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L I N D O C # : 1844
SG1844/SG1845 Series
CURRENT MODE PWM CONTROLLER
T
HE
I
NFINITE
P
OWER
OF
I
NNOVATION
P
RODUCTION
D
ATA
S
HEET
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-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 -55C to 125C. The SG2844/45 is specified for the industrial range of -25C to 85C, and the SG3844/45 is designed for the commercial range of 0C to 70C.
K E Y F E AT U R E S
s OPTIMIZED FOR OFF-LINE CONTROL s LOW START-UP CURRENT (<1mA) s AUTOMATIC FEED FORWARD COMPENSATION s TRIMMED OSCILLATOR DISCHARGE CURRENT s PULSE-BY-PULSE CURRENT LIMITING s ENHANCED LOAD RESPONSE CHARACTERISTICS s UNDER-VOLTAGE LOCKOUT WITH 6V HYSTERESIS (SG1844 only) s DOUBLE PULSE SUPPRESSION s HIGH-CURRENT TOTEM-POLE OUTPUT s INTERNALLY TRIMMED BANDGAP REFERENCE s 500kHz OPERATION s UNDERVOLTAGE LOCKOUT SG1844 - 16 volts SG1845 - 8.4 volts s LOW SHOOT-THROUGH CURRENT <75mA OVER TEMPERATURE
PRODUCT HIGHLIGHT
T Y P I C A L AP P L I C AT I O N
OF
S G 3 8 4 4 I N A F LY B A C K CO N V E R T E R
RST
HIGH RELIABILITY FEATURES
s AVAILABLE TO MIL-STD-883B s LINFINITY LEVEL "S" PROCESSING AVAILABLE
AC INPUT
IST VCC
SG3844
TA (C)
0 to 70 -25 to 85 -55 to 125 MIL-STD/883 DESC
M
Plastic DIP 8-pin
N
Plastic DIP 14-pin
SG3844N SG3845N SG2844N SG2845N -- -- -- -- --
PA C K A G E O R D E R I N F O R M AT I O N Plastic SOIC Plastic SOIC Ceramic DIP Ceramic DIP DM 8-pin D 14-pin Y 8-pin J 14-pin
SG3844DM SG3845DM SG2844DM SG2845DM -- -- -- -- -- SG3844D SG3845D SG2844D SG2845D -- -- -- -- -- SG3844Y SG3845Y SG2844Y SG2845Y SG1844Y SG1845Y SG3844J SG3845J SG2844J SG2845J SG1844J SG1845J
F 10-pin
-- -- -- -- -- -- -- --
Cer. Flatpack
L 20-pin
-- -- -- --
Ceramic LCC
SG3844M SG3845M SG2844M SG2845M -- -- -- -- --
SG1844L SG1845L SG1844L/883B SG1845L/883B
SG1844Y/883B SG1844J/883B SG1845Y/883B SG1845J/883B SG1844Y/DESC SG1844J/DESC
SG1844F/DESC SG1844L/DESC SG1845F/DESC SG1845L/DESC
-- -- SG1845Y/DESC SG1845J/DESC Note: All surface-mount packages are available in Tape & Reel.
F O R F U R T H E R I N F O R M AT I O N C A L L ( 7 1 4 ) 8 9 8 - 8 1 2 1
Copyright (c) 2000 Rev. 1.4 4/00
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841
1
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
A B S O L U T E M A X I M U M R AT I N G S
(Notes 1 & 2)
PACKAGE PIN OUTS
COM VFB ISENSE RT/CT
1 2 3 4 8 7 6 5
Supply Voltage (ICC < 30mA) ............................................................... Self Limiting Supply Voltage (Low Impedance Source) ........................................................ 30V Output Current (Peak) ....................................................................................... 1A Output Current (Continuous) ....................................................................... 350mA Output Energy (Capacitive Load) ....................................................................... 5J 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) ................................................................... 150C Plastic (N, M, D, DM Packages) ................................................................ 150C Storage Temperature Range .......................................................... -65C to +150C Lead Temperature (Soldering, 10 Seconds) .................................................. 300C
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.
VREF VCC OUT UT GND
M & Y PACKAGE (Top View)
COM VFB ISENSE RT/CT
1 2 3 4 8 7 6 5
VREF VCC OUT UT GND
DM PACKAGE (Top View)
COM N.C. VFB N.C. ISENSE N.C. RT/CT
1 2 3 4 5 6 7 14 13 12 11 10 9 8
T H E R M A L D ATA
M PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA N PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA DM PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA D PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA Y PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA J PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA F PACKAGE: THERMAL RESISTANCE-JUNCTION TO CASE, JC THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA L PACKAGE: THERMAL RESISTANCE-JUNCTION TO CASE, JC THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA 35C/W 120C/W
3 2
VREF N.C. VCC VC OUT UT GND WR GND
95C/W 65C/W 165C/W 120C/W 130C/W 80C/W 80C/W 145C/W
D PACKAGE (Top View)
COM N.C. VFB N.C. ISENSE N.C. RT/CT
1 2 3 4 5 6 7 14 13 12 11 10 9 8
VREF N.C. VCC VC OUT UT GROUND OWER GND
J & N PACKAGE (Top View)
1. COM 2. VFB 3. ISENSE 4. RT/CT 5. OWER GND
1 2 3 4 5 10 9 8 7 6
10.VREF 9. VCC 8. VC 7. OUT UT 6. GND
F PACKAGE (Top View)
1 20 19
Junction Temperature Calculation: TJ = TA + (PD x JA). The JA numbers are guidelines for the thermal performance of the device/pc-board system. All of the above assume no ambient airflow.
4 5 6 7 8
18 17 16 15 14
9 10 11 12 13
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.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
N.C. WR GND GND N.C. OUT UT N.C. VC VCC N.C. VREF
L PACKAGE (Top View)
2
Copyright (c) 2000 Rev. 1.4 4/00
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
R E C O M M E N D E D O P E R AT I N G C O N D I T I O N S Parameter
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 Oscillator Timing Capacitor Operating Ambient Temperature Range: SG1844/45 SG2844/45 SG3844/45 Note 3. Range over which the device is functional.
(Note 3)
Symbol
Recommended Operating Conditions Min. Typ. Max.
30 1 200 0 5 0.1 0.52 0.1 -55 -25 0 2.6 500 150 1.0 125 85 70
Units
V A mA V mA kHz k F C C C
RT CT
ELECTRICAL CHARACTERISTICS
(Unless otherwise specified, these specifications apply over the operating ambient temperatures for SG1844/SG1845 with -55C TA 125C, SG2844/ SG2845 with -25C TA 85C, SG3844/SG3845 with 0C TA 70C, VCC = 15V (Note 7), RT = 10k, and CT = 3.3nF. Low duty cycle pulse testing techniques are used which maintains junction and case temperatures equal to the ambient temperature.)
Parameter Reference Section
Output Voltage Line Regulation Load Regulation Temperature Stability (Note 4) Total Output Variation (Note 4) Output Noise Voltage (Note 4) Long Term Stability (Note 4) Output Short Circuit
Symbol
Test Conditions
SG1844/45 SG2844/45 SG3844/45 Units Min. Typ. Max. Min. Typ. Max. Min. Typ. Max.
4.95 5.00 5.05 4.95 5.00 5.05 4.90 5.00 5.10 6 20 6 20 6 20 6 25 6 25 6 25 0.2 0.4 0.2 0.4 0.2 0.4 4.90 5.10 4.90 5.10 4.82 5.18 50 50 50 5 25 5 25 5 25 -30 -100 -180 -30 -100 -180 -30 -100 -180 47 52 .02 5 1.7 8.3 57 1 47 52 0.2 5 1.7 8.4 57 1 47 52 0.2 5 1.7 8.4 57 1 V mV mV mV/C V V mV mA kHz % % V mA mA V A dB MHz dB mA mA V V
TJ = 25C, IO = 1mA 12 VIN 25V 1 IO 20mA Line, Load, Temp. 10Hz f 10kHz, TJ = 25C TA = 125C, 1000hrs
VN
Oscillator Section
Initial Accuracy (Note 8) Voltage Stability Temperature Stability (Note 4) Amplitude Discharge Current TJ = 25C 12V VCC 25V TMIN TA TMAX VRT/CT (Peak to Peak) TJ = 25C TMIN TA TMAX VCOMP = 2.5V AVOL PSRR 2 VO 4V TJ = 25C 12 VCC 25V VVFB = 2.7V, VCOMP = 1.1V VVFB = 2.3V, VCOMP = 5V VVFB = 2.3V, RL = 15K to gnd VVFB = 2.7V, RL = 15K to VREF
7.8 6.8
9.1 9.3
7.5 7.2
9.3 9.5
7.5 7.2
9.3 9.5
Error Amp Section
Input Voltage Input Bias Current Open Loop Gain Unity Gain Bandwidth (Note 4) Power Supply Rejection Ratio Output Sink Current Output Source Current VOUT High VOUT Low 2.45 2.50 2.55 2.45 2.50 2.55 2.42 2.50 2.58 -0.3 -1 -0.3 1 -0.3 -2 65 90 65 90 65 90 0.7 1 0.7 1 0.7 1 60 70 60 70 60 70 2 6 2 6 2 6 -0.5 -0.8 -0.5 -0.8 -0.5 -0.8 5 6 5 6 5 6 0.7 1.1 0.7 1.1 0.7 1.1
(Electrical Characteristics continue next page.)
Copyright (c) 2000 Rev. 1.4 4/00
3
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
ELECTRICAL CHARACTERISTICS Parameter Current Sense Section
Gain (Notes 5 & 6) Maximum Input Signal (Note 5) Power Supply Rejection Ratio (Note 5) PSRR Input Bias Current Delay to Output (Note 4) VCOMP = 5V 12 VCC 25V 2.85 0.9 3 1 70 -2 150 0.1 1.5 13.5 13.5 50 50 16 8.4 10 7.6 48
(Cont'd.)
Symbol
Test Conditions
SG1844/45 SG2844/45 SG3844/45 Units Min. Typ. Max. Min. Typ. Max. Min. Typ. Max.
3.15 2.85 1.1 0.9 -10 300 0.4 2.2 13 12 150 150 17 9.0 11 8.3 50 0 1 17 15 7.8 9 7.0 46 3 1 70 -2 150 0.1 1.5 13.5 13.5 50 50 16 8.4 10 7.6 48 3.15 2.85 1.1 0.9 -10 300 0.4 2.2 13 12 150 150 17 9.0 11 8.2 50 0 14.5 7.8 8.5 7.0 46 3 1 70 -2 150 0.1 1.5 13.5 13.5 50 50 16 8.4 10 7.6 48 3.15 1.1 -10 300 0.4 2.2 V/V V dB A ns V V V V ns ns V V V V % %
Output Section
Output Low Level Output High Level Rise Time (Note 4) Fall Time (Note 4) ISINK = 20mA ISINK = 200mA ISOURCE = 20mA ISOURCE = 200mA T J = 25C, CL = 1nF TJ = 25C, CL = 1nF 1844 1845 1844 1845
13 12
150 150 17.5 9.0 11.5 8.2 50 0
Under-Voltage Lockout Section
Start Threshold Min. Operation Voltage After Turn-On 15 7.8 9 7.0 46
PWM Section
Maximum Duty Cycle Minimum Duty Cycle
Power Consumption Section
Start-Up Current Operating Supply Current VCC Zener Voltage VFB = VISENSE = 0V ICC = 25mA 0.5 1 0.5 1 mA 11 17 11 17 mA 34 34 V VCOMP 6. Gain defined as: A = V ; 0 VISENSE 0.8V. ISENSE 7. Adjust VCC above the start threshold before setting at 15V. 8. Output frequency equals one half of oscillator frequency. 0.5 11 34
Notes: 4. These parameters, although guaranteed, are not 100% tested in production. 5. Parameter measured at trip point of latch with VVFB = 0.
4
Copyright (c) 2000 Rev. 1.4 4/00
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
BLOCK DIAGRAM
VCC* 34V
UVLO S/R 5V REF
GROUND** 16V (1844) 8.4 (1845) 2.5V 6V (1844) 0.8V (1845)
VREF 5.0V 50mA
INTERNAL BIAS
VREF GOOD LOGIC RT/CT OSCILLATOR T ERROR AMP
VC*
OUTPUT
2R R 1V
S R
CURRENT SENSE COMPARATOR PWM LATCH POWER GROUND**
VFB COMP CURRENT SENSE
* - VCC and VC are internally connected for 8-pin packages. ** - POWER GROUND and GROUND are internally connected for 8-pin packages.
Copyright (c) 2000 Rev. 1.4 4/00
5
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
GRAPH / CURVE INDEX
FIGURE INDEX
Characteristic Curves
FIGURE # 1. 2. 3. 4. 5. 6. 7. 8. 9. DROPOUT VOLTAGE vs. TEMPERATURE OSCILLATOR TEMPERATURE STABILITY CURRENT SENSE TO OUTPUT DELAY vs. TEMPERATURE START-UP CURRENT vs. TEMPERATURE REFERENCE VOLTAGE vs. TEMPERATURE START-UP VOLTAGE THRESHOLD vs. TEMPERATURE START-UP VOLTAGE THRESHOLD vs. TEMPERATURE OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT AND TEMPERATURE (SINK TRANSISTOR) FIGURE # FIGURE #
Application Information
12. OSCILLATOR TIMING CIRCUIT 13. OSCILLATOR FREQUENCY vs. RT FOR VARIOUS CT
Typical Applications Section
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
10. CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT 11. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT AND TEMPERATURE (SOURCE TRANSISTOR)
6
Copyright (c) 2000 Rev. 1.4 4/00
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
CHARACTERISTIC CURVES
FIGURE 1. -- DROPOUT VOLTAGE vs. TEMPERATURE
FIGURE 2. -- OSCILLATOR TEMPERATURE STABILITY
2
Minimum Operating Voltage - (V)
10.0
0
SG1844
VIN = 15V Duty Cycle = 50%
Frequency Drift - (%)
SG1845
9.6 9.2 8.8
-2 -4 -6
8.4 8.0
-8 -10
-75
-50
-25
0
25
50
75
100
125
-75
-50
-25
0
25
50
75
100
125
Junction Temperature - (C)
Junction Temperature - (C)
FIGURE 3. -- CURRENT SENSE TO OUTPUT DELAY vs. TEMPERATURE
FIGURE 4. -- START-UP CURRENT vs. TEMPERATURE
220
0.7
Current Sense Delay - (nS)
SG1844
200 180
Start-Up Current - (mA)
0.6 0.5 0.4
VPIN3 = 1.1V
160
140 120
0.3
SG1845
0.2
-75
-50
-25
0
25
50
75
100
125
-75
-50
-25
0
25
50
75
100
125
Junction Temperature - (C)
Junction Temperature - (C)
Copyright (c) 2000 Rev. 1.4 4/00
7
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
CHARACTERISTIC CURVES
FIGURE 5. -- REFERENCE VOLTAGE vs. TEMPERATURE
5.02
FIGURE 6. -- START-UP VOLTAGE THRESHOLD vs. TEMPERATURE
8.32
5.01
SG1845
Reference Voltage - (V)
VCC = 15V
5.00
Reference Voltage - (V)
50 75 100 125
8.30 8.28 8.26 8.24 8.22 8.20 8.18 -75
4.99
4.98
-75
-50
-25
0
25
-50
-25
0
25
50
75
100
125
Junction Temperature - (C)
Junction Temperature - (C)
FIGURE 7. -- START-UP VOLTAGE THRESHOLD vs. TEMPERATURE
16.08
FIGURE 8. -- OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE
8.2
16.06
Oscillator Discharge Current - (mA)
8.0
Start Up Voltage - (V)
SG1844
16.04
7.8
16.02
7.6
16.00
7.4
15.98 -75 -50 -25 0 25 50 75 100 125
7.2 -75 -50 -25 0 25 50 75 100 125
Junction Temperature - (C)
Junction Temperature - (C)
8
Copyright (c) 2000 Rev. 1.4 4/00
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
CHARACTERISTIC CURVES
FIGURE 9. -- OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT & TEMPERATURE
2.5
FIGURE 10. -- CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT
1.0 0.9
2.0
Current Sense Threshold - (V)
Saturation Voltage - (V)
C +25
1.5
0.7 0.6 0.5 0.4 0.3 0.2 0.1
5C +12
1.0
VIN = 15V Duty Cycle < 5%
0.5
0 100 200 300 400 500
0 1.0 2.0 3.0 4.0 5.0
Output Current - (mA)
Error Amp Output Voltage - (V)
FIGURE 11. -- OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT & TEMPERATURE
4.0
Saturation Voltage - (V)
VIN = 15V Duty Cycle < 5%
C 25 +1 5C +2
3.0
C -55
C +25
2.0
5C +12
1.0
0 100 200 300 400 500
Output Current - (mA)
Copyright (c) 2000 Rev. 1.4 4/00
12 5C 25 C -55 C
C -55
0.8
9
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
A P P L I C AT I O N I N F O R M AT I O N 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.
VREF RT RT/CT CT GND
RT - ( )
100K
47 nF
F 0n 10
F 7n 4. nF 10 nF 22
F 1n F 2n 2.
10K
F
1.86 RTC T
where RT 5k
1K 100
1K
10K
100K
1M
Oscillator Frequency - (Hz)
FIGURE 13 -- OSCILLATOR FREQUENCY vs. RT FOR VARIOUS CT
FIGURE 12 -- OSCILLATOR TIMING CIRCUIT
10
Copyright (c) 2000 Rev. 1.4 4/00
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
T Y P I C A L A P P L I C AT I O N C I R C U I T S
Pin numbers referenced are for 8-pin package and pin numbers in parenthesis are for 14-pin package.
VCC VIN VCC VIN
7 (12)
7 (12)
7 (11) 6 (10) 5 (8) R 3 (5) C
Q1 SG1844/45 IPK
7 (11) R1 6 (10) Q1
SG1844/45
RS
IPK(MAX) = 1.0V RS 3 (5)
5 (8) RS
FIGURE 14. -- CURRENT SENSE SPIKE SUPPRESSION The RC low-pass filter will eliminate the leading edge current spike caused by parasitics of Power MOSFET.
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.)
VCC VIN Isolation Boundary
Q1
IB VC + R2 VC1 R1| R2 _ | VC VC1 7 (11) R2 SG1844/45 6 (10) C1 R1 Q1
SG1844/45 5 (8)
VIN
7 (12) 7 (11) 6 (10)
VGS Waveforms + 0 _ 50% DC
R
+ 0 _ 25% DC
3 (5)
5 (8)
C
RS
NS
NP
3 (5) RS
IPK =
V (PIN 1) - 1.4 3RS
N P NS
FIGURE 16. -- BIPOLAR TRANSISTOR DRIVE 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.
FIGURE 17. -- ISOLATED MOSFET 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.
Copyright (c) 2000 Rev. 1.4 4/00
11
PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
CURRENT-MODE PWM CONTROLLER
P
RODUCTION
D
ATA
S
HEET
T Y P I C A L A P P L I C AT I O N C I R C U I T S
VCC VIN
(continued)
8 (14)
7 (12) 8 (14) 7 (11) 4 (7) 2 (3) Q1
RA 6
8
4
SG1844/45
RB 2 1 5 (9) 555 TIMER 3 4 (7)
SG1844/45
6 (10) 5 (8)
1N4148
1 (1) R2 2N2907 R1 5 (9)
3 (5) RS
C
C
IPK =
V1 Where: 0 V1 1.0V RS 1.43 - 0.23
R1 R2 R
VC R 1 R2 tSOFTSTART = -ln 1 C V2 R1+R 2
where: V2 = 0.5 1+
R1 R2
f= f=
1.44 (RA + 2RB)C RB RA + 2RB
To other SG1844/45
and V1 =
1+ R1 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.
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.
2.8V 1.1V
5V
7 (11)
2.5V
RT
SG1844/45 0.5mA
6 (10) CT SG1844/45
2 (3) Ri RF 1 (1) RF 10K
Discharge Current
Id = 8.2mA
FIGURE 20. -- OSCILLATOR CONNECTION 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.
FIGURE 21. -- ERROR AMPLIFIER CONNECTION Error amplifier is capable of sourcing and sinking current up to 0.5mA.
12
Copyright (c) 2000 Rev. 1.4 4/00

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