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LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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DESCRIPTION The LX155X family of ultra-low start-up current (250A max.), current mode control IC's offer new levels of energy efficiency for offline converter applications. 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 excellent value for cost conscious consumer applications. Optimizing energy efficiency, the LX155X series demonstrates a significant 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 dissipation. Additionally, the precise oscillator discharge current gives the power supply designer considerable flexibility in optimizing system duty cycle consistency. The current mode architecture 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 comparator 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 250A of supply current permitting very efficient bootstrap designs.
K E Y F E AT U R E S
s ULTRA-LOW START-UP CURRENT (150A typ.) s TRIMMED OSCILLATOR DISCHARGE CURRENT (2% typ.) s INITIAL OSCILLATOR FREQUENCY BETTER THAN 4% s OUTPUT PULLDOWN DURING UVLO s 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 p ENHANCED LOAD RESPONSE CHARACTERISTICS p UNDER-VOLTAGE LOCKOUT WITH HYSTERESIS p DOUBLE PULSE SUPPRESSION p HIGH CURRENT TOTEM POLE OUTPUT (1Amp peak) p 500kHz OPERATION
A P P L I C AT I O N S PRODUCT HIGHLIGHT
s ECONOMY OFF-LINE FLYBACK OR FORWARD CONVERTERS s DC-DC BUCK OR BOOST CONVERTERS s LOW COST DC MOTOR CONTROL
TYPICAL APPLICATION OF LX155X USING ITS MICROPOWER START-UP FEATURE
TABLE 1
R ST
Design Using
SG384x UC384xA LX155x 250A
A VA I L A B L E O P T I O N S Part # LX1552 LX1553 LX1554 LX1555
PER
P A RT #
AC INPUT
I ST VCC
Max. Start-up Current 1000A 500A Specification (IST) Typical Start-Up Resistor Value (RST) 62K
Start-Up Hysteresis Max. Duty Voltage Cycle 16V 8.4V 16V 8.4V 6V 0.8V 6V 0.8V <100% <100% <50% <50%
124K 248K
LX1552
or
Max. Start-Up Resistor 2.26W 1.13W 0.56W Power Dissipation (PR)
Note: Calculation is done for universal AC input specification of V ACMIN= 90VRMS to VACMAX= 265V RMS using the following equation: (Resistor curr ent is selected to be 2 * I ST at V ACMIN.)
LX1554
RST =
VAC MIN 2V AC2 MAX , PR = 2 * IST RST
PA C K A G E O R D E R I N F O R M AT I O N TA (C) 0 to 70 -40 to 85 -55 to 125
M Plastic DIP 8-pin
LX155xCM LX155xIM --
DM Plastic SOIC 8-pin
LX155xCDM LX155xIDM --
D Plastic SOIC 14-pin
LX155xCD LX155xID --
Y Ceramic DIP 8-pin
-- -- LX155xMY
PW TSSOP 20-pin
LX155xCPW -- --
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 (c) 1994 Rev. 1.0a 1/01
11861 WESTERN AVENUE, GARDEN GROVE, CA. 92841
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PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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A B S O L U T E M A X I M U M R AT I N G S
(Note 1)
PACKAGE PIN OUTS
COMP VFB ISENSE RT/CT
1 2 3 4 8 7 6 5
Supply Voltage (Low Impedance Source) .................................................................. 30V Supply Voltage (ICC < 30mA) ......................................................................... Self Limiting Output Current ............................................................................................................. 1A Output Energy (Capacitive Load) ................................................................................ 5J Analog Inputs (Pins 2, 3) ........................................................................... -0.3V to +6.3V Error Amp Output Sink Current ............................................................................... 10mA Power Dissipation at TA = 25C (DIL-8) ...................................................................... 1W Operating Junction Temperature Ceramic (Y Package) ............................................................................................ 150C Plastic (M, DM, D, PW 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. All voltages are with respect to Ground. Currents are positive into, negative out of the specified terminal. Pin numbers refer to DIL packages only.
VREF VCC OUTPUT GND
M & Y PACKAGE (Top View)
COMP VFB ISENSE RT/CT
1 2 3 4
8 7 6 5
VREF VCC OUTPUT GND
DM PACKAGE (Top View)
T H E R M A L D ATA
M 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 PW PACKAGE: THERMAL RESISTANCE-JUNCTION TO AMBIENT, JA 144C/W 130C/W 120C/W 165C/W 95C/W
COMP 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 OUTPUT GND PWR GND
D PACKAGE (Top View)
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 2 3 4 5 6 7 8 9 10
20 19 18 17 16 15 14 13 12 11
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
PW PACKAGE (Top View)
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Copyright (c) 1994 Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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ELECTRICAL
CHARACTERISTICS
(Unless otherwise specified, these specifications apply over the operating ambient temperatures for LX155xC with 0C T A 70C, LX155xI with -40C TA 85C, LX155xM with -55C T A 125C; 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.)
Parameter Reference Section
Output Voltage Line Regulation Load Regulation Temperature Stability (Note 2 & 7) Total Output Variation Output Noise Voltage (Note 2) Long Term Stability (Note 2) Output Short Circuit
Symbol
Test Conditions
LX155xI/155xM LX155xC Units Min. Typ. Max. Min. Typ. Max.
4.95 5.00 5.05 4.95 5.00 5.05 6 20 6 20 6 25 6 25 0.2 0.4 0.2 0.4 4.9 5.1 4.9 5.1 50 50 5 25 5 25 -30 -100 -180 -30 -100 -180 48.5 50.5 52.5 48.5 50.5 52.5 56 58 60 56 58 60 0.2 1 0.2 1 5 5 1.7 1.7 8.0 8.3 8.6 8.0 8.3 8.6 7.6 8.8 7.8 8.8 2.45 2.50 2.55 2.45 2.50 2.55 -0.1 -1 -0.1 -0.5 65 90 65 90 0.6 0.6 60 70 60 70 2 4 2 4 -0.5 -0.8 -0.5 -0.8 5 6.5 5 6.5 0.7 1.1 0.7 1.1 2.85 0.9 3 1 70 -2 150 0.1 1.5 13.5 13.5 50 50 0.7 3.15 2.85 1.1 0.9 -10 300 0.4 2.2 13 12 100 100 1.2 3 1 70 -2 150 0.1 1.5 13.5 13.5 50 50 0.7 3.15 1.1 -5 300 0.4 2.2 V mV mV mV/C V V mV mA kHz kHz % % V mA mA V A dB MHz dB mA mA V V V/V V dB A ns V V V V ns ns V
V REF
TA = 25C, IL = 1mA 12 VIN 25V 1 IO 20mA Over Line, Load, and Temperature 10Hz f 10kHz, TA = 25C TA = 125C, t = 1000hrs
VN ISC
Oscillator Section
Initial Accuracy (Note 6) Voltage Stability Temperature Stability (Note 2) Amplitude (Note 2) Discharge Current TA = 25C TA = 25C, R T = 698, CT = 22nF, LX1552/3 only 12 VCC 25V TMIN TA TMAX VPIN 4 peak to peak TA = 25C, VPIN 4 = 2V VPIN 4 = 2V, TMIN TA TMAX VPIN 1 = 2.5V IB AVOL UGBW PSRR IOL IOH V OH V OL AVOL PSRR IB TPD V OL V OH TR TF VSAT VPIN 1 = 5V 12 VCC 25V VPIN 3 = 0 to 2V ISINK = 20mA ISINK = 200mA ISOURCE = 20mA ISOURCE = 200mA TA = 25C, CL = 1nF TA = 25C, CL = 1nF VCC = 5V, ISINK = 10mA 2 VO 4V TA = 25C 12 VCC 25V VPIN 2 = 2.7V, VPIN 1 = 1.1V VPIN 2 = 2.3V, VPIN 1 = 5V VPIN 2 = 2.3V, RL = 15K to ground VPIN 2 = 2.7V, RL = 15K to VREF
ID
Error Amp Section
Input Voltage Input Bias Current Open Loop Gain 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
Current Sense Section
Gain (Note 3 & 4) Maximum Input Signal (Note 3) Power Supply Rejection Ratio (Note 3) Input Bias Current Delay to Output (Note 2)
Output Section
Output Voltage Low Level Output Voltage High Level Rise Time (Note 2) Fall Time (Note 2) UVLO Saturation
13 12
100 100 1.2
( E l e c tr i c a l Cha r a ct er i st i cs cont i nu e next pa g e.)
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ELECTRICAL CHARACTERISTICS Parameter Under-Voltage Lockout Section
Start Threshold Min. Operation Voltage After Turn-On VST 1552/1554 1553/1555 1552/1554 1553/1555 1552/1553 1552/1553, RT = 698, CT = 22nF 1554/1555
(Con't.)
Symbol
Test Conditions
LX155xI/155xM LX155xC Units Min. Typ. Max. Min. Typ. Max.
15 7.8 9 7.0 94 47 16 8.4 10 7.6 96 50 48 0 17 9.0 11 8.2 15 7.8 9 7.0 94 47 16 8.4 10 7.6 96 50 48 0 150 11 35 250 17 17 9.0 11 8.2 V V V V % % % % A mA V
PWM Section
Maximum Duty Cycle
Minimum Duty Cycle
Power Consumption Section
Start-Up Current Operating Supply Current VCC Zener Voltage IST I CC VZ 150 11 35 250 17 30
ICC = 25mA
30
Notes: 2. These parameters, although guaranteed, are not 100% tested in production. 3. Parameter measured at trip point of latch with VFB = 0. V 4. Gain defined as: A = V COMP ; 0 VISENSE 0.8V. ISENSE 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.
7. Temperature stability, sometimes referred to as average temperature coefficient, is described by the equation: Temp Stability = V REF (max.) - VREF (min.) TA (max.) - TA (min.)
V REF (max.) & V REF (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.
BLOCK DIAGRAM
VCC* 34V
UVLO S/R 5V REF VREF
GROUND** 16V (1552/1554) 8.4V (1553/1555) 16V (1552/1554) 8.4V (1553/1555)
INTERNAL BIAS
VREF GOOD LOGIC RT/CT OSCILLATOR *** ERROR AMP 2R R 1V T
VC*
OUTPUT
S R
CURRENT SENSE COMPARATOR PWM LATCH POWER GROUND**
VFB COMP ISENSE
* - 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.
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Copyright (c) 1994 Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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GRAPH / CURVE INDEX
FIGURE INDEX
Characteristic Curves
FIGURE # 1. 2. 3. 4. 5. 6. 7. 8. 9. OSCILLATOR FREQUENCY vs. TIMING RESISTOR MAXIMUM DUTY CYCLE vs. TIMING RESISTOR OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE OSCILLATOR FREQUENCY vs. TEMPERATURE OUTPUT INITIAL ACCURACY vs. TEMPERATURE OUTPUT DUTY CYCLE vs. TEMPERATURE REFERENCE VOLTAGE vs. TEMPERATURE REFERENCE SHORT CIRCUIT CURRENT vs. TEMPERATURE E.A. INPUT VOLTAGE vs. TEMPERATURE FIGURE # FIGURE #
Theory of Operation Section
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
Typical Applications Section
30. CURRENT SENSE SPIKE SUPPRESSION 31. MOSFET PARASITIC OSCILLATIONS 32. ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL WITH SOFT-START 33. EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT SYCHRONIZATION 34. SLOPE COMPENSATION 35. OPEN LOOP LABORATORY FIXTURE 36. OFF-LINE FLYBACK REGULATOR
10. START-UP CURRENT vs. TEMPERATURE 11. START-UP CURRENT vs. SUPPLY VOLTAGE 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 17. START-UP THRESHOLD vs. TEMPERATURE 18. MINIMUM OPERATING VOLTAGE vs. TEMPERATURE 19. MINIMUM OPERATING VOLTAGE vs. TEMPERATURE 20. LOW LEVEL OUTPUT SATURATION VOLTAGE DURING UNDERVOLTAGE LOCKOUT 21. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and TEMPERATURE 22. OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and TEMPERATURE
Copyright (c) 1994 Rev. 1.0a 1/01
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ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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CHARACTERISTIC
C U RV E S
FIGURE 1. -- OSCILLATOR FREQUENCY vs. TIMING RESISTOR
1000 CT = 1nF
FIGURE 2. -- MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
100 90 80
Oscillator Frequency - (kHz)
CT = 3.3nF CT = 6.8nF
Maximum Duty Cycle - (%)
100
70 60 50 40 30 20 10
10 CT = 22nF 1 CT = 47nF CT = 0.1F
0.1 0.1
VCC = 15V TA = 25C
1 10 100
VCC = 15V TA = 25C
1 10 100
0 0.1
(RT) Timing Resistor - (k )
(RT) Timing Resistor - (k )
FIGURE 3. -- OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE
8.50
FIGURE 4. -- OSCILLATOR FREQUENCY vs. TEMPERATURE
55
(Id) Oscillator Discharge Current - (mA)
8.40 8.30 8.20 8.10 8.00 7.90 7.80 7.70 -75
Oscillator Frequency - (KHz)
VCC = 15V VPIN4 = 2V
54 53 52 51 50 49 48 47 46 45 -75
VCC = 15V RT = 10k CT = 3.3nF
-50
-25
0
25
50
75
100
125
-50
-25
0
25
50
75
100
125
(TA) Ambient Temperature - (C)
(TA) Ambient Temperature - (C)
6
Copyright (c) 1994 Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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CHARACTERISTIC
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FIGURE 5. -- OUTPUT INITIAL ACCURACY vs. TEMPERATURE
65.0 63.5
FIGURE 6. -- OUTPUT DUTY CYCLE vs. TEMPERATURE
48
LX1552 and LX1553 only
Output Initial Accuracy - (kHz)
62.0 60.5 59.0 57.5 56.0 54.5 53.0 51.5 50.0 -75
Output Duty Cycle - (%)
VCC = 15V RT = 698W CT = 22nF
47 46 45 44 43 42 41 40 -75
VCC = 15V RT = 698W CT = 22nF
-50
-25
0
25
50
75
100
125
-50
-25
0
25
50
75
100
125
(TA) Ambient Temperature - (C)
(TA) Ambient Temperature - (C)
FIGURE 7. -- REFERENCE VOLTAGE vs. TEMPERATURE
5.03 5.02
FIGURE 8. -- REFERENCE SHORT CIRCUIT CURRENT vs. TEMPERATURE
(VREF) Reference Voltage - (V)
VCC = 15V IL = 1mA
(ISC) Reference Short Circuit Current - (mA)
125
180 165 150 135 120 105 90 75 60 45 30 -75
5.01 5.00 4.99 4.98 4.97 4.96 4.95 -75
-50
-25
0
25
50
75
100
-50
-25
0
25
50
75
100
125
(TA) Ambient Temperature - (C)
(TA) Ambient Temperature - (C)
Copyright (c) 1994 Rev. 1.0a 1/01
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LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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FIGURE 9. -- E.A. INPUT VOLTAGE vs. TEMPERATURE
2.55 2.54 2.53
FIGURE 10. -- START-UP CURRENT vs. TEMPERATURE
250
VCC = 15V
225
(IST) Start-Up Current - (A)
200 175 150 125 100 75 50 25 0 -75
E.A. Input Voltage - (V)
2.52 2.51 2.50 2.49 2.48 2.47 2.46 2.45 -75
LX1552/LX1554
LX1553/LX1555
-50
-25
0
25
50
75
100
125
-50
-25
0
25
50
75
100
125
(TA) Ambient Temperature - (C)
(TA) Ambient Temperature - (C)
FIGURE 11. -- START-UP CURRENT vs. SUPPLY VOLTAGE
250 225
FIGURE 12. -- START-UP CURRENT vs. SUPPLY VOLTAGE
250
LX1553/LX1555 TA = 25C
225
LX1552/LX1554 TA = 25C
(IST) Start-Up Current - (A)
175 150 125 100 75 50 25 0 0 2 4 6 8 10 12 14 16 18 20
(IST) Start-Up Current - (A)
200
200 175 150 125 100 75 50 25 0 0 1 2 3 4 5 6 7 8 9 10
(VCC) Supply Voltage - (V)
(VCC) Supply Voltage - (V)
8
Copyright (c) 1994 Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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FIGURE 13. -- DYNAMIC SUPPLY CURRENT vs. OSCILLATOR FREQUENCY
30
FIGURE 14. -- CURRENT SENSE DELAY TO OUTPUT vs. TEMPERATURE
300
(ICC) Dynamic Supply Current - (mA)
27 24 21 18 15 12 9 6 3 0 10
(Tpd) C.S. Delay to Output - (ns)
TA = 25C RT = 10k CL = 1000pF
270
VIN = 16V VIN = 12V VIN = 10V
240 210 180 150 120 90 60 30 0 -75
VCC = 15V VPIN3 = 0V to 2V CL = 1nF
100
1000
-50
-25
0
25
50
75
100
125
Oscillator Frequency - (kHz)
(TA) Ambient Temperature - (C)
FIGURE 15. -- CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT
1.1 1.0 TA = 125C
FIGURE 16. -- START-UP THRESHOLD vs. TEMPERATURE
8.8 8.7 8.6
LX1553 LX1555
Current Sense Threshold - (V)
Start-Up Trheshold - (V)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.5 1.0 1.5 2.0 TA = 25C
8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 -75
TA = -55C
2.5
3.0 3.5
4.0
4.5
5.0
-50
-25
0
25
50
75
100
125
Error Amplifier Output Voltage - (V)
(TA) Ambient Temperature - (C)
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FIGURE 17. -- START-UP THRESHOLD vs. TEMPERATURE
17.0 16.8 16.6
FIGURE 18. -- MINIMUM OPERATING VOLTAGE vs. TEMPERATURE
11.0
Minimum Operating Voltage - (V)
LX1552 LX1554
10.8 10.6 10.4 10.2 10.0 9.8 9.6 9.4 9.2 9.0 -75
LX1552 LX1554
Start-Up Trheshold - (V)
16.4 16.2 16.0 15.8 15.6 15.4 15.2 15.0 -75
-50
-25
0
25
50
75
100
125
-50
-25
0
25
50
75
100
125
(TA) Ambient Temperature - (C)
(TA) Ambient Temperature - (C)
FIGURE 19. -- MINIMUM OPERATING VOLTAGE vs. TEMPERATURE
8.0
FIGURE 20. -- LOW LEVEL OUTPUT SATURATION VOLTAGE DURING UNDER-VOLTAGE LOCKOUT
1.20
7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7.0 -75
(VSAT) Output Saturation Voltage - (V)
7.9
Minimum Operating Voltage - (V)
LX1553 LX1555
1.08 0.96 0.84 0.72 0.60 0.48 0.36 0.24 0.12 0.00 0.1
VCC = 5V
TA = -55C TA = 25C
TA = 125C
-50
-25
0
25
50
75
100
125
1
10
(TA) Ambient Temperature - (C)
Output Sink Current - (mA)
10
Copyright (c) 1994 Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
P
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CHARACTERISTIC
C U RV E S
FIGURE 21. -- OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and TEMPERATURE
6.0
FIGURE 22. -- OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and TEMPERATURE
6.00
(VSAT) Output Saturation Voltage - (V)
5.0
(VSAT) Output Saturation Voltage - (V)
VCC = 5V Sink Transistor
5.40 4.80 4.20 3.60 3.00 2.40 1.80 1.20 0.60 0.00
VCC = 15V Source Transistor
4.0
3.0
2.0
TA = -55C TA = 25C TA = 125C
TA = 25C
TA = -55C TA = 125C
1.0
0.00 10 100 1000
10
100
1000
Output Sink Current - (mA)
Output Source Current - (mA)
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LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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THEORY OF OPERATION IC DESCRIPTION The LX1552/3/4/5 series of current mode PWM controller IC's are designed to offer substantial improvements in the areas of startup 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 startup 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 summarized in Table 1. 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 250A 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. Table 2 below shows a comparison of start-up resistor power dissipation vs. maximum start-up current for different devices.
TABLE 1
UVLO
PART # LX1552 LX1553 LX1554 LX1555
Start-up Voltage Hysterises Voltage (VHYS) (VST)
TABLE 2
Design Using SG384x 1000A 62K 2.26W UC384xA 500A 124K 1.13W LX155x 250A 248K 0.56W Max. Start-up Current Specification (IST ) Typical Start-Up Resistor Value (RST ) Max. Start-Up Resistor Power Dissipation (PR)
MAXIMUM DUTY CYCLE <100% <100% <50% <50%
16V 8.4V 16V 8.4V
6V 0.8V 6V 0.8V
UNDERVOLTAGE LOCKOUT The LX155x undervoltage lock-out is designed to maintain an ultra low quiescent current of less than 250A, while guaranteeing 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 > 250A 1ST < 250A REF C1 RT LX1554 RT/CT CT GND GND RS VO VIN D1
(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. Example assumes 90 to 265VAC universal input application.)
FIGURE 23 -- TYPICAL APPLICATION OF START-UP CIRCUITRY
12
Copyright (c) 1994 Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
P
RODUCTION
D
ATA
S
HEET
T H E O R Y O F O P E R AT I O N VOLTAGE REFERENCE 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.
5.03 5.02
REF IR RT
5V
VP VV S2
2.8V
1.1V
RT/CT S1
TO OUTPUT STAGE A1 1 OPEN
(VREF) Reference Voltage - (V)
VCC = 15V IL = 1mA
CT
2 ID = 8.3mA
5.01 5.00
FIGURE 25 -- SIMPLIFIED SCHEMATIC OF OSCILLATOR SECTION
4.99 4.98 4.97 4.96 4.95 -75
-50
-25
0
25
50
75
100
125
(TA) Ambient Temperature - (C)
FIGURE 24 -- REFERENCE VOLTAGE vs. TEMPERATURE
OSCILLATOR 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 components (RT) and (CT). A simplified schematic of the oscillator 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
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 8.30.3mA 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 devices.
100 90
Oscillator Duty Cycle - (%)
80 70 60 50
TA = 25C VP = 2.7V V = 1V VREF = 5V
Id = 9.3mA Id = 8.6mA
SG384x Upper Limit
Id = 8.0mA 40 30 20 600
LX155x Limits SG384x Lower Limit
700 800
Id = 7.5mA
900
1000
(RT) Timing Resistor - ( )
FIGURE 26 -- DUTY CYCLE VARIATION vs. DISCHARGE CURRENT
Copyright (c) 1994 Rev. 1.0a 1/01
13
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
P
RODUCTION
D
ATA
S
HEET
THEORY OF OPERATION OSCILLATOR (continued) Given: frequency f; maximum duty-cycle Dm Calculate: (1.74) (1.74)
1 Dm 1-Dm Dm
The oscillator is designed such that many values of RT and CT will give the same frequency, but only one combination will yield 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 cycle.
1000 CT = 1nF
1)
RT = 277
-1 -1
(), 0.3 Dm 0.95
Oscillator Frequency - (kHz)
CT = 3.3nF 100 CT = 6.8nF
Note: RT must always be greater than 520 for proper operation of oscillator circuit. 2) CT = 1.81 * Dm (f) f * RT
10 CT = 22nF 1 CT = 47nF CT = 0.1F
for duty cycles above 95% use: f 1.81 R TC T where RT 5k
3)
0.1 0.1
VCC = 15V TA = 25C
1 10 100
Example: A flyback power supply design requires the duty cycle to be limited to less than 45%. If the output switching frequency is selected to be 100kHz, what are the values of RT and C T for the a) LX1552/53, and the b) LX1554/55 ? a) LX1552/53 Given: f = 100kHz Dm = 0.45 (1.74) (1.74) CT =
1 .45 .55 .45
(RT) Timing Resistor - (k )
FIGURE 27 -- OSCILLATOR FREQUENCY vs. TIMING RESISTOR
100 90 80
RT = 267
-1 -1
= 669
Maximum Duty Cycle - (%)
70 60 50 40 30 20 10 0 0.1
1.81 * 0.45 = .012 f 100x10 3 * 669
b) LX1554/55 fOUT = 1/2 fOSC (due to internal flip flop) fOSC = 200kHz
VCC = 15V TA = 25C
1 10 100
select CT = 1000pf using Figure 27 or Equation 3: RT = 9.1k
(RT) Timing Resistor - (k )
FIGURE 28 -- MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
14
Copyright (c) 1994 Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
P
RODUCTION
D
ATA
S
HEET
T H E O R Y O F O P E R AT I O N CURRENT SENSE COMPARATOR AND PWM LATCH 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: VZ (1) ISP = where: ISP Peak switch current RS VZ internal zener 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: (1) ISP = VE - 2VF 3 * RS where: VE Voltage at pin 1 VF Diode - Forward voltage 0.7V at TA = 25C ERROR AMPLIFIER 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 0.5A, while LX155XI/155XM devices are rated for 1A 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: RFMIN = 3(1.1) + 1.8 10K 0.5mA
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 temperature 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.
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 = 125C
Current Sense Threshold - (V)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.5 1.0 1.5 2.0 TA = 25C
TA = -55C
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 (c) 1994 Rev. 1.0a 1/01
15
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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
Unless otherwise specified, pin numbers refer to 8-pin package. FIGURE 30. -- CURRENT SENSE SPIKE SUPPRESSION
VCC DC BUS
7
FIGURE 31. -- MOSFET PARASITIC OSCILLATIONS
VCC DC BUS
7
Q1
LX155x
LX155x
R1 6
Q1
6 5 IPK IPK(MAX) = 1.0V RS
RS 5
3 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 33. -- EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT SYNCHRONIZATION
FIGURE 32. -- ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL WITH SOFT-START
VCC VIN
8
7 8 4 Q1 2
1N4148
RA 7 RB 6
8
4
LX155x
6
IPK VCS RS
555 TIMER
LX155x
3 4
1 R2 MPSA63 R1 5 3
2 5 0.01 1 5 To other LX155x devices
C
IPK =
V CS Where: VCS = 1.67 RS VEAO - 1.3 5(
R1 R 1+R2
( R +R ) and V
1 2
R1
= 1V (Typ.) C.S.MAX
f = (R 1.44 )C + 2RB A R f = R + B2R A B
tSOFTSTART = -ln 1 -
)
(
R1 R2 R1+R2
)C
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.
16
Copyright (c) 1994 Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
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
FIGURE 34. -- SLOPE COMPENSATION
(continued)
VCC
DC BUS
LX155x
5V 8(14)
7(12) VO
UVLO S R 5V REF INTERNAL BIAS 2.5V VREF GOOD LOGIC
RT 2N222A
7(11)
RSLOPE From VO CT
4(7) OSCILLATOR 6(10) Q1 C.S. COMP 1V R PWM LATCH 5(8) R 3(5) 5(9) C RS
Ri
2R 2(3) CF RF 1(1) ERROR AMP
Rd
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 2N2222 4.7K 100K 1K ERROR AMP ADJUST 4.7K 5K ISENSE ADJUST 3 ISENSE OUTPUT 6 1 COMP
LX155x
VREF 8
A
VCC
2
VFB
VCC
7
0.1F
0.1F
1K OUTPUT
4
RTCT
GROUND
5
CT
GROUND
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 (c) 1994 Rev. 1.0a 1/01
17
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
ULTRA-LOW START-UP CURRENT, CURRENT-MODE PWM
P
RODUCTION
D
ATA
S
HEET
TYPICAL APPLICATION CIRCUITS
FIGURE 36. -- OFF-LINE FLYBACK REGULATOR
(continued)
4.7 1W 220F 250V 250k 1/2W AC INPUT 1N4004 1N4004 1N4935 16V 20k 2 150k 3.6k 1 4.7k 2W 3600pF 400V
TI
MBR735
1N4004
1N4004
4700F 10V
5V 2-5A
1N4935
LX1554
VFB COMP
7 VCC
1N4935
0.01F
10F 20V
820pF 2.5k
27k
IRF830
OUT 6
100pF 8 10k 4 RT/CT VREF CUR 3 SEN GND 5 1k 470pF
0.85k ISOLATION BOUNDARY
0.01F
.0022F
SPECIFICATIONS
Input line voltage: Input frequency: Switching frequency: Output power: Output voltage: Output current: Line regulation: Load regulation: Efficiency @ 25 Watts, VIN = 90VAC: VIN = 130VAC: Output short-circuit current: 90VAC to 130VAC 50 or 60Hz 40KHz 10% 25W maximum 5V +5% 2 to 5A 0.01%/V 8%/A* 70% 65% 2.5Amp average * 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.
18
Copyright (c) 1994 Rev. 1.0a 1/01

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