IN
7 (12)
7 (11)
6 (10)
5 (8)
R
S
Q1
SG1844/45
3 (5)
R
1
SG1844/45
3 (5)
5 (8)
6 (10)
7 (11)
R
S
Q1
V
C1
C
1
R
1
R
2
V
IN
V
C
I
B
+
V
C
R
2
»
V
C1
R
1
|| R
2
»_
V
CC
V
IN
7 (12)
7 (11)
6 (10)
5 (8)
3 (5)
SG1844/45
Isolation
Boundary
C
R
R
S
N
S
N
P
Q1
V
GS
Waveforms
+
_
0
+
_
0
50% DC
25% DC
IPK = V (PIN 1) - 1.4
3RS
NP
NS
C
URRENT
-M
ODE
PWM C
ONTROLLER
SG1844/SG1845 Series
PRODUCT DATABOOK 1996/1997
Copyright © 2000
Rev. 1.6c
12
PRODUCTION DATA SHEET
TYPICAL APPLICATION CIRCUITS (continued)
FIGURE 19. — EXTERNAL DUTY CYCLE CLAMP AND
MULTI-UNIT SYNCHRONIZATION
FIGURE 18. — ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL
WITH SOFTSTART
Precision duty cycle limiting for a duty cycle of <50%, as well as
synchronizing several 1844/45's is possible with the above circuitry.
Softstart and adjustable peak current can be done with the external
circuitry shown above.
IPK = Where: 0 ≤ V1 ≤ 1.0V tSOFTSTART = -ln1 - C
7 (12)
6 (10)
7 (11)
5 (8)
3 (5)
SG1844/45
V
CC
V
IN
Q1
R
S
5 (9)
8 (14)
4 (7)
2 (3)
1 (1)
2N2907
R
2
R
1
1N4148
C
2
6
R
B
R
A
1
84
3
555
TIMER 4 (7)
5 (9)
8 (14)
SG1844/45
To other
SG1844/45
C
FIGURE 21. — ERROR AMPLIFIER CONNECTIONFIGURE 20. — OSCILLATOR CONNECTION
7 (11)
6 (10)
5V
C
T
2.8V
1.1V
SG1844/45
Discharge
Current I
d
= 8.2mA
R
T
1 (1)
R
F
R
i
2 (3)
R
F
³ 10K
2.5V
SG1844/45
0.5mA
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.
Error amplifier is capable of sourcing and sinking current up to 0.5mA.
f = (RA + 2RB)C
1.44
f = RA + 2RB
RB
V1
RS
R1 R2
R1+R2
1.43 - 0.23
1+
VC
V2
and V1 = where: V2 =
R1
R2
R1
R2
0.5
1+ R1
R2
