SG1845L [MICROSEMI]
CURRENT MODE PWM CONTROLLER; 电流模式PWM控制器
| 型号： | SG1845L |
| 厂家： | 
Microsemi |
| 描述： | CURRENT MODE PWM CONTROLLER |
| 文件： | 总12页 (文件大小：216K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LIN DOC #: 1844
SG1844/SG1845 Series
U R R E N T M O D E P W M C O N T R O L L E R
C
T H E I N F I N I T E P O W E R O F I N N O V A T I O N
P R O D U C T I O N D A T A S H E E T
DESCRIPTION
KEY FEATURES
■ OPTIMIZED FOR OFF-LINE CONTROL
■ LOW START-UP CURRENT (<1mA)
■ AUTOMATIC FEED FORWARD
COMPENSATION
■ TRIMMED OSCILLATOR DISCHARGE
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
1ꢀ over temperature. Oscillator
discharge current is trimmed to less than
1ꢁꢀ. The SG1844/45 has under-
voltage 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 <5ꢁꢀ 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 ꢁ°C to 7ꢁ°C.
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
PRODUCT HIGHLIGHT
■ LOW SHOOT-THROUGH CURRENT <75mA
OVER TEMPERATURE
TYPICAL APPLICATION OF SG3844 IN A FLYBACK CONVERTER
HIGH RELIABILITY FEATURES
RST
■ AVAILABLE TO MIL-STD-883B
■
LINFINITY LEVEL "S" PROCESSING AVAILABLE
IST
AC
INPUT
VCC
SG3844
PACKAGE ORDER INFORMATION
Plastic DIP
8-pin
Plastic DIP
14-pin
Plastic SOIC
8-pin
Plastic SOIC
14-pin
Ceramic DIP
8-pin
Ceramic DIP
14-pin
Cer. Flatpack Ceramic LCC
L
1ꢁ-pin 2ꢁ-pin
TA (°C)
0 to 70
M
N
DM
D
Y
J
F
SG3844M
SG3844N
SG3844DM
SG3844D
SG3844Y
SG3845Y
SG2844Y
SG2845Y
SG1844Y
SG1845Y
SG3844J
SG3845J
SG2844J
SG2845J
SG1844J
SG1845J
—
—
—
—
—
—
—
—
—
—
SG3845M
SG3845N
SG3845DM
SG3845D
SG2844M
SG2844N
SG2844DM
SG2844D
—
-25 to 85
-55 to 125
MIL-STD/883
DESC
SG2845M
SG2845N
SG2845DM
SG2845D
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
SG1844L
SG1845L
SG1844Y/883B SG1844J/883B
SG1845Y/883B SG1845J/883B
SG1844L/883B
SG1845L/883B
SG1844Y/DESC SG1844J/DESC SG1844F/DESC SG1844L/DESC
SG1845Y/DESC SG1845J/DESC SG1845F/DESC SG1845L/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 © 2000
Rev. 1.4 4/00
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841
1
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
PACKAGE PIN OUTS
ABSOLUTE MAXIMUM RATINGS (Notes 1 & 2)
Supply Voltage (ICC < 3ꢁmA) ...............................................................Self Limiting
Supply Voltage (Low Impedance Source) ........................................................3ꢁV
Output Current (Peak) ....................................................................................... 1A
Output Current (Continuous) .......................................................................35ꢁmA
Output Energy (Capacitive Load)....................................................................... 5µJ
Analog Inputs (Pins 2, 3) ................................................................. -ꢁ.3V to +6.3V
Error Amp Output Sink Current .....................................................................1ꢁmA
Operating Junction Temperature
Hermetic (J, Y, F, L Packages)................................................................... 15ꢁ°C
Plastic (N, M, D, DM Packages) ................................................................ 15ꢁ°C
Storage Temperature Range.......................................................... -65°C to +15ꢁ°C
Lead Temperature (Soldering, 1ꢁ Seconds).................................................. 3ꢁꢁ°C
1
2
3
4
8
7
6
5
COM
VFB
VREF
VCC
ISENSE
OUT UT
GND
RT/CT
M & Y PACKAGE
(Top View)
1
2
3
4
8
7
6
5
COM
VFB
VREF
VCC
ISENSE
OUT UT
GND
RT/CT
DM PACKAGE
(Top View)
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
13
COM
VREF
2
N.C.
VFB
N.C.
312
4
VCC
11
10
9
N.C.
ISENSE
N.C.
RT/CT
VC
THERMAL DATA
5
OUT UT
GND
WR GND
6
M PACKAGE:
7
8
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
N PACKAGE:
95°C/W
65°C/W
D PACKAGE
(Top View)
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
DM PACKAGE:
COM
N.C.
1
14
13
VREF
2
N.C.
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
D PACKAGE:
165°C/W
120°C/W
130°C/W
80°C/W
VFB
N.C.
ISENSE
N.C.
RT/CT
312
4
VCC
11
10
9
VC
5
OUT UT
GROUND
OWER GND
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
Y PACKAGE:
6
7
8
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
J PACKAGE:
J & N PACKAGE
(Top View)
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
F PACKAGE:
1. COM
10.VREF
1
2
3
4
5
10
9
8
7
6
2. VFB
9. VCC
3. ISENSE
8. VC
THERMAL RESISTANCE-JUNCTION TO CASE, θJC
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
L PACKAGE:
80°C/W
4. RT/CT
7. OUT UT
6. GND
5. OWER GND
145°C/W
F PACKAGE
(Top View)
THERMAL RESISTANCE-JUNCTION TO CASE, θJC
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
35°C/W
120°C/W
3
2
1 20 19
1. N.C.
2. N.C.
11. N.C.
Junction Temperature Calculation: TJ = TA + (PD x θJA).
12. WR GND
The θJA numbers are guidelines for the thermal performance of the device/pc-board
3. COM . 13. GND
4
18
4. N.C.
5. VFB
14. N.C.
15. OUT UT
16. N.C.
17. VC
system. All of the above assume no ambient airflow.
5
6
7
8
17
16
15
14
6. N.C.
7. ISENSE
8. RT/CT
9. N.C.
10. N.C.
18. VCC
19. N.C.
20. VREF
9
10 11 12 13
L PACKAGE
(Top View)
Copyright © 2000
Rev. 1.4 4/00
2
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
RECOMMENDED OPERATING CONDITIONS
(Note 3)
Recommended Operating Conditions
Parameter
Symbol
Units
Min.
Typ.
Max.
Supply Voltage Range
Output Current (Peak)
30
1
V
A
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
200
mA
V
0
2.6
5
mA
kHz
kΩ
µF
0.1
0.52
0.1
500
150
1.0
RT
CT
-55
-25
0
125
85
°C
°C
°C
SG2844/45
SG3844/45
70
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 ꢁ°C ≤ TA ≤ 7ꢁ°C, VCC = 15V (Note 7), RT = 1ꢁkΩ, and CT = 3.3nF. Low duty cycle pulse testing
techniques are used which maintains junction and case temperatures equal to the ambient temperature.)
SG1844/45
SG2844/45
SG3844/45
Parameter
Symbol
Test Conditions
Units
Min. Typ. Max. Min. Typ. Max. Min. Typ. Max.
Reference Section
Output Voltage
Line Regulation
TJ = 25°C, IO = 1mA
12 ≤ VIN ≤ 25V
1 ≤ IO ≤ 20mA
4.95 5.00 5.05 4.95 5.00 5.05 4.90 5.00 5.10
V
6
6
20
25
6
6
20
25
6
6
20
25
mV
mV
Load Regulation
0.2 0.4
5.10 4.90
0.2 0.4
5.10 4.82
0.2 0.4 mV/°C
Temperature Stability (Note 4)
Total Output Variation (Note 4)
Output Noise Voltage (Note 4)
Long Term Stability (Note 4)
Output Short Circuit
Oscillator Section
Initial Accuracy (Note 8)
Voltage Stability
4.90
5.18
V
Line, Load, Temp.
50
5
50
5
50
5
µV
mV
mA
VN
10Hz ≤ f ≤ 10kHz, TJ = 25°C
TA = 125°C, 1000hrs
25
25
25
-30 -100 -180 -30 -100 -180 -30 -100 -180
TJ = 25°C
47
52
.02
5
57
1
47
52
0.2
5
57
1
47
52
0.2
5
57
1
kHz
%
%
12V ≤ VCC ≤ 25V
TMIN ≤ TA ≤ TMAX
VRT/CT (Peak to Peak)
TJ = 25°C
Temperature Stability (Note 4)
Amplitude
1.7
1.7
1.7
V
Discharge Current
7.8 8.3 9.1 7.5 8.4 9.3 7.5 8.4 9.3
6.8 9.3 7.2 9.5 7.2 9.5
mA
mA
TMIN ≤ TA ≤ TMAX
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
VCOMP = 2.5V
2.45 2.50 2.55 2.45 2.50 2.55 2.42 2.50 2.58
V
µA
dB
MHz
dB
mA
mA
V
-0.3 -1
-0.3
90
1
70
6
1
-0.3 -2
AVOL
2 ≤ VO ≤ 4V
TJ = 25°C
65
0.7
60
2
90
1
70
6
65
0.7
60
2
65
0.7
60
2
90
1
70
6
PSRR 12 ≤ VCC ≤ 25V
VVFB = 2.7V, VCOMP = 1.1V
-0.5 -0.8
-0.5 -0.8
-0.5 -0.8
VVFB = 2.3V, VCOMP = 5V
5
6
5
6
5
6
VVFB = 2.3V, RL = 15K to gnd
VVFB = 2.7V, RL = 15K to VREF
0.7 1.1
0.7 1.1
0.7 1.1
V
VOUT Low
(Electrical Characteristics continue next page.)
Copyright © 2000
Rev. 1.4 4/00
3
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
ELECTRICAL CHARACTERISTICS (Cont'd.)
SG1844/45 SG2844/45
SG3844/45
Min. Typ. Max. Min. Typ. Max. Min. Typ. Max.
Parameter
Symbol
Test Conditions
Units
Current Sense Section
Gain (Notes 5 & 6)
Maximum Input Signal (Note 5)
2.85
0.9
3
1
3.15 2.85
1.1 0.9
3
1
3.15 2.85
1.1 0.9
3
1
3.15
1.1
V/V
V
VCOMP = 5V
Power Supply Rejection Ratio (Note 5) PSRR
Input Bias Current
Delay to Output (Note 4)
Output Section
12 ≤ VCC ≤ 25V
70
-2
70
-2
70
-2
dB
µA
ns
-10
-10
-10
150 300
150 300
150 300
Output Low Level
ISINK = 20mA
0.1 0.4
1.5 2.2
0.1 0.4
1.5 2.2
0.1 0.4
1.5 2.2
V
V
ISINK = 200mA
13 13.5
12 13.5
13 13.5
12 13.5
50 150
50 150
13 13.5
12 13.5
50 150
50 150
V
Output High Level
ISOURCE = 20mA
ISOURCE = 200mA
T J = 25°C, CL = 1nF
TJ = 25°C, CL = 1nF
V
ns
ns
50 150
50 150
Rise Time (Note 4)
Fall Time (Note 4)
Under-Voltage Lockout Section
Start Threshold
1844
1845
1844
1845
15
7.8 8.4 9.0 7.8 8.4 9.0 7.8 8.4 9.0
10 11 10 11 8.5 10 11.5
7.0 7.6 8.3 7.0 7.6 8.2 7.0 7.6 8.2
16
17
15
16
17 14.5 16 17.5
V
V
V
V
9
9
Min. Operation Voltage After Turn-On
PWM Section
Maximum Duty Cycle
Minimum Duty Cycle
46
48
50
0
46
48
50
0
46
48
50
0
%
%
Power Consumption Section
Start-Up Current
Operating Supply Current
VCC Zener Voltage
0.5
11
34
1
17
0.5
11
34
1
17
0.5
11
34
1
17
mA
mA
V
VFB = VISENSE = 0V
ICC = 25mA
∆ VCOMP
∆ VISENSE
Notes: 4. These parameters, although guaranteed, are not 1ꢁꢁꢀ tested in
production.
6. Gain defined as: A =
; ꢁ ≤ VISENSE ≤ ꢁ.8V.
7. Adjust VCC above the start threshold before setting at 15V.
8. Output frequency equals one half of oscillator frequency.
5. Parameter measured at trip point of latch with VVFB = ꢁ.
Copyright © 2000
Rev. 1.4 4/00
4
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
BLOCK DIAGRAM
VCC*
34V
UVLO
VREF
5.0V
50mA
5V
S / R
REF
GROUND**
16V (1844)
6V (1844)
8.4 (1845)
0.8V (1845)
INTERNAL
BIAS
2.5V
VREF
VC*
GOOD LOGIC
OSCILLATOR
ERROR AMP
RT/CT
T
OUTPUT
S
2R
POWER GROUND**
R
PWM
VFB
COMP
R
LATCH
1V
CURRENT SENSE
COMPARATOR
CURRENT SENSE
- VCC and VC are internally connected for 8-pin packages.
- POWER GROUND and GROUND are internally connected for 8-pin packages.
*
**
Copyright © 2000
Rev. 1.4 4/00
5
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
GRAPH / CURVE INDEX
FIGURE INDEX
Characteristic Curves
Application Information
FIGURE #
FIGURE #
1. DROPOUT VOLTAGE vs. TEMPERATURE
12. OSCILLATOR TIMING CIRCUIT
2. OSCILLATOR TEMPERATURE STABILITY
13. OSCILLATOR FREQUENCY vs. RT FOR VARIOUS CT
3. CURRENT SENSE TO OUTPUT DELAY vs. TEMPERATURE
4. START-UP CURRENT vs. TEMPERATURE
Typical Applications Section
FIGURE #
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
14. CURRENT SENSE SPIKE SUPPRESSION
15. MOSFET PARASITIC OSCILLATIONS
16. BIPOLAR TRANSISTOR DRIVE
17. ISOLATED MOSFET DRIVE
9. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT AND
TEMPERATURE (SINK TRANSISTOR)
18. ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL WITH
10. CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT
SOFTSTART
11. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT AND
19. EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT
TEMPERATURE (SOURCE TRANSISTOR)
SYNCHRONIZATION
20. OSCILLATOR CONNECTION
21. ERROR AMPLIFIER CONNECTION
Copyright © 2000
Rev. 1.4 4/00
6
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
CHARACTERISTIC CURVES
FIGURE 1. — DROPOUT VOLTAGE vs. TEMPERATURE
FIGURE 2. — OSCILLATOR TEMPERATURE STABILITY
2
0
VIN = 15V
10.0
Duty Cycle = 50%
SG1844
-2
-4
-6
9.6
9.2
8.8
-8
8.4
8.0
-10
SG1845
-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.
FIGURE 4. — START-UP CURRENT vs. TEMPERATURE
TEMPERATURE
220
0.7
SG1844
200
180
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 © 2000
Rev. 1.4 4/00
7
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
CHARACTERISTIC CURVES
FIGURE 5. — REFERENCE VOLTAGE vs. TEMPERATURE
FIGURE 6. — START-UP VOLTAGE THRESHOLD vs.
TEMPERATURE
5.02
8.32
5.01
SG1845
8.30
VCC = 15V
8.28
8.26
8.24
8.22
8.20
8.18
5.00
4.99
4.98
-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 7. — START-UP VOLTAGE THRESHOLD vs.
FIGURE 8. — OSCILLATOR DISCHARGE CURRENT vs.
TEMPERATURE
TEMPERATURE
16.08
8.2
8.0
7.8
7.6
16.06
SG1844
16.04
16.02
16.00
15.98
7.4
7.2
-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 © 2000
Rev. 1.4 4/00
8
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
CHARACTERISTIC CURVES
FIGURE 9. — OUTPUT SATURATION VOLTAGE vs.
FIGURE 10. — CURRENT SENSE THRESHOLD vs.
OUTPUT CURRENT & TEMPERATURE
ERROR AMPLIFIER OUTPUT
2.5
1.0
0.9
0.8
0.7
2.0
C
°
-55
C
°
+25
125°
25°C
-55°C
1.5
1.0
0.6
C
°
+125
0.5
0.4
0.3
0.2
VIN = 15V
Duty Cycle < 5%
0.5
0
0.1
0
100
200
300
400
500
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
VIN = 15V
4.0
Duty Cycle < 5%
3.0
C
°
-55
2.0
1.0
0
100
200
300
400
500
Output Current - (mA)
Copyright © 2000
Rev. 1.4 4/00
9
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
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.
VREF
100K
10K
RT
RT/CT
CT
GND
1K
100
1K
10K
100K
1M
1.86
F ≈
where RT ≥ 5kΩ
RTCT
Oscillator Frequency - (Hz)
FIGURE 13 — OSCILLATOR FREQUENCY vs. RT FOR VARIOUS CT
FIGURE 12 — OSCILLATOR TIMING CIRCUIT
Copyright © 2000
Rev. 1.4 4/00
1ꢁ
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
TYPICAL APPLICATION CIRCUITS
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)
7 (11)
Q1
Q1
R1
6 (10)
5 (8)
SG1844/45
SG1844/45 6 (10)
IPK
R
1.0V
RS
3 (5)
IPK(MAX)
=
5 (8)
3 (5)
RS
C
RS
FIGURE 15. — MOSFET PARASITIC OSCILLATIONS
FIGURE 14. — CURRENT SENSE SPIKE SUPPRESSION
The RC low-pass filter will eliminate the leading edge current spike
caused by parasitics of Power MOSFET.
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
IB
VC
R2
+
_
»
VIN
Isolation
Boundary
7 (12)
VC1
R1|| R 2
»
7 (11)
Q1
VC1
C1
R1
6 (10)
VGS Waveforms
+
VC
7 (11)
6 (10)
SG1844/45
0
_
R2
5 (8)
3 (5)
50% DC
+
Q1
SG1844/45
R
0
_
25% DC
RS
NS
NP
C
5 (8)
3 (5)
RS
NP
V (PIN 1) - 1.4
3RS
IPK
=
NS
FIGURE 16. — BIPOLAR TRANSISTOR DRIVE
FIGURE 17. — ISOLATED MOSFET 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.
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 © 2000
Rev. 1.4 4/00
11
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1844/SG1845 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
TYPICAL APPLICATION CIRCUITS (continued)
VCC
VIN
8 (14)
4 (7)
7 (12)
8
4
RA
RB
8 (14)
4 (7)
2 (3)
1 (1)
7 (11)
6
Q1
SG1844/45
555
TIMER
3
6 (10)
5 (8)
SG1844/45
R2
R1
2
C
2N2907
1
3 (5)
RS
C
5 (9)
5 (9)
1.44
(RA + 2RB)C
To other
SG1844/45
VC
V2
R1 R2
V1
RS
f =
IPK
=
Where: ꢁ ≤ V1 ≤ 1.ꢁV
tSOFTSTART = -ln1 -
C
R1+R2
RB
RA + 2RB
R1
1.43 - ꢁ.23
R2
f =
ꢁ.5
and V1 =
where: V2 =
R1
R2
R1
1+
1+
R2
FIGURE 18. — ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL
FIGURE 19. — EXTERNAL DUTY CYCLE CLAMP AND
WITH SOFTSTART
MULTI-UNIT SYNCHRONIZATION
Softstart and adjustable peak current can be done with the external
circuitry shown above.
Precision duty cycle limiting for a duty cycle of <5ꢁꢀ, as well as
synchronizing several 1844/45's is possible with the above circuitry.
5V
7 (11)
6 (10)
2.8V
1.1V
2.5V
SG1844/45
0.5mA
RT
2 (3)
CT
Ri
SG1844/45
1 (1)
RF
Discharge
Current
RF ³ 10K
Id = 8.2mA
FIGURE 20. — OSCILLATOR CONNECTION
FIGURE 21. — ERROR AMPLIFIER 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.
Error amplifier is capable of sourcing and sinking current up to ꢁ.5mA.
Copyright © 2000
Rev. 1.4 4/00
12
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