LX1555CDT [MICROSEMI]
Switching Controller, Current-mode, 1A, 500kHz Switching Freq-Max, BIPolar, PDSO14, PLASTIC, SOIC-14;
| 型号： | LX1555CDT |
| 厂家： | 
Microsemi |
| 描述： | Switching Controller, Current-mode, 1A, 500kHz Switching Freq-Max, BIPolar, PDSO14, PLASTIC, SOIC-14 光电二极管 |
| 文件： | 总18页 (文件大小：378K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LIN DOC #: 1552
LX1552/3/4/5
U
LTRA-LOW
S
TART-U
P
C
URRENT, CURRENT-MODE PWM
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
■ ULTRA-LOW START-UP CURRENT
The LX155X family of ultra-low start-up
current (250µA max.), current mode
control IC's offer new levels of energy
efficiency for offline converter applica-
tions. They are ideally optimized for
personal computer and CRT power
supplies although they can be used in
any number of off-line applications
where energy efficiency is critical.
Coupled with the fact that the LX155X
series requires a minimal set of external
components, the series offers an
tion. Additionally, the precise oscillator
discharge current gives the power
supply designer considerable flexibility
in optimizing system duty cycle
consistency.
(150µA typ.)
■ TRIMMED OSCILLATOR DISCHARGE
CURRENT (±±2 typ.)
■ INITIAL OSCILLATOR FREQUENCY BETTER
THAN ±±4
The current mode architecture
■ OUTPUT PULLDOWN DURING UVLO
■ PRECISION 2.5V REFERENCE (±±2 max.)
p CURRENT SENSE DELAY TO OUTPUT
(150ns typ.)
p AUTOMATIC FEED FORWARD
COMPENSATION
p PULSE-BY-PULSE CURRENT LIMITING
demonstrates improved load regulation,
pulse by pulse current limiting and
inherent protection of the power supply
output switch. The LX155X includes a
bandgap reference trimmed to 1%, an
error amplifier, a current sense com-
parator internally clamped to 1V, a high
current totem pole output stage for fast
switching of power mosfet's, and an
externally programmable oscillator to
set operating frequency and maximum
duty cycle. The undervoltage lock-out
circuitry is designed to operate with as
little as 250µA of supply current
permitting very efficient bootstrap
designs.
excellent value for cost conscious
consumer applications.
p ENHANCED LOAD RESPONSE
CHARACTERISTICS
p UNDER-VOLTAGE LOCKOUT WITH
HYSTERESIS
p DOUBLE PULSE SUPPRESSION
p HIGH CURRENT TOTEM POLE OUTPUT
Optimizing energy efficiency, the
LX155X series demonstrates a signifi-
cant power reduction as compared with
other similar off-line controllers. Table 1
compares the SG384X, UC384XA and
the LX155X start-up resistor power
dissipation. The LX155X offers an
overall 4X reduction in power dissipa-
(±1Amp peak)
p 500kHz OPERATION
APPLICATIONS
PRODUCT HIGHLIGHT
■ ECONOMY OFF-LINE FLYBACK OR
FORWARD CONVERTERS
TYPICAL APPLICATION OF LX155X USING ITS
MICROPOWER START-UP FEATURE
■ DC-DC BUCK OR BOOST CONVERTERS
■ LOW COST DC MOTOR CONTROL
T A B L E 1
A V A I L A B L E O P T I O N S P E R P A R T #
Design Using
SG384x UC384xA LX155x
R
ST
Part #
Start-Up Hysteresis Max. Duty
Max. Start-up Current
Specification (IST)
1000µA 500µA 250µA
Voltage
16V
Cycle
<100%
<100%
<50%
<50%
I
ST
AC
Typical Start-Up
Resistor Value (RST)
INPUT
LX1552
LX1553
LX1554
LX1555
6V
0.8V
6V
62KΩ 124KΩ 248KΩ
V
CC
8.4V
16V
Max. Start-Up Resistor
Power Dissipation (PR)
2.26W 1.13W 0.56W
LX1552
or
Note: Calculation is done for universal AC input speci-
fication of VACMIN= 90VRMS to VACMAX= 265VRMS using the
following equation: (Resistor current is selected to be
LX1554
8.4V
0.8V
2
IST at VACMIN.)
*
2
VAC MIN
2VAC
MAX
RST
=
, PR =
√2
I
RST
*
ST
PACKAGE ORDER INFORMATION
Plastic DIP
8-pin
Plastic SOIC
8-pin
Plastic SOIC
14-pin
Ceramic DIP
8-pin
TSSOP
PW
20-pin
TA (°C)
M
DM
D
Y
0 to 70
-40 to 85
-55 to 125
LX155xCM
LX155xCDM
LX155xIDM
—
LX155xCD
LX155xID
—
—
—
LX155xCPW
LX155xIM
—
—
—
LX155xMY
Note: All surface-mount packages are available in Tape & Reel. Append the letter "T" to part number. (i.e. LX1552CDMT)
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 © 1994
Rev. 1.0a 1/01
1
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U
P
CURRENT, CURRENT-MODE PWM
P R O D U C T I O N D A T A S H E E T
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE PIN OUTS
Supply Voltage (Low Impedance Source) .................................................................. 30V
Supply Voltage (ICC < 30mA) ......................................................................... Self Limiting
Output Current............................................................................................................. ±1A
Output Energy (Capacitive Load) ................................................................................ 5µJ
Analog Inputs (Pins 2, 3) ........................................................................... -0.3V to +6.3V
Error Amp Output Sink Current............................................................................... 10mA
Power Dissipation at TA = 25°C (DIL-8) ...................................................................... 1W
Operating Junction Temperature
1
2
3
4
8
7
6
5
COMP
VFB
ISENSE
RT/CT
VREF
VCC
OUTPUT
GND
M & Y PACKAGE
(Top View)
Ceramic (Y Package) ............................................................................................ 150°C
Plastic (M, DM, D, PW Packages) ........................................................................ 150°C
Storage Temperature Range .................................................................... -65°C to +150°C
Lead Temperature (Soldering, 10 Seconds) ............................................................ 300°C
1
2
3
4
8
7
6
5
COMP
VFB
ISENSE
RT/CT
VREF
VCC
OUTPUT
GND
Note 1. Exceeding these ratings could cause damage to the device. All voltages are with respect
to Ground. Currents are positive into, negative out of the specified terminal. Pin
numbers refer to DIL packages only.
DM PACKAGE
(Top View)
1
2
3
4
5
6
7
14
13
12
11
10
9
COMP
N.C.
VFB
N.C.
ISENSE
N.C.
RT/CT
VREF
N.C.
VCC
VC
OUTPUT
GND
PWR GND
THERMAL DATA
M PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
DM PACKAGE:
95°C/W
165°C/W
120°C/W
130°C/W
144°C/W
8
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
D PACKAGE:
D PACKAGE
(Top View)
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
Y PACKAGE:
N.C.
N.C.
COMP
VFB
N.C.
ISENSE
N.C.
RT/CT
N.C.
N.C.
N.C.
N.C.
VREF
N.C.
VCC
VC
OUTPUT
GND
PWR GND
N.C.
1
20
19
2
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
PW PACKAGE:
3
18
17
16
15
14
13
12
11
4
5
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
6
Junction Temperature Calculation: TJ = T + (PD x θJA).
7
The θJA numbers are guidelines for the theArmal performance of the device/pc-board system.
8
9
All of the above assume no ambient airflow
10
PW PACKAGE
(Top View)
Copyright © 1994
Rev. 1.0a 1/01
2
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
ELECTRICAL CHARACTERISTICS
(Unless otherwise specified, these specifications apply over the operating ambient temperatures for LX155xC with 0°C ≤TA ≤70°C, LX155xI with -40°C ≤ TA ≤ 85°C, LX155xM
with -55°C ≤TA ≤ 125°C; VCC=15V (Note 5); RT=10K; CT=3.3nF. Low duty cycle pulse testing techniques are used which maintains junction and case temperatures equal to the
ambient temperature.)
LX155xI/155xM
Min. Typ. Max. Min. Typ. Max.
LX155xC
Parameter
Symbol
Test Conditions
Units
Reference Section
Output Voltage
VREF
TA = 25°C, IL = 1mA
4.95 5.00 5.05 4.95 5.00 5.05
V
Line Regulation
12 ≤ VIN ≤ 25V
1 ≤ IO ≤ 20mA
6
6
20
25
6
6
20
25
mV
mV
Load Regulation
0.2 0.4
5.1 4.9
0.2 0.4 mV/°C
Temperature Stability (Note 2 & 7)
Total Output Variation
Output Noise Voltage (Note 2)
Long Term Stability (Note 2)
Output Short Circuit
Over Line, Load, and Temperature
10Hz ≤ f ≤ 10kHz, TA = 25°C
TA = 125°C, t = 1000hrs
4.9
5.1
V
50
5
50
5
µV
mV
mA
VN
ISC
25
25
-30 -100 -180 -30 -100 -180
Oscillator Section
Initial Accuracy (Note 6)
TA = 25°C
48.5 50.5 52.5 48.5 50.5 52.5
kHz
kHz
%
TA = 25°C, RT = 698Ω, CT = 22nF, LX1552/3 only
12 ≤ VCC ≤ 25V
56
58
0.2
5
60
1
56
58
0.2
5
60
1
Voltage Stability
%
Temperature Stability (Note 2)
Amplitude (Note 2)
Discharge Current
TMIN ≤ TA ≤ TMAX
VPIN 4 peak to peak
1.7
1.7
V
mA
mA
8.0 8.3 8.6 8.0 8.3 8.6
7.6 8.8 7.8 8.8
ID
TA = 25°C, VPIN 4 = 2V
VPIN 4 = 2V, TMIN ≤ TA ≤ TMAX
Error Amp Section
Input Voltage
Input Bias Current
VPIN 1 = 2.5V
2.45 2.50 2.55 2.45 2.50 2.55
V
µA
dB
MHz
dB
mA
mA
V
IB
-0.1
90
0.6
70
4
-1
-0.1 -0.5
65
65
90
0.6
70
4
Open Loop Gain
AVOL
2 ≤ VO ≤ 4V
UGBW TA = 25°C
Unity Gain Bandwidth (Note 2)
Power Supply Rejection Ratio (Note 3)
Output Sink Current
Output Source Current
Output Voltage High Level
Output Voltage Low Level
60
2
60
2
PSRR
IOL
IOH
12 ≤ VCC ≤ 25V
VPIN 2 = 2.7V, VPIN 1 = 1.1V
VPIN 2 = 2.3V, VPIN 1 = 5V
-0.5 -0.8
-0.5 -0.8
5
6.5
0.7 1.1
5
6.5
VOH
VOL
VPIN 2 = 2.3V, RL = 15K to ground
VPIN 2 = 2.7V, RL = 15K to VREF
0.7 1.1
V
Current Sense Section
Gain (Note 3 & 4)
AVOL
2.85
0.9
3
1
3.15 2.85
1.1 0.9
3
1
3.15
1.1
V/V
V
Maximum Input Signal (Note 3)
Power Supply Rejection Ratio (Note 3)
Input Bias Current
VPIN 1 = 5V
12 ≤ VCC ≤ 25V
PSRR
IB
TPD
70
-2
70
-2
dB
µA
ns
-10
-5
Delay to Output (Note 2)
VPIN 3 = 0 to 2V
150 300
150 300
Output Section
Output Voltage Low Level
VOL
VOH
ISINK = 20mA
0.1 0.4
1.5 2.2
0.1 0.4
1.5 2.2
13 13.5
V
V
V
ISINK = 200mA
13 13.5
12 13.5
50
Output Voltage High Level
ISOURCE = 20mA
ISOURCE = 200mA
TA = 25°C, CL = 1nF
TA = 25°C, CL = 1nF
VCC = 5V, ISINK = 10mA
12 13.5
V
100
100
50
50
100
100
ns
ns
V
Rise Time (Note 2)
Fall Time (Note 2)
UVLO Saturation
TR
TF
50
0.7 1.2
0.7 1.2
VSAT
(Electrical Characteristics continue next page.)
Copyright © 1994
Rev. 1.0a 1/01
3
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U
P
CURRENT, CURRENT-MODE PWM
P R O D U C T I O N D A T A S H E E T
ELECTRICAL CHARACTERISTICS
(Con't.)
LX155xI/155xM
Min. Typ. Max. Min. Typ. Max.
LX155xC
Parameter
Symbol
Test Conditions
Units
Under-Voltage Lockout Section
Start Threshold
VST
1552/1554
1553/1555
1552/1554
1553/1555
15
7.8 8.4 9.0 7.8 8.4 9.0
10 11 10 11
7.0 7.6 8.2 7.0 7.6 8.2
16
17
15
16
17
V
V
V
V
Min. Operation Voltage After Turn-On
9
9
PWM Section
Maximum Duty Cycle
1552/1553
1552/1553, RT = 698Ω, CT = 22nF
94
47
96
50
48
94
47
96
50
48
%
%
%
%
1554/1555
0
0
Minimum Duty Cycle
Power Consumption Section
Start-Up Current
Operating Supply Current
VCC Zener Voltage
IST
ICC
VZ
150 250
150 250
µA
mA
V
11
35
17
11
35
17
ICC = 25mA
30
30
Notes: 2. These parameters, although guaranteed, are not 100% tested in
7. Temperature stability, sometimes referred to as average temperature
coefficient, is described by the equation:
production.
3. Parameter measured at trip point of latch with VFB = 0.
VREF (max.) - VREF (min.)
Temp Stability =
∆ VCOMP
4. Gain defined as: A =
; 0 ≤ V
≤ 0.8V.
TA (max.) - TA (min.)
ISENSE
∆ V
ISENSE
VREF (max.) & VREF (min.) are the maximum & minimum reference
voltage measured over the appropriate temperature range. Note that the
extremes in voltage do not necessarily occur at the extremes in
temperature.
5. Adjust VCC above the start threshold before setting at 15V.
6. Output frequency equals oscillator frequency for the LX1552 and
LX1553. Output frequency is one half oscillator frequency for the
LX1554 and LX1555.
BLOCK DIAGRAM
VCC*
34V
UVLO
5V
REF
VREF
S / R
GROUND**
16V (1552/1554)
8.4V (1553/1555)
16V (1552/1554)
8.4V (1553/1555)
INTERNAL
BIAS
VREF
GOOD LOGIC
VC*
OSCILLATOR
ERROR AMP
RT/CT
T
OUTPUT
***
S
R
2R
POWER GROUND**
1V
PWM
LATCH
VFB
COMP
ISENSE
R
CURRENT SENSE
COMPARATOR
- VCC and VC are internally connected for 8 pin packages.
- POWER GROUND and GROUND are internally connected for 8 pin packages.
- Toggle flip flop used only in 1554 and 1555.
*
**
***
Copyright © 1994
Rev. 1.0a 1/01
4
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U
P
CURRENT, CURRENT-MODE PWM
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
Theory of Operation Section
FIGURE #
FIGURE #
1. OSCILLATOR FREQUENCY vs. TIMING RESISTOR
2. MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
3. OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE
4. OSCILLATOR FREQUENCY vs. TEMPERATURE
5. OUTPUT INITIAL ACCURACY vs. TEMPERATURE
6. OUTPUT DUTY CYCLE vs. TEMPERATURE
23. TYPICAL APPLICATION OF START-UP CIRCUITRY
24. REFERENCE VOLTAGE vs. TEMPERATURE
25. SIMPLIFIED SCHEMATIC OF OSCILLATOR SECTION
26. DUTY CYCLE VARIATION vs. DISCHARGE CURRENT
27. OSCILLATOR FREQUENCY vs. TIMING RESISTOR
28. MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
29. CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT
7. REFERENCE VOLTAGE vs. TEMPERATURE
8. REFERENCE SHORT CIRCUIT CURRENT vs. TEMPERATURE
9. E.A. INPUT VOLTAGE vs. TEMPERATURE
Typical Applications Section
10. START-UP CURRENT vs. TEMPERATURE
FIGURE #
11. START-UP CURRENT vs. SUPPLY VOLTAGE
30. CURRENT SENSE SPIKE SUPPRESSION
31. MOSFET PARASITIC OSCILLATIONS
12. START-UP CURRENT vs. SUPPLY VOLTAGE
13. DYNAMIC SUPPLY CURRENT vs. OSCILLATOR FREQUENCY
14. CURRENT SENSE DELAY TO OUTPUT vs. TEMPERATURE
15. CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT
16. START-UP THRESHOLD vs. TEMPERATURE
32. ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL
WITH SOFT-START
33. EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT SYCHRONIZATION
34. SLOPE COMPENSATION
17. START-UP THRESHOLD vs. TEMPERATURE
35. OPEN LOOP LABORATORY FIXTURE
36. OFF-LINE FLYBACK REGULATOR
18. MINIMUM OPERATING VOLTAGE vs. TEMPERATURE
19. MINIMUM OPERATING VOLTAGE vs. TEMPERATURE
20. LOW LEVEL OUTPUT SATURATION VOLTAGE DURING UNDER-
VOLTAGE LOCKOUT
21. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and
TEMPERATURE
22. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and
TEMPERATURE
Copyright © 1994
Rev. 1.0a 1/01
5
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
CHARACTERISTIC CURVES
FIGURE 1. — OSCILLATOR FREQUENCY vs. TIMING RESISTOR
FIGURE 2. — MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
100
90
80
70
60
50
40
30
20
1000
CT = 1nF
CT = 3.3nF
100
CT = 6.8nF
10
CT = 22nF
CT = 47nF
1
CT = 0.1µF
VCC = 15V
TA = 25°C
VCC = 15V
TA = 25°C
10
0.1
0.1
0
1
10
100
0.1
1
10
100
(RT) Timing Resistor - (k )
(RT) Timing Resistor - (k )
FIGURE 3. — OSCILLATOR DISCHARGE CURRENT vs.
FIGURE 4. — OSCILLATOR FREQUENCY vs. TEMPERATURE
TEMPERATURE
8.50
55
VCC = 15V
VPIN4 = 2V
54
53
52
51
50
49
48
47
46
45
VCC = 15V
RT = 10k
CT = 3.3nF
8.40
8.30
8.20
8.10
8.00
7.90
7.80
7.70
-75
-50
-25
0
25
50
75
100 125
-75
-50
-25
0
25
50
75
100 125
(TA) Ambient Temperature - (°C)
(TA) Ambient Temperature - (°C)
Copyright © 1994
Rev. 1.0a 1/01
6
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
CHARACTERISTIC CURVES
FIGURE 5. — OUTPUT INITIAL ACCURACY vs. TEMPERATURE
FIGURE 6. — OUTPUT DUTY CYCLE vs. TEMPERATURE
65.0
48
47
46
45
44
43
42
41
40
LX1552 and LX1553 only
VCC = 15V
RT = 698W
CT = 22nF
63.5
62.0
60.5
59.0
57.5
56.0
54.5
53.0
51.5
50.0
VCC = 15V
RT = 698W
CT = 22nF
-75
-50
-25
0
25
50
75
100 125
-75
-50
-25
0
25
50
75
100 125
(TA) Ambient Temperature - (°C)
(TA) Ambient Temperature - (°C)
FIGURE 7. — REFERENCE VOLTAGE vs. TEMPERATURE
FIGURE 8. — REFERENCE SHORT CIRCUIT CURRENT vs.
TEMPERATURE
5.03
180
165
150
135
120
105
90
VCC = 15V
IL = 1mA
5.02
5.01
5.00
4.99
4.98
4.97
4.96
4.95
75
60
45
30
-75
-50
-25
0
25
50
75
100 125
-75
-50
-25
0
25
50
75
100 125
(TA) Ambient Temperature - (°C)
(TA) Ambient Temperature - (°C)
Copyright © 1994
Rev. 1.0a 1/01
7
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
CHARACTERISTIC CURVES
FIGURE 9. — E.A. INPUT VOLTAGE vs. TEMPERATURE
FIGURE 10. — START-UP CURRENT vs. TEMPERATURE
2.55
250
225
200
2.54
2.53
2.52
2.51
2.50
2.49
2.48
2.47
2.46
2.45
VCC = 15V
LX1552/LX1554
175
150
125
100
75
50
25
0
LX1553/LX1555
-75
-50
-25
0
25
50
75
100 125
-75
-50
-25
0
25
50
75
100 125
(TA) Ambient Temperature - (°C)
(TA) Ambient Temperature - (°C)
FIGURE 11. — START-UP CURRENT vs. SUPPLY VOLTAGE
FIGURE 12. — START-UP CURRENT vs. SUPPLY VOLTAGE
250
250
LX1553/LX1555
TA = 25°C
LX1552/LX1554
TA = 25°C
225
225
200
200
175
150
125
100
75
175
150
125
100
75
50
50
25
25
0
0
0
2
4
6
8
10 12 14
16 18 20
0
1
2
3
4
5
6
7
8
9
10
(VCC) Supply Voltage - (V)
(VCC) Supply Voltage - (V)
Copyright © 1994
Rev. 1.0a 1/01
8
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
CHARACTERISTIC CURVES
FIGURE 13. — DYNAMIC SUPPLY CURRENT vs.
FIGURE 14. — CURRENT SENSE DELAY TO OUTPUT vs.
TEMPERATURE
OSCILLATOR FREQUENCY
30
300
270
240
210
180
150
120
90
VCC = 15V
VPIN3 = 0V to 2V
CL = 1nF
27
24
21
18
15
12
9
TA = 25°C
RT = 10k
CL = 1000pF
VIN = 16V
VIN = 12V
VIN = 10V
60
6
30
3
0
0
-75
-50
-25
0
25
50
75
100 125
10
100
1000
(TA) Ambient Temperature - (°C)
Oscillator Frequency - (kHz)
FIGURE 15. — CURRENT SENSE THRESHOLD vs.
FIGURE 16. — START-UP THRESHOLD vs. TEMPERATURE
ERROR AMPLIFIER OUTPUT
8.8
1.1
LX1553
LX1555
8.7
1.0
TA = 125°C
8.6
8.5
8.4
8.3
8.2
8.1
8.0
7.9
7.8
0.9
0.8
TA = 25°C
0.7
0.6
TA = -55°C
0.5
0.4
0.3
0.2
0.1
0
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
-75
-50
-25
0
25
50
75
100 125
Error Amplifier Output Voltage - (V)
(TA) Ambient Temperature - (°C)
Copyright © 1994
Rev. 1.0a 1/01
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U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
CHARACTERISTIC CURVES
FIGURE 17. — START-UP THRESHOLD vs. TEMPERATURE
FIGURE 18. — MINIMUM OPERATING VOLTAGE vs.
TEMPERATURE
17.0
11.0
LX1552
LX1554
LX1552
LX1554
16.8
10.8
16.6
16.4
16.2
16.0
15.8
15.6
15.4
15.2
15.0
10.6
10.4
10.2
10.0
9.8
9.6
9.4
9.2
9.0
-75
-50
-25
0
25
50
75
100 125
-75
-50
-25
0
25
50
75
100 125
(TA) Ambient Temperature - (°C)
(TA) Ambient Temperature - (°C)
FIGURE 19. — MINIMUM OPERATING VOLTAGE vs.
FIGURE 20. — LOW LEVEL OUTPUT SATURATION VOLTAGE
TEMPERATURE
DURING UNDER-VOLTAGE LOCKOUT
8.0
1.20
LX1553
7.9
1.08
0.96
0.84
0.72
0.60
0.48
0.36
0.24
0.12
0.00
VCC = 5V
LX1555
7.8
7.7
7.6
7.5
7.4
7.3
7.2
7.1
7.0
TA = -55°C
TA = 25°C
TA = 125°C
-75
-50
-25
0
25
50
75
100 125
0.1
1
10
(TA) Ambient Temperature - (°C)
Output Sink Current - (mA)
Copyright © 1994
Rev. 1.0a 1/01
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LX1552/3/4/5
U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
CHARACTERISTIC CURVES
FIGURE 21. — OUTPUT SATURATION VOLTAGE vs.
FIGURE 22. — OUTPUT SATURATION VOLTAGE vs.
OUTPUT CURRENT and TEMPERATURE
OUTPUT CURRENT and TEMPERATURE
6.00
5.40
4.80
4.20
3.60
3.00
2.40
1.80
1.20
0.60
0.00
6.0
VCC = 5V
Sink Transistor
5.0
VCC = 15V
Source Transistor
4.0
3.0
TA = -55°C
TA = 25°C
TA = -55°C
2.0
TA = 25°C
TA = 125°C
TA = 125°C
1.0
0.00
10
100
1000
10
100
1000
Output Sink Current - (mA)
Output Source Current - (mA)
Copyright © 1994
Rev. 1.0a 1/01
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U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
THEORY OF OPERATION
IC DESCRIPTION
The start-up capacitor (C1) is charged by current through resistor
(R1) minus the start-up current. Resistor (R1) is designed such
that it provides more than 250µA of current (typically 2x IST(max)).
Once this voltage reaches the start-up threshold, the IC turns on,
starting the switching cycle. This causes an increase in IC
operating current, resulting in discharging the start-up capacitor.
During this time, the auxiliary winding flyback voltage gets
rectified & filtered via (D1) and (C1) and provides sufficient
voltage to continue to operate the IC and support its required
supply current. The start-up capacitor must be large enough such
that during the discharge period, the bootsrap voltage exceeds
the shutdown threshold of the IC.
The LX1552/3/4/5 series of current mode PWM controller IC's are
designed to offer substantial improvements in the areas of start-
up current and oscillator accuracy when compared to the first
generation products, the UC184x series. While they can be used
in most DC-DC applications, they are optimized for single-ended
designs such as Flyback and Forward converters. The LX1552/
54 series are best suited for off-line applications, whereas the
1553/55 series are mostly used in power supplies with low input
voltages. The IC can be divided into six main sections as shown
in the Block Diagram (page 4): undervoltage lockout and start-
up circuit; voltage reference; oscillator; current sense comparator
and PWM latch; error amplifier; and the output stage. The
operation of each section is described in the following sections.
The differences between the members of this family are summa-
rized in Table 1.
Table 2 below shows a comparison of start-up resistor power
dissipation vs. maximum start-up current for different devices.
TABLE 2
TABLE 1
Design Using
SG384x UC384xA LX155x
UVLO
MAXIMUM
DUTY CYCLE
Start-up Voltage Hysterises Voltage
Max. Start-up Current
Specification (IST)
PART #
1000µA
62KΩ
500µA
124KΩ
1.13W
250µA
248KΩ
0.56W
(VST)
(VHYS
)
LX1552
LX1553
LX1554
LX1555
16V
8.4V
16V
6V
<100%
<100%
<50%
Typical Start-Up
Resistor Value (RST)
0.8V
6V
Max. Start-Up Resistor
Power Dissipation (PR)
8.4V
0.8V
<50%
2.26W
UNDERVOLTAGE LOCKOUT
(Resistor R1 is designed such that it provides 2X maximum
start-up current under low line conditions. Maximum power
dissipation is calculated under maximum line conditions. Ex-
ample assumes 90 to 265VAC universal input application.)
The LX155x undervoltage lock-out is designed to maintain an
ultra low quiescent current of less than 250µA, while guarantee-
ing the IC is fully functional before the output stage is activated.
Comparing this to the SG384x series, a 4x reduction in start-up
current is achieved resulting in 75% less power dissipation in the
start-up resistor. This is especially important in off-line power
supplies which are designed to operate for universal input
voltages of 90 to 265V AC.
Figure 23 shows an efficient supply voltage using the ultra low
start-up current of the LX1554 in conjunction with a bootstrap
winding off of the power transformer. Circuit operation is as
follows.
DC BUS
I1 > 250µA
D1
1ST < 250µA
REF
VIN
C1
RT
LX1554
VO
RT/CT
CT
RS
GND
GND
FIGURE 23 — TYPICAL APPLICATION OF START-UP CIRCUITRY
Copyright © 1994
Rev. 1.0a 1/01
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U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
THEORY OF OPERATION
VOLTAGE REFERENCE
REF
5V
VP
VV
The voltage reference is a low drift bandgap design which
provides +5.0V to supply charging current to the oscillator timing
capacitor, as well as supporting internal circuitries. Initial
accuracy for all devices are specified at ±1% max., which is a 2x
improvement for the commercial product when compared to the
SG384x series. The reference is capable of providing in excess
of 20mA for powering any external control circuitries and has
built-in short circuit protection.
IR
S2
RT
2.8V
1.1V
TO OUTPUT
STAGE
RT/CT
A1
S1
5.03
CT
2
1
VCC = 15V
IL = 1mA
5.02
OPEN
ID = 8.3mA
5.01
5.00
4.99
4.98
4.97
4.96
4.95
FIGURE 25 — SIMPLIFIED SCHEMATIC OF OSCILLATOR SECTION
variation is more pronounced when maximum duty cycle has to
be limited to 50% or less. This is due to the fact that for longer
output off time, capacitor discharge current (ID - IR) must be
decreased by increasing IR. Consequently, this increases the
sensitivity of the frequency and duty cycle to any small variations
of the internal current source (ID), making this parameter more
critical under those conditions. Because this is a desired feature
in many applications, this parameter is trimmed to a nominal
current value of 8.3±0.3mA at room temperature, and guaranteed
to a maximum range of 7.8 to 8.8mA over the specified ambient
temperature range. Figure 26 shows variation of oscillator duty
cycle versus discharge current for LX155x and SG384x series
-75
-50
-25
0
25
50
75
100 125
(TA) Ambient Temperature - (°C)
FIGURE 24 — REFERENCE VOLTAGE vs. TEMPERATURE
devices.
OSCILLATOR
100
The oscillator circuit is designed such that discharge current and
valley voltage are trimmed independently. This results in more
accurate initial oscillator frequency and maximum output duty
cycle, especially important in LX1552/53 applications. The
oscillator is programmed by the values selected for the timing
TA = 25°C
VP = 2.7V
V = 1V
VREF = 5V
90
Id = 9.3mA
80
Id = 8.6mA
70
components (RT) and (C ). A simplified schematic of the oscillator
T
is shown in Figure 25. The operation is as follows; Capacitor (CT)
is charged from the 5V reference thru resistor (RT) to a peak
voltage of 2.7V nominally. Once the voltage reaches this
threshold, comparator (A1) changes state, causing (S1) to switch
to position (2) and (S2) to (VV) position. This will allow the
capacitor to discharge with a current equal to the difference
between a constant discharge current (ID) and current through
charging resistor (IR), until the voltage drops down to 1V
nominally and the comparator changes state again, repeating the
cycle. Oscillator charge time results in the output to be in a high
state (on time) and discharge time sets it to a low state (off time).
Since the oscillator period is the sum of the charge and discharge
time, any variations in either of them will ultimately affect stability
of the output frequency and the maximum duty cycle. In fact, this
60
50
40
30
20
SG384x Upper Limit
Id = 8.0mA
Id = 7.5mA
LX155x Limits
SG384x Lower Limit
600
700
800
900
1000
(RT) Timing Resistor - ( )
FIGURE 26 — DUTY CYCLE VARIATION vs. DISCHARGE CURRENT
Copyright © 1994
Rev. 1.0a 1/01
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P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
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U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
THEORY OF OPERATION
OSCILLATOR (continued)
Given: frequency f; maximum duty-cycle Dm
Calculate:
The oscillator is designed such that many values of RT and C will
give the same frequency, but only one combination will yiTeld a
specific duty cycle at a given frequency. A set of charts as well
as the timing equations are given to determine approximate
values of timing components for a given frequency and duty
1
(1.74) Dm -1
1)
R = 277
(Ω), 0.3 ≤ Dm ≤ 0.95
1-Dm
T
Dm
cycle.
(1.74)
-1
1000
CT = 1nF
Note: RT must always be greater than 520Ω for proper
CT = 3.3nF
operation of oscillator circuit.
100
CT = 6.8nF
1.81 Dm
*
*
2)
CT =
(µf)
f
R
T
10
for duty cycles above 95% use:
CT = 22nF
1.81
RTCT
CT = 47nF
1
3)
f ≈
where RT ≥ 5kΩ
CT = 0.1µF
Example: A flyback power supply design requires the duty cycle
to be limited to less than 45%. If the output switching frequency
VCC = 15V
TA = 25°C
is selected to be 100kHz, what are the values of R and CT for the
0.1
0.1
T
1
10
100
a) LX1552/53, and the b) LX1554/55 ?
a) LX1552/53
(RT) Timing Resistor - (k )
FIGURE 27 — OSCILLATOR FREQUENCY vs. TIMING RESISTOR
Given: f = 100kHz
Dm = 0.45
1
.45
100
90
80
70
60
50
40
30
20
(1.74)
(1.74)
-1
R = 267
= 669Ω
.55
.45
T
-1
= .012 µf
1.81 0.45
*
CT =
100x103 669
*
b) LX1554/55
fOUT = ½ fOSC (due to internal flip flop)
fOSC = 200kHz
select CT = 1000pf
using Figure 27 or Equation 3: RT = 9.1k
VCC = 15V
TA = 25°C
10
0
0.1
1
10
100
(RT) Timing Resistor - (k )
FIGURE 28 — MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
Copyright © 1994
Rev. 1.0a 1/01
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P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
THEORY OF OPERATION
CURRENT SENSE COMPARATOR AND PWM LATCH
ERROR AMPLIFIER
Switch current is sensed by an external sense resistor (or a current
transformer), monitored by the C.S. pin and compared internally
with voltage from error amplifier output. The comparator output
resets the PWM latch ensuring that a single pulse appears at the
output for any given oscillator cycle. The LX1554/55 series has
an additional flip flop stage that limits the output to less than 50%
duty cycle range as well as dividing its output frequency to half
of the oscillator frequency. The current sense comparator
threshold is internally clamped to 1V nominally which would
limit peak switch current to:
The error amplifier has a PNP input differential stage with access
to the Inverting input and the output pin. The N.I. input is
internally biased to 2.5 volts and is not available for any external
connections. The maximum input bias current for the LX155XC
series is 0.5µA, while LX155XI/155XM devices are rated for 1µA
maximum over their specified range of ambient temperature.
Low value resistor dividers should be used in order to avoid
output voltage errors caused by the input bias current. The error
amplifier can source 0.5mA and sink 2mA of current. A minimum
feedback resistor (RF) value of is given by:
V
Z
(1) ISP
=
where:
ISP ≡ Peak switch current
3(1.1) + 1.8
RS
R
=
≈ 10K
V ≡ internal zener
FMIN
Z
0.5mA
0.9V ≤ VZ ≤ 1.1V
Equation 1 is used to calculate the value of sense resistor during
the current limit condition where switch current reaches its
maximum level. In normal operation of the converter, the
relationship between peak switch current and error voltage
(voltage at pin 1) is given by:
OUTPUT STAGE
The output section has been specifically designed for direct drive
of power MOSFETs. It has a totempole configuration which is
capable of high peak current for fast charging and discharging of
external MOSFET gate capacitance. This typically results in a rise
and fall time of 50ns for a 1000pf capacitive load. Each output
transistor (source and sink) is capable of supplying 200mA of
continuous current with typical saturation voltages versus tem-
perature as shown in Figures 21 & 22 of the characteristic curve
section. All devices are designed to minimize the amount of
shoot-thru current which is a result of momentary overlap of
output transistors. This allows more efficient usage of the IC at
higher frequencies, as well as improving the noise susceptibility
of the device. Internal circuitry insures that the outputs are held
off during VCC ramp-up. Figure 20, in the characteristic curves
section, shows output sink saturation voltage vs. current at 5V.
V - 2VF
E
(1) ISP
=
where: VE ≡ Voltage at pin 1
V ≡ Diode - Forward voltage
3
R
S
*
F
0.7V at TA = 25°C
The above equation is plotted in Figure 29. Notice that the gain
becomes non-linear above current sense voltages greater than ≈
0.95 volts. It is therefore recommended to operate below this
range during normal operation. This would insure that the overall
closed loop gain of the system will not be affected by the change
in the gain of the current sense stage.
1.1
1.0
TA = 125°C
0.9
0.8
TA = 25°C
0.7
0.6
TA = -55°C
0.5
0.4
0.3
0.2
0.1
0
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Error Amplifier Output Voltage - (V)
FIGURE 29 — CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT
Copyright © 1994
Rev. 1.0a 1/01
15
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U
LTRA-LOW
S
TART-U CURRENT, CURRENT-MODE PWM
P
P R O D U C T I O N D A T A S H E E T
TYPICAL APPLICATION CIRCUITS
Unless otherwise specified, pin numbers refer to 8-pin package.
FIGURE 30. — CURRENT SENSE SPIKE SUPPRESSION FIGURE 31. — MOSFET PARASITIC OSCILLATIONS
VCC
DC BUS
VCC
DC BUS
7
7
Q1
RS
R1
Q1
LX155x
LX155x
6
6
5
IPK
3
1.0V
RS
IPK(MAX)
=
5
C
RS
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 reduces overshoot &
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.)
FIGURE 32. — ADJUSTABLE BUFFERED REDUCTION OF CLAMP
FIGURE 33. — EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT
LEVEL WITH SOFT-START
SYNCHRONIZATION
VCC
VIN
8
7
8
4
RA
RB
8
4
2
1
7
6
LX155x
Q1
IPK
555
TIMER
3
4
LX155x
6
3
R2
R1
VCS
RS
2
5
1
MPSA63
0.01
5
C
5
To other
LX155x devices
R1
VCS
RS
1.44
f =
IPK
=
Where: VCS = 1.67 ( R +R ) and VC.S.MAX = 1V (Typ.)
(RA + 2RB)C
1
2
RB
R1 R2
R1+R2
VEAO - 1.3
f =
tSOFTSTART = -ln 1 -
(
) C
RA + 2RB
R1
5 (
)
R1+R2
where; VEAO ≡ voltage at the Error Amp Output under
minimum line and maximum load conditions.
Soft start and adjustable peak current can be done with the external
circuitry shown above.
Precision duty cycle limiting as well as synchronizing several parts is
possible with the above circuitry.
Copyright © 1994
Rev. 1.0a 1/01
16
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U
P
CURRENT, CURRENT-MODE PWM
P R O D U C T I O N D A T A S H E E T
TYPICAL APPLICATION CIRCUITS (continued)
FIGURE 34. — SLOPE COMPENSATION
VCC
DC BUS
LX155x
7(12)
VO
5V
8(14)
RT
UVLO
S
5V
REF
R
INTERNAL
BIAS
2.5V
2N222A
From VO
VREF
GOOD LOGIC
7(11)
6(10)
RSLOPE
4(7)
OSCILLATOR
Q1
CT
C.S.
COMP
2R
Ri
2(3)
RF
1V
ERROR
AMP
R
5(8)
3(5)
PWM
LATCH
Rd
CF
R
1(1)
C
RS
5(9)
Due to inherent instability of fixed frequency current mode converters running above 50% duty cycle, slope compensation should be
added to either the current sense pin or the error amplifier. Figure 34 shows a typical slope compensation technique. Pin numbers
inside parenthesis refer to 14-pin package.
FIGURE 35. — OPEN LOOP LABORATORY FIXTURE
VREF
RT
VCC
A
LX155x
2N2222
100K
4.7K
1K
COMP
VFB
VREF
1
2
3
4
8
7
6
5
VCC
0.1µF
0.1µF
ERROR AMP
ADJUST
1K
5K
OUTPUT
4.7K
ISENSE
OUTPUT
ISENSE
ADJUST
RTCT
GROUND
GROUND
CT
High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypass capacitors should be
connected to pin 5 in a single point ground. The transistor and 5k potentiometer are used to sample the oscillator waveform and apply an
adjustable ramp to pin 3.
Copyright © 1994
Rev. 1.0a 1/01
17
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
LX1552/3/4/5
U
LTRA-LOW
S
TART-U
P
CURRENT, CURRENT-MODE PWM
P R O D U C T I O N D A T A S H E E T
TYPICAL APPLICATION CIRCUITS (continued)
FIGURE 36. — OFF-LINE FLYBACK REGULATOR
TI
MBR735
4.7Ω 1W
220µF
250V
4.7k
2W
Ω
3600pF
400V
1N4004
1N4004
5V
2-5A
4700µF
10V
250kΩ
1/2W
1N4935
AC
INPUT
1N4004
1N4004
1N4935
16V
LX1554
10µF
20V
20k
Ω
820pF
7
0.01µF
VFB
VCC
2
1
2.5k
Ω
27k
Ω
150k
Ω
COMP
OUT
6
3
3.6kΩ
100pF
VREF
8
4
1kΩ
CUR
SEN
10k
Ω
470pF
0.85kΩ
RT/CT
GND
0.01µF
.0022µF
5
ISOLATION
BOUNDARY
SPECIFICATIONS
Input line voltage:
Input frequency:
90VAC to 130VAC
50 or 60Hz
40KHz 10%
25W maximum
5V +5%
2 to 5A
0.01%/V
8%/A*
* This circuit uses a low-cost feedback scheme in which the DC
voltage developed from the primary-side control winding is
sensed by the LX1554 error amplifier. Load regulation is
therefore dependent on the coupling between secondary and
control windings, and on transformer leakage inductance.
Switching frequency:
Output power:
Output voltage:
Output current:
Line regulation:
Load regulation:
Efficiency @ 25 Watts,
VIN = 90VAC:
70%
65%
VIN = 130VAC:
Output short-circuit current: 2.5Amp average
Copyright © 1994
Rev. 1.0a 1/01
18
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LX1555IDMT
Switching Controller, Current-mode, 1A, 500kHz Switching Freq-Max, BIPolar, PDSO8, PLASTIC, SOIC-8
MICROSEMI
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