Microsemi LX1552 Ultra-low start-up current, current-mode pwm Datasheet

L I N D O C # : 1552
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
T
I
H E
N F I N I T E
P
O W E R
O F
I
P
N N O VA T I O N
R O D U C T I O N
DESCRIPTION
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 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 dissipa-
D
S
A T A
H E E T
K E Y F E AT U R E S
■ ULTRA-LOW START-UP CURRENT
(150µA typ.)
■ TRIMMED OSCILLATOR DISCHARGE
CURRENT (±2% typ.)
■ INITIAL OSCILLATOR FREQUENCY BETTER
THAN ±4%
■ 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
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
tion. 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 250µA of supply current
permitting very efficient bootstrap
designs.
A P P L I C AT I O N S
PRODUCT HIGHLIGHT
■ ECONOMY OFF-LINE FLYBACK OR
FORWARD CONVERTERS
■ DC-DC BUCK OR BOOST CONVERTERS
■ LOW COST DC MOTOR CONTROL
T YPICAL A PPLICATION OF LX155X U SING I TS
M ICRO P OWER S TART -U P F EATURE
TABLE 1
R ST
I ST
AC
INPUT
VCC
LX1552
or
LX1554
Design Using
A VA I L A B L E O P T I O N S
SG384x UC384xA LX155x
Max. Start-up Current 1000µA 500µA
Specification (IST)
Typical Start-Up
Resistor Value (RST)
62KΩ
Ω
Part #
250µA
Ω
124KΩ
Ω 248KΩ
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.)
RST =
PER
P A RT #
Start-Up Hysteresis Max. Duty
Voltage
Cycle
LX1552
16V
6V
<100%
LX1553
8.4V
0.8V
<100%
LX1554
16V
6V
<50%
LX1555
8.4V
0.8V
<50%
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
DIP
M Plastic
8-pin
TA (°C)
SOIC
DM Plastic
8-pin
SOIC
D Plastic
14-pin
DIP
Y Ceramic
8-pin
PW TSSOP
20-pin
0 to 70
LX155xCM
LX155xCDM
LX155xCD
—
LX155xCPW
-40 to 85
-55 to 125
LX155xIM
—
LX155xIDM
—
LX155xID
—
—
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
11861 WESTERN A VENUE , G ARDEN G ROVE , CA. 92841
1
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
A B S O L U T E M A X I M U M R AT I N G S
D
A T A
S
H E E T
PACKAGE PIN OUTS
(Note 1)
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
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
COMP
VFB
ISENSE
RT/CT
M PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA
95°C/W
DM PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA
165°C/W
6
4
5
1
8
2
7
3
6
4
5
VREF
VCC
OUTPUT
GND
DM PACKAGE
(Top View)
COMP
N.C.
VFB
N.C.
ISENSE
N.C.
RT/CT
1
14
2
13
3
12
4
11
5
10
6
9
7
8
VREF
N.C.
VCC
VC
OUTPUT
GND
PWR GND
120°C/W
130°C/W
PW PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA
3
VREF
VCC
OUTPUT
GND
D PACKAGE
(Top View)
Y PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA
7
COMP
VFB
ISENSE
RT/CT
D PACKAGE:
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θ JA
8
2
M & Y PACKAGE
(Top View)
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.
T H E R M A L D ATA
1
144°C/W
Junction Temperature Calculation: TJ = TA + (PD x θJA).
The θ JA numbers are guidelines for the thermal performance of the device/pc-board system.
All of the above assume no ambient airflow
N.C.
N.C.
COMP
VFB
N.C.
ISENSE
N.C.
RT/CT
N.C.
N.C.
1
20
2
19
3
18
4
17
5
16
6
15
7
14
8
13
9
12
10
11
N.C.
N.C.
VREF
N.C.
VCC
VC
OUTPUT
GND
PWR GND
N.C.
PW PACKAGE
(Top View)
2
Copyright © 1994
Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
ELECTRICAL
D
A T A
S
H E E T
CHARACTERISTICS
(Unless otherwise specified, these specifications apply over the operating ambient temperatures for LX155xC with 0°C ≤ T A ≤ 70°C, LX155xI with -40°C ≤ TA ≤ 85°C, LX155xM
with -55°C ≤ T A ≤ 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.)
Parameter
Symbol
Test Conditions
LX155xI/155xM
LX155xC
Units
Min. Typ. Max. Min. Typ. Max.
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
V REF
VN
TA = 25°C, IL = 1mA
12 ≤ VIN ≤ 25V
1 ≤ IO ≤ 20mA
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
V
mV
mV
mV/°C
V
µV
mV
mA
TA = 25°C
TA = 25°C, R T = 698Ω, CT = 22nF, LX1552/3 only
12 ≤ VCC ≤ 25V
TMIN ≤ TA ≤ TMAX
VPIN 4 peak to peak
TA = 25°C, VPIN 4 = 2V
VPIN 4 = 2V, TMIN ≤ TA ≤ TMAX
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
kHz
kHz
%
%
V
mA
mA
VPIN 1 = 2.5V
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
V
µA
dB
MHz
dB
mA
mA
V
V
Over Line, Load, and Temperature
10Hz ≤ f ≤ 10kHz, TA = 25°C
TA = 125°C, t = 1000hrs
ISC
Oscillator Section
Initial Accuracy (Note 6)
Voltage Stability
Temperature Stability (Note 2)
Amplitude (Note 2)
Discharge Current
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
IB
AVOL
UGBW
PSRR
IOL
IOH
V OH
V OL
2 ≤ VO ≤ 4V
TA = 25°C
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
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)
AVOL
PSRR
IB
TPD
3.15 2.85
1.1 0.9
VPIN 3 = 0 to 2V
3
1
70
-2
150
ISINK = 20mA
ISINK = 200mA
ISOURCE = 20mA
ISOURCE = 200mA
TA = 25°C, CL = 1nF
TA = 25°C, CL = 1nF
VCC = 5V, ISINK = 10mA
0.1
1.5
13.5
13.5
50
50
0.7
0.4
2.2
VPIN 1 = 5V
12 ≤ VCC ≤ 25V
2.85
0.9
-10
300
3
1
70
-2
150
3.15
1.1
0.1
1.5
13.5
13.5
50
50
0.7
0.4
2.2
-5
300
V/V
V
dB
µA
ns
Output Section
Output Voltage Low Level
V OL
Output Voltage High Level
V OH
Rise Time (Note 2)
Fall Time (Note 2)
UVLO Saturation
TR
TF
VSAT
13
12
13
12
100
100
1.2
100
100
1.2
V
V
V
V
ns
ns
V
( 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.)
Copyright © 1994
Rev. 1.0a 1/01
3
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
D
R O D U C T I O N
A T A
S
H E E T
ELECTRICAL CHARACTERISTICS
Parameter
Symbol
(Con't.)
LX155xI/155xM
LX155xC
Units
Min. Typ. Max. Min. Typ. Max.
Test Conditions
Under-Voltage Lockout Section
Start Threshold
VST
Min. Operation Voltage After Turn-On
1552/1554
1553/1555
1552/1554
1553/1555
15
7.8
9
7.0
16
8.4
10
7.6
1552/1553
1552/1553, RT = 698Ω, CT = 22nF
1554/1555
94
96
50
48
17
9.0
11
8.2
15
7.8
9
7.0
16
8.4
10
7.6
94
96
50
48
17
9.0
11
8.2
V
V
V
V
0
%
%
%
%
PWM Section
Maximum Duty Cycle
47
47
0
Minimum Duty Cycle
Power Consumption Section
Start-Up Current
Operating Supply Current
VCC Zener Voltage
IST
I CC
VZ
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.
150
11
35
250
17
30
150
11
35
250
17
µA
mA
V
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
GROUND**
16V (1552/1554)
8.4V (1553/1555)
5V
REF
VREF
16V (1552/1554)
8.4V (1553/1555)
INTERNAL
BIAS
VREF
GOOD LOGIC
RT/CT
VC*
OSCILLATOR
T
***
ERROR AMP
VFB
COMP
ISENSE
OUTPUT
S
2R
R
R
1V
PWM
LATCH
POWER GROUND**
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.
4
Copyright © 1994
Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
S
GRAPH / CURVE INDEX
H E E T
FIGURE INDEX
Characteristic Curves
Theory of Operation Section
FIGURE #
FIGURE #
1.
OSCILLATOR FREQUENCY vs. TIMING RESISTOR
23. TYPICAL APPLICATION OF START-UP CIRCUITRY
2.
MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
24. REFERENCE VOLTAGE vs. TEMPERATURE
3.
OSCILLATOR DISCHARGE CURRENT vs. TEMPERATURE
25. SIMPLIFIED SCHEMATIC OF OSCILLATOR SECTION
4.
OSCILLATOR FREQUENCY vs. TEMPERATURE
26. DUTY CYCLE VARIATION vs. DISCHARGE CURRENT
5.
OUTPUT INITIAL ACCURACY vs. TEMPERATURE
27. OSCILLATOR FREQUENCY vs. TIMING RESISTOR
6.
OUTPUT DUTY CYCLE vs. TEMPERATURE
28. MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
7.
REFERENCE VOLTAGE vs. TEMPERATURE
29. CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT
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
12. START-UP CURRENT vs. SUPPLY VOLTAGE
31. MOSFET PARASITIC OSCILLATIONS
13. DYNAMIC SUPPLY CURRENT vs. OSCILLATOR FREQUENCY
14. CURRENT SENSE DELAY TO OUTPUT vs. TEMPERATURE
32. ADJUSTABLE BUFFERED REDUCTION OF CLAMP LEVEL
WITH SOFT-START
15. CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT
33. EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT SYCHRONIZATION
16. START-UP THRESHOLD vs. TEMPERATURE
34. SLOPE COMPENSATION
17. START-UP THRESHOLD vs. TEMPERATURE
35. OPEN LOOP LABORATORY FIXTURE
18. MINIMUM OPERATING VOLTAGE vs. TEMPERATURE
36. OFF-LINE FLYBACK REGULATOR
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 © 1994
Rev. 1.0a 1/01
5
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
CHARACTERISTIC
FIGURE 1. — OSCILLATOR FREQUENCY vs. TIMING RESISTOR
H E E T
C U RV E S
FIGURE 2. — MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
100
1000
CT = 1nF
90
CT = 3.3nF
80
100
Maximum Duty Cycle - (%)
Oscillator Frequency - (kHz)
S
CT = 6.8nF
10
CT = 22nF
CT = 47nF
1
CT = 0.1µF
0.1
0.1
VCC = 15V
TA = 25°C
70
60
50
40
30
20
VCC = 15V
TA = 25°C
10
10
1
0
0.1
100
1
FIGURE 3. — OSCILLATOR DISCHARGE CURRENT vs.
TEMPERATURE
FIGURE 4. — OSCILLATOR FREQUENCY vs. TEMPERATURE
55
VCC = 15V
VPIN4 = 2V
8.40
VCC = 15V
RT = 10k
CT = 3.3nF
54
Oscillator Frequency - (KHz)
(Id) Oscillator Discharge Current - (mA)
8.50
8.30
8.20
8.10
8.00
7.90
7.80
53
52
51
50
49
48
47
46
-50
-25
0
25
50
75
100
(TA) Ambient Temperature - (°C)
6
100
(RT) Timing Resistor - (k )
(RT) Timing Resistor - (k )
7.70
-75
10
125
45
-75
-50
-25
0
25
50
75
100
125
(TA) Ambient Temperature - (°C)
Copyright © 1994
Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
CHARACTERISTIC
FIGURE 5. — OUTPUT INITIAL ACCURACY vs. TEMPERATURE
C U RV E S
48
LX1552 and LX1553 only
VCC = 15V
RT = 698W
CT = 22nF
62.0
60.5
59.0
57.5
56.0
54.5
46
45
44
43
42
53.0
41
51.5
50.0
-75
-50
-25
0
25
50
75
100
40
-75
125
(TA) Ambient Temperature - (°C)
(VREF) Reference Voltage - (V)
(ISC) Reference Short Circuit Current - (mA)
VCC = 15V
IL = 1mA
5.01
5.00
4.99
4.98
4.97
4.96
4.95
-75
-50
-25
0
25
50
75
100
(TA) Ambient Temperature - (°C)
-25
0
25
50
75
100
125
FIGURE 8. — REFERENCE SHORT CIRCUIT CURRENT vs.
TEMPERATURE
5.03
5.02
-50
(TA) Ambient Temperature - (°C)
FIGURE 7. — REFERENCE VOLTAGE vs. TEMPERATURE
Copyright © 1994
Rev. 1.0a 1/01
VCC = 15V
RT = 698W
CT = 22nF
47
Output Duty Cycle - (%)
Output Initial Accuracy - (kHz)
H E E T
FIGURE 6. — OUTPUT DUTY CYCLE vs. TEMPERATURE
65.0
63.5
S
125
180
165
150
135
120
105
90
75
60
45
30
-75
-50
-25
0
25
50
75
100
125
(TA) Ambient Temperature - (°C)
7
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
CHARACTERISTIC
FIGURE 9. — E.A. INPUT VOLTAGE vs. TEMPERATURE
H E E T
C U RV E S
FIGURE 10. — START-UP CURRENT vs. TEMPERATURE
2.55
250
VCC = 15V
2.54
225
(IST) Start-Up Current - (µA)
2.53
E.A. Input Voltage - (V)
S
2.52
2.51
2.50
2.49
2.48
2.47
2.46
200
LX1552/LX1554
175
150
125
100
LX1553/LX1555
75
50
25
2.45
-75
-50
-25
0
25
50
75
100
0
-75
125
-50
(TA) Ambient Temperature - (°C)
25
50
75
100
125
FIGURE 12. — START-UP CURRENT vs. SUPPLY VOLTAGE
250
250
LX1553/LX1555
TA = 25°C
225
LX1552/LX1554
TA = 25°C
225
200
(IST) Start-Up Current - (µA)
(IST) Start-Up Current - (µA)
0
(TA) Ambient Temperature - (°C)
FIGURE 11. — START-UP CURRENT vs. SUPPLY VOLTAGE
175
150
125
100
75
50
25
200
175
150
125
100
75
50
25
0
0
0
2
4
6
8
10
12
14
16
(VCC) Supply Voltage - (V)
8
-25
18
20
0
1
2
3
4
5
6
7
8
9
10
(VCC) Supply Voltage - (V)
Copyright © 1994
Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
CHARACTERISTIC
FIGURE 13. — DYNAMIC SUPPLY CURRENT vs.
OSCILLATOR FREQUENCY
H E E T
C U RV E S
FIGURE 14. — CURRENT SENSE DELAY TO OUTPUT vs.
TEMPERATURE
30
300
TA = 25°C
RT = 10k
CL = 1000pF
27
24
270
(Tpd) C.S. Delay to Output - (ns)
(ICC) Dynamic Supply Current - (mA)
S
VIN = 16V
VIN = 12V
VIN = 10V
21
18
15
12
9
6
240
210
180
150
120
90
60
30
3
0
-75
0
10
1000
100
FIGURE 15. — CURRENT SENSE THRESHOLD vs.
ERROR AMPLIFIER OUTPUT
-25
0
25
50
75
100
125
FIGURE 16. — START-UP THRESHOLD vs. TEMPERATURE
8.8
1.1
LX1553
LX1555
8.7
1.0
TA = 125°C
0.9
8.6
Start-Up Trheshold - (V)
Current Sense Threshold - (V)
-50
(TA) Ambient Temperature - (°C)
Oscillator Frequency - (kHz)
0.8
TA = 25°C
0.7
0.6
TA = -55°C
0.5
0.4
0.3
8.5
8.4
8.3
8.2
8.1
8.0
0.2
7.9
0.1
0
0
0.5
1.0
1.5
2.0
2.5
3.0 3.5
4.0
4.5
Error Amplifier Output Voltage - (V)
Copyright © 1994
Rev. 1.0a 1/01
VCC = 15V
VPIN3 = 0V to 2V
CL = 1nF
5.0
7.8
-75
-50
-25
0
25
50
75
100
125
(TA) Ambient Temperature - (°C)
9
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
CHARACTERISTIC
FIGURE 17. — START-UP THRESHOLD vs. TEMPERATURE
C U RV E S
11.0
LX1552
LX1554
16.6
16.4
16.2
16.0
15.8
15.6
15.4
10.6
10.4
10.2
10.0
9.8
9.6
9.4
9.2
15.2
-50
-25
0
25
50
75
100
9.0
-75
125
FIGURE 19. — MINIMUM OPERATING VOLTAGE vs.
TEMPERATURE
0
25
50
75
100
125
1.20
(VSAT) Output Saturation Voltage - (V)
LX1553
LX1555
7.9
Minimum Operating Voltage - (V)
-25
FIGURE 20. — LOW LEVEL OUTPUT SATURATION VOLTAGE
DURING UNDER-VOLTAGE LOCKOUT
8.0
7.8
7.7
7.6
7.5
7.4
7.3
7.2
7.1
-50
-25
0
25
50
75
100
(TA) Ambient Temperature - (°C)
10
-50
(TA) Ambient Temperature - (°C)
(TA) Ambient Temperature - (°C)
7.0
-75
LX1552
LX1554
10.8
Minimum Operating Voltage - (V)
16.8
Start-Up Trheshold - (V)
H E E T
FIGURE 18. — MINIMUM OPERATING VOLTAGE vs.
TEMPERATURE
17.0
15.0
-75
S
125
1.08
0.96
0.84
VCC = 5V
TA = -55°C
TA = 25°C
0.72
0.60
TA = 125°C
0.48
0.36
0.24
0.12
0.00
0.1
1
10
Output Sink Current - (mA)
Copyright © 1994
Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
CHARACTERISTIC
FIGURE 21. — OUTPUT SATURATION VOLTAGE vs.
OUTPUT CURRENT and TEMPERATURE
C U RV E S
6.00
VCC = 5V
Sink Transistor
5.0
4.0
3.0
TA = -55°C
2.0
TA = 25°C
TA = 125°C
1.0
0.00
(VSAT) Output Saturation Voltage - (V)
(VSAT) Output Saturation Voltage - (V)
H E E T
FIGURE 22. — OUTPUT SATURATION VOLTAGE vs.
OUTPUT CURRENT and TEMPERATURE
6.0
VCC = 15V
Source Transistor
5.40
4.80
4.20
3.60
3.00
2.40
TA = 25°C
1.80
TA = -55°C
1.20
TA = 125°C
0.60
0.00
10
100
Output Sink Current - (mA)
Copyright © 1994
Rev. 1.0a 1/01
S
1000
10
100
1000
Output Source Current - (mA)
11
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
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 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.
TABLE 1
UVLO
Start-up Voltage Hysterises Voltage
(VHYS)
(VST)
PART #
LX1552
LX1553
LX1554
LX1555
16V
8.4V
16V
8.4V
6V
0.8V
6V
0.8V
MAXIMUM
DUTY CYCLE
<100%
<100%
<50%
<50%
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.
Table 2 below shows a comparison of start-up resistor power
dissipation vs. maximum start-up current for different devices.
TABLE 2
Design Using
SG384x
UC384xA
LX155x
Max. Start-up Current
Specification (IST )
1000µA
500µA
250µA
Typical Start-Up
Resistor Value (RST )
62KΩ
Ω
124KΩ
Ω
248KΩ
Ω
Max. Start-Up Resistor
Power Dissipation (PR)
2.26W
1.13W
0.56W
UNDERVOLTAGE LOCKOUT
The LX155x undervoltage lock-out is designed to maintain an
ultra low quiescent current of less than 250µA, 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.
(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.)
DC BUS
I1 > 250µA
D1
1ST < 250µA
VIN
REF
C1
RT
LX1554
VO
RT/CT
CT
GND
RS
GND
FIGURE 23 — TYPICAL APPLICATION OF START-UP CIRCUITRY
12
Copyright © 1994
Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
S
A T A
H E E T
T H E O R Y O F O P E R AT I O N
VOLTAGE REFERENCE
REF
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.
5V
VP
IR
VV
RT
2.8V
1.1V
TO OUTPUT
STAGE
RT/CT
A1
S1
5.03
2
CT
VCC = 15V
IL = 1mA
5.02
ID = 8.3mA
1
OPEN
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
Copyright © 1994
Rev. 1.0a 1/01
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
devices.
100
90
Oscillator Duty Cycle - (%)
(VREF) Reference Voltage - (V)
S2
80
TA = 25°C
VP = 2.7V
V = 1V
VREF = 5V
Id = 9.3mA
Id = 8.6mA
70
60
SG384x Upper Limit
50
Id = 8.0mA
40
LX155x Limits
30
Id = 7.5mA
SG384x Lower Limit
20
600
700
800
900
1000
(RT) Timing Resistor - ( )
FIGURE 26 — DUTY CYCLE VARIATION vs. DISCHARGE CURRENT
13
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
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H E E T
THEORY OF OPERATION
OSCILLATOR
(continued)
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.
Given: frequency ≅ f; maximum duty-cycle ≅ Dm
Calculate:
1)
RT = 277
(1.74)
1000
1
Dm
(1.74)
-1
1-Dm
Dm
(Ω), 0.3 ≤ Dm ≤ 0.95
-1
Oscillator Frequency - (kHz)
CT = 1nF
Note: RT must always be greater than 520Ω for proper
operation of oscillator circuit.
CT = 3.3nF
100
CT = 6.8nF
2)
10
CT = 1.81 * Dm (µf)
f * RT
for duty cycles above 95% use:
CT = 22nF
CT = 47nF
1
3)
f ≈ 1.81
R TC T
where RT ≥ 5kΩ
CT = 0.1µF
0.1
0.1
VCC = 15V
TA = 25°C
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
(RT) Timing Resistor - (k )
FIGURE 27 — OSCILLATOR FREQUENCY vs. TIMING RESISTOR
Given: f = 100kHz
Dm = 0.45
100
RT = 267
90
(1.74)
80
Maximum Duty Cycle - (%)
(1.74)
70
CT =
60
50
1
.45
.55
.45
-1
= 669Ω
-1
1.81 * 0.45
= .012 µf
100x10 3 * 669
b) LX1554/55
40
fOUT = ½ fOSC (due to internal flip flop)
fOSC = 200kHz
30
20
0
0.1
select CT = 1000pf
using Figure 27 or Equation 3: RT = 9.1k
VCC = 15V
TA = 25°C
10
1
10
100
(RT) Timing Resistor - (k )
FIGURE 28 — MAXIMUM DUTY CYCLE vs. TIMING RESISTOR
14
Copyright © 1994
Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
S
H E E T
T H E O R Y O F O P E R AT I O N
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:
VZ
(1) ISP =
where:
ISP ≡ Peak switch current
RS
VZ ≡ internal zener
0.9V ≤ VZ ≤ 1.1V
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:
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:
VE - 2VF
(1) ISP =
VE ≡ Voltage at pin 1
VF ≡ Diode - Forward voltage
0.7V at TA = 25°C
where:
3 * RS
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
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.
Current Sense Threshold - (V)
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
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
S
H E E T
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
FIGURE 31. — MOSFET PARASITIC OSCILLATIONS
DC BUS
VCC
DC BUS
7
7
Q1
LX155x
LX155x
6
5
Q1
R1
6
IPK
3
C
IPK(MAX) =
RS
RS
1.0V
RS
5
The RC low pass filter will eliminate the leading edge current spike
caused by parasitics of Power MOSFET.
FIGURE 32. — ADJUSTABLE BUFFERED REDUCTION OF CLAMP
LEVEL WITH SOFT-START
VCC
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
VIN
8
7
8
RA
8
4
7
4
Q1
2
LX155x
6
RB
6
IPK
LX155x
555
TIMER
3
4
1N4148
1
R2
3
MPSA63
R1
C
IPK =
5
R1
V CS
Where: VCS = 1.67
RS
tSOFTSTART = -ln 1 -
( R +R ) and V
1
VEAO - 1.3
5(
R1
R 1+R2
)
2
(
R1 R2
R1+R2
VCS
2
RS
0.01
5
= 1V (Typ.)
C.S.MAX
)C
f = (R 1.44
+ 2RB)C
A
1
5
To other
LX155x devices
R
f = R + B2R
A
B
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.
16
Precision duty cycle limiting as well as synchronizing several parts is
possible with the above circuitry.
Copyright © 1994
Rev. 1.0a 1/01
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
R O D U C T I O N
D
A T A
S
H E E T
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
(continued)
FIGURE 34. — SLOPE COMPENSATION
VCC
LX155x
DC BUS
7(12)
VO
5V
8(14)
UVLO
S
R
5V
REF
RT
INTERNAL
BIAS
2.5V
2N222A
VREF
GOOD LOGIC
RSLOPE
7(11)
4(7)
OSCILLATOR
From VO
2(3)
CF
RF
6(10)
C.S.
COMP
2R
Ri
Rd
Q1
CT
1V
ERROR
AMP
R
5(8)
PWM
LATCH
R
3(5)
1(1)
5(9)
C
RS
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
4.7K
100K
1K
ERROR AMP
ADJUST
4.7K
1
COMP
VREF
8
2
VFB
VCC
7
5K
ISENSE
ADJUST
3
ISENSE
OUTPUT
6
4
RTCT
GROUND
5
CT
VCC
A
LX155x
2N2222
0.1µF
0.1µF
1K
OUTPUT
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 © 1994
Rev. 1.0a 1/01
17
PRODUCT DATABOOK 1996/1997
LX1552/3/4/5
U LTRA-LOW S TART-U P CURRENT, C URRENT-MODE PWM
P
D
R O D U C T I O N
A T A
S
H E E T
TYPICAL APPLICATION CIRCUITS
(continued)
FIGURE 36. — OFF-LINE FLYBACK REGULATOR
TI
4.7Ω 1W
1N4004
220µF
250V
1N4004
4.7kΩ
2W
3600pF
400V
250kΩ
1/2W
AC
INPUT
1N4004
MBR735
5V
2-5A
4700µF
10V
1N4935
1N4004
1N4935
16V
150kΩ
3.6kΩ
VCC
VFB
1
COMP
8
VREF
4
RT/CT
10µF
20V
0.01µF
27kΩ
OUT 6
100pF
CUR
3
SEN
10kΩ
0.01µF
.0022µF
820pF
1N4935
2
7
GND
5
2.5kΩ
IRF830
LX1554
20kΩ
1kΩ
470pF
0.85kΩ
ISOLATION
BOUNDARY
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:
18
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.
70%
65%
2.5Amp average
Copyright © 1994
Rev. 1.0a 1/01