Datasheet LX1554CPW, LX1554CM, LX1554CDM, LX1553IM, LX1553IDM Datasheet (Microsemi Corporation)

Copyright © 1994

Rev. 1.0a 1/01

FOR FURTHER INFORMATION CALL [phone]

11861 WESTERN AVENUE , GARDEN GROVE, CA. 92841

1

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

PRODUCTION DATA SHEET

THE INFINITE POWER OF INNOVATION

LX1552/3/4/5

LIN DOC #:

1552

DESCRIPTION KEY FEATURES

■■

■■

■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.)

pCURRENT SENSE DELAY TO OUTPUT

(150ns typ.)

pAUTOMATIC FEED FORWARD

COMPENSATION

pPULSE-BY-PULSE CURRENT LIMITING
pENHANCED LOAD RESPONSE

CHARACTERISTICS

pUNDER-VOLTAGE LOCKOUT WITH

HYSTERESIS

pDOUBLE PULSE SUPPRESSION
pHIGH CURRENT TOTEM POLE OUTPUT

(±1Amp peak)

p500kHz OPERATION

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-

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.

PRODUCT HIGHLIGHT

PACKAGE ORDER INFORMATION

T

A

(°C)

Plastic DIP
8-pin

0 to 70 LX155xCM LX155xCDM LX155xCD — LX155xCPW

-40 to 85 LX155xIM LX155xIDM LX155xID — —

-55 to 125 — — — LX155xMY —

M

Plastic SOIC
8-pin

DM

Plastic SOIC
14-pin

D

Ceramic DIP
8-pin

Y

TSSOP
20-pin

PW

TYPICAL APPLICATION OF LX155X USING ITS

MICROPOWER START-UP FEATURE

Max. Start-up Current
Specification (I

ST

)

Typical Start-Up
Resistor Value (R

ST

)

Max. Start-Up Resistor
Power Dissipation (PR)

Design Using

SG384xUC384xALX155x

1000µA

500µA 250µA

2.26W1.13W0.56W

62K

ΩΩ

ΩΩ

Ω 124K

ΩΩ

ΩΩ

Ω248K

ΩΩ

ΩΩ

Ω

Note:Calculation is done for universal AC input speci­fication of V

ACMIN

= 90V

RMS

to V

ACMAX

= 265V

RMS

using the

following equation: (Resistor current is selected to be

2 * I

ST

at V

ACMIN

.)

RST =, P

R

=

V

AC MIN

√2 * I

ST

2V

AC2 MAX

R

ST

TABLE 1

APPLICATIONS

■■

■■

■ECONOMY OFF-LINE FLYBACK OR

FORWARD CONVERTERS

■DC-DC BUCK OR BOOST CONVERTERS

■LOW COST DC MOTOR CONTROL

AVAILABLE OPTIONS PER PART #

Part # Start-UpHysteresisMax.Duty

Voltage Cycle

LX1552 16V 6V <100%

LX1553 8.4V 0.8V <100%

LX1554 16V 6V <50%

LX1555 8.4V 0.8V <50%

I

ST

R

ST

V

CC

AC

INPUT

LX1552

or

LX1554

Note: All surface-mount packages are available in Tape & Reel. Append the letter "T" to part number. (i.e. LX1552CDMT)

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

Copyright © 1994

Rev. 1.0a 1/01

2

P

RODUCTION DATA SHEET

ABSOLUTE MAXIMUM RATINGS (Note 1)

Supply Voltage (Low Impedance Source)..................................................................30V

Supply Voltage (I

CC

< 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

PACKAGE PIN OUTS

V

REF

V

CC

OUTPUT
GND

COMP

V

FB

I

SENSE

RT/C

T

1 8

27

36

45

M & Y PACKAGE

(Top View)

DM PACKAGE

(Top View)

V

REF

V

CC

OUTPUT
GND

COMP

V

FB

I

SENSE

RT/C

T

1 8

27

36

45

V

REF

N.C.
V

CC

V

C

OUTPUT
GND
PWR GND

COMP

N.C.

V

FB

N.C.
I

SENSE

N.C.

RT/C

T

1 14

213

312

411

510

69

78

D PACKAGE

(Top View)

PW PACKAGE

(Top View)

1 20
219

318

417

516

615

714

813

912
10 11

N.C.
N.C.

COMP

V

FB

N.C.
I

SENSE

N.C.

RT/C

T

N.C.
N.C.

N.C.
N.C.
V

REF

N.C.
V

CC

V

C

OUTPUT
GND
PWR GND
N.C.

M PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

θθ

θθ

θ

JA

95°C/W

DM PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

θθ

θθ

θ

JA

165°C/W

D PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

θθ

θθ

θ

JA

120°C/W

Y PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

θθ

θθ

θ

JA

130°C/W

PW PACKAGE:

THERMAL RESISTANCE-JUNCTION TO AMBIENT,

θθ

θθ

θ

JA

144°C/W

Junction Temperature Calculation: T

J

= 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

THERMAL DATA

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.

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

3

Copyright © 1994

Rev. 1.0a 1/01

PRODUCTION DATA SHEET

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 ≤ 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.)

Reference Section

Parameter

Symbol

Test Conditions

Output Voltage V

REFTA

= 25°C, IL = 1mA

Line Regulation 12 ≤ VIN ≤ 25V
Load Regulation 1 ≤ I

O

≤ 20mA

Temperature Stability (Note 2 & 7)
Total Output Variation Over Line, Load, and Temperature
Output Noise Voltage (Note 2) V

N

10Hz ≤ f ≤ 10kHz, TA = 25°C

Long Term Stability (Note 2) TA = 125°C, t = 1000hrs
Output Short Circuit I

SC

LX155xC

Units

Min.Typ.Max.Min.Typ.Max.

LX155xI/155xM

4.955.005.054.955.005.05 V
620 620mV
625 625mV

0.2 0.4 0.2 0.4 mV/°C

4.9 5.1 4.9 5.1 V
50 50 µV

525 525mV

-30 -100-180 -30 -100-180 mA

Oscillator Section

Initial Accuracy (Note 6) TA = 25°C

TA = 25°C, RT = 698Ω, CT = 22nF, LX1552/3 only

Voltage Stability 12 ≤ V

CC

≤ 25V

Temperature Stability (Note 2) T

MIN

≤ TA ≤ T

MAX

Amplitude (Note 2) V

PIN 4

peak to peak

Discharge Current I

D

TA = 25°C, V

PIN 4

= 2V

V

PIN 4

= 2V, T

MIN

≤ TA ≤ T

MAX

48.550.552.548.550.552.5 kHz
56 58 60 56 58 60 kHz

0.2 1 0.2 1 %
55%

1.7 1.7 V

8.0 8.3 8.6 8.0 8.3 8.6 mA

7.6 8.8 7.8 8.8 mA

Output Voltage Low Level V

OLISINK

= 20mA

I

SINK

= 200mA

Output Voltage High Level V

OHISOURCE

= 20mA

I

SOURCE

= 200mA

Rise Time (Note 2) T

R

TA = 25°C, CL = 1nF
Fall Time (Note 2) TFTA = 25°C, CL = 1nF
UVLO Saturation V

SATVCC

= 5V, I

SINK

= 10mA

Error Amp Section

Current Sense Section

Gain (Note 3 & 4) A

VOL

Maximum Input Signal (Note 3) V

PIN 1

= 5V

Power Supply Rejection Ratio (Note 3) PSRR 12 ≤ VCC ≤ 25V
Input Bias Current I

B

Delay to Output (Note 2) T

PDVPIN 3

= 0 to 2V

Output Section

2.452.502.552.452.502.55 V

-0.1 -1 -0.1 -0.5 µA

65 90 65 90 dB

0.6 0.6 MHz

60 70 60 70 dB

24 24 mA

-0.5 -0.8 -0.5 -0.8 mA
5 6.5 5 6.5 V

0.7 1.1 0.7 1.1 V

2.85 3 3.152.85 3 3.15 V/V

0.9 1 1.1 0.9 1 1.1 V
70 70 dB

-2 -10 -2 -5 µA

150 300 150 300 ns

0.1 0.4 0.1 0.4 V

1.5 2.2 1.5 2.2 V
13 13.5 13 13.5 V
12 13.5 12 13.5 V

50 100 50 100 ns
50 100 50 100 ns

0.7 1.2 0.7 1.2 V

(Electrical Characteristics continue next page.)

Input Voltage V

PIN 1

= 2.5V

Input Bias Current I

B

Open Loop Gain A

VOL

2 ≤ VO ≤ 4V

Unity Gain Bandwidth (Note 2) UGBW TA = 25°C
Power Supply Rejection Ratio (Note 3) PSRR 12 ≤ V

CC

≤ 25V

Output Sink Current I

OLVPIN 2

= 2.7V, V

PIN 1

= 1.1V

Output Source Current I

OHVPIN 2

= 2.3V, V

PIN 1

= 5V

Output Voltage High Level V

OHVPIN 2

= 2.3V, RL = 15K to ground

Output Voltage Low Level V

OLVPIN 2

= 2.7V, RL = 15K to V

REF

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

Copyright © 1994

Rev. 1.0a 1/01

4

P

RODUCTION DATA SHEET

ELECTRICAL CHARACTERISTICS (Con't.)

Under-Voltage Lockout Section

Parameter

Symbol

Test Conditions

Start Threshold VST1552/1554

1553/1555

Min. Operation Voltage After Turn-On 1552/1554

1553/1555

LX155xC

Units

Min.Typ.Max.Min.Typ.Max.

LX155xI/155xM

15 16 17 15 16 17 V

7.8 8.4 9.0 7.8 8.4 9.0 V

9 10 11 9 10 11 V

7.0 7.6 8.2 7.0 7.6 8.2 V

PWM Section

Maximum Duty Cycle 1552/1553

1552/1553, RT = 698Ω, CT = 22nF
1554/1555

Minimum Duty Cycle

94 96 94 96 %

50 50 %

47 48 47 48 %

00%

Power Consumption Section

Start-Up Current I

ST

Operating Supply Current I

CC

VCC Zener Voltage VZICC = 25mA

150 250 150 250 µA

11 17 11 17 mA

30 35 30 35 V

Notes:2.These parameters, although guaranteed, are not 100% tested in

production.

3.Parameter measured at trip point of latch with V

FB

= 0.

4.Gain defined as:A = ; 0 ≤ V

ISENSE

≤ 0.8V.

5.Adjust V

CC

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.) & 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.

V

REF

(max.) - V

REF

(min.)

TA (max.) - TA (min.)

∆ V

COMP

∆ V

ISENSE

BLOCK DIAGRAM

*

- V

CC

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.

OSCILLATOR

S

R

***

V

REF

GOOD LOGIC

INTERNAL

BIAS

S / R

5V

REF

PWM
LATCH

CURRENT SENSE
COMPARATOR

1V

R

2R

ERROR AMP

16V (1552/1554)

8.4V (1553/1555)

16V (1552/1554)

8.4V (1553/1555)

UVLO

34V

GROUND**

V

CC

*

R

T/CT

V

FB

T

COMP

I

SENSE

POWER GROUND**

OUTPUT

V

C

*

V

REF

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

5

Copyright © 1994

Rev. 1.0a 1/01

PRODUCTION DATA SHEET

GRAPH / CURVE INDEX

Characteristic Curves

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

7. REFERENCE VOLTAGE vs. TEMPERATURE

8. REFERENCE SHORT CIRCUIT CURRENT vs. TEMPERATURE

9. E.A. INPUT VOLTAGE vs. TEMPERATURE

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 UNDER-

VOLTAGE LOCKOUT

21.OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and
TEMPERATURE

22.OUTPUT SATURATION VOLTAGE vs. OUTPUT CURRENT and
TEMPERATURE

FIGURE INDEX

Theory of Operation Section

FIGURE #

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

FIGURE #

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

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

Copyright © 1994

Rev. 1.0a 1/01

6

P

RODUCTION DATA SHEET

CHARACTERISTIC CURVES

FIGURE 2. — MAXIMUM DUTY CYCLE vs. TIMING RESISTOR

FIGURE 3. — OSCILLATOR DISCHARGE CURRENT vs.

TEMPERATURE

FIGURE 4. — OSCILLATOR FREQUENCY vs. TEMPERATURE

FIGURE 1. — OSCILLATOR FREQUENCY vs. TIMING RESISTOR

0.1

0

40

(RT) Timing Resistor - (k)

100

Maximum Duty Cycle - (%)

20

50

80

110100

10

60

70

90

30

VCC = 15V
T

A

= 25°C

100

0.1

0.1

1

1000

Oscillator Frequency - (kHz)

(RT) Timing Resistor - (k)

100

10

1

10

VCC = 15V
T

A

= 25°C

CT = 3.3nF

CT = 1nF

CT = 6.8nF

CT = 22nF

CT = 47nF

CT = 0.1µF

7.70

8.10

(TA) Ambient Temperature - (°C)

(I

d

) Oscillator Discharge Current - (mA)

7.90

8.20

7.80

8.30

8.40

8.00

-75

-50 -25 0 25 50 75 100 125

8.50

VCC = 15V
V

PIN4

= 2V

45

49

(TA) Ambient Temperature - (°C)

Oscillator Frequency - (KHz)

47

50

46

51

52

48

-75

-50 -25 0 25 50 75 100 125

53

VCC = 15V
R

T

= 10k

C

T

= 3.3nF

54

55

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

7

Copyright © 1994

Rev. 1.0a 1/01

PRODUCTION DATA SHEET

CHARACTERISTIC CURVES

FIGURE 6. — OUTPUT DUTY CYCLE vs. TEMPERATURE

FIGURE 7. — REFERENCE VOLTAGE vs. TEMPERATURE FIGURE 8. — REFERENCE SHORT CIRCUIT CURRENT vs.

TEMPERATURE

FIGURE 5. — OUTPUT INITIAL ACCURACY vs. TEMPERATURE

40

44

(TA) Ambient Temperature - (°C)

Output Duty Cycle - (%)

42

45

41

46

47

43

-75

-50 -25 0 25 50 75 100 125

48

VCC = 15V
R

T

= 698

W

C

T

= 22nF

50.0

56.0

(TA) Ambient Temperature - (°C)

Output Initial Accuracy - (kHz)

53.0

57.5

51.5

59.0

60.5

54.5

-75

-50 -25 0 25 50 75 100 125

62.0

VCC = 15V
R

T

= 698

W

C

T

= 22nF

63.5

65.0

LX1552 and LX1553 only

4.95

4.99

(TA) Ambient Temperature - (°C)

(V

REF

) Reference Voltage - (V)

4.97

5.00

4.96

5.01

5.02

4.98

-75

-50 -25 0 25 50 75 100 125

5.03

VCC = 15V
I

L

= 1mA

30

90

(TA) Ambient Temperature - (°C)

(I

SC

) Reference Short Circuit Current - (mA)

60

105

45

120

135

75

-75

-50 -25 0 25 50 75 100 125

180

150

165

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

Copyright © 1994

Rev. 1.0a 1/01

8

P

RODUCTION DATA SHEET

CHARACTERISTIC CURVES

FIGURE 10. — START-UP CURRENT vs. TEMPERATURE

FIGURE 11. — START-UP CURRENT vs. SUPPLY VOLTAGE FIGURE 12. — START-UP CURRENT vs. SUPPLY VOLTAGE

FIGURE 9. — E.A. INPUT VOLTAGE vs. TEMPERATURE

0

100

(TA) Ambient Temperature - (°C)

(I

ST

) Start-Up Current - (µA)

50

125

25

150

175

75

-75

-50 -25 0 25 50 75 100 125

250

200

225

LX1552/LX1554

LX1553/LX1555

2.45

2.49

(TA) Ambient Temperature - (°C)

E.A. Input Voltage - (V)

2.47

2.50

2.46

2.51

2.52

2.48

-75

-50 -25 0 25 50 75 100 125

2.55

2.53

2.54

VCC = 15V

0

100

(VCC) Supply Voltage - (V)

(I

ST

) Start-Up Current - (µA)

50

125

25

150

175

75

0

2 4 6 8 10 12 14 20

250

200

225

16 18

LX1553/LX1555

T

A

= 25°C

0

100

(VCC) Supply Voltage - (V)

(I

ST

) Start-Up Current - (µA)

50

125

25

150

175

75

0

1234567 10

250

200

225

89

LX1552/LX1554

T

A

= 25°C

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

9

Copyright © 1994

Rev. 1.0a 1/01

PRODUCTION DATA SHEET

CHARACTERISTIC CURVES

FIGURE 14. — CURRENT SENSE DELAY TO OUTPUT vs.

TEMPERATURE

FIGURE 15. — CURRENT SENSE THRESHOLD vs.

ERROR AMPLIFIER OUTPUT

FIGURE 16. — START-UP THRESHOLD vs. TEMPERATURE

FIGURE 13. — DYNAMIC SUPPLY CURRENT vs.

OSCILLATOR FREQUENCY

0

120

(TA) Ambient Temperature - (°C)

(T

pd

) C.S. Delay to Output - (ns)

60

150

30

180

210

90

-75

-50 -25 0 25 50 75 100 125

300

240

270

VCC = 15V
V

PIN3

= 0V to 2V

C

L

= 1nF

100

10

0

12

Oscillator Frequency - (kHz)

30

(I

CC

) Dynamic Supply Current - (mA)

6

15

24

1000

3

18

21

27

9

TA = 25°C
R

T

= 10k

C

L

= 1000pF

VIN = 16V
V

IN

= 12V

V

IN

= 10V

7.8

8.2

(TA) Ambient Temperature - (°C)

Start-Up Trheshold - (V)

8.0

8.3

7.9

8.4

8.5

8.1

-75

-50 -25 0 25 50 75 100 125

8.6

LX1553
LX1555

8.7

8.8

0

0.4

Error Amplifier Output Voltage - (V)

Current Sense Threshold - (V)

0.2

0.5

0.1

0.6

0.7

0.3

0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 5.0

1.0

0.8

0.9

4.0 4.5

TA = 25°C

1.1

TA = 125°C

TA = -55°C

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

Copyright © 1994

Rev. 1.0a 1/01

10

P

RODUCTION DATA SHEET

CHARACTERISTIC CURVES

FIGURE 18. — MINIMUM OPERATING VOLTAGE vs.

TEMPERATURE

FIGURE 19. — MINIMUM OPERATING VOLTAGE vs.

TEMPERATURE

FIGURE 20. — LOW LEVEL OUTPUT SATURATION VOLTAGE

DURING UNDER-VOLTAGE LOCKOUT

FIGURE 17. — START-UP THRESHOLD vs. TEMPERATURE

15.0

15.8

(TA) Ambient Temperature - (°C)

Start-Up Trheshold - (V)

15.4

16.0

15.2

16.2

16.4

15.6

-75

-50 -25 0 25 50 75 100 125

16.6

LX1552
LX1554

16.8

17.0

7.0

7.4

(TA) Ambient Temperature - (°C)

Minimum Operating Voltage - (V)

7.2

7.5

7.1

7.6

7.7

7.3

-75

-50 -25 0 25 50 75 100 125

7.8

LX1553
LX1555

7.9

8.0

1

0.1

0.00

0.48

Output Sink Current - (mA)

1.20

(V

SAT

) Output Saturation Voltage - (V)

0.24

0.60

0.96

10

0.12

0.72

0.84

1.08

0.36

VCC = 5V

TA = -55°C

TA = 25°C

TA = 125°C

9.0

9.8

(TA) Ambient Temperature - (°C)

Minimum Operating Voltage - (V)

9.4

10.0

9.2

10.2

10.4

9.6

-75

-50 -25 0 25 50 75 100 125

10.6

LX1552
LX1554

10.8

11.0

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

11

Copyright © 1994

Rev. 1.0a 1/01

PRODUCTION DATA SHEET

CHARACTERISTIC CURVES

FIGURE 21. — OUTPUT SATURATION VOLTAGE vs.

OUTPUT CURRENT and TEMPERATURE

FIGURE 22. — OUTPUT SATURATION VOLTAGE vs.

OUTPUT CURRENT and TEMPERATURE

100

10

0.00

Output Sink Current - (mA)

6.0

(V

SAT

) Output Saturation Voltage - (V)

3.0

100

0

1.0

4.0

2.0

VCC = 5V
Sink Transistor

TA = -55°C

TA = 25°C

TA = 125°C

5.0

100

10

0.00

2.40

Output Source Current - (mA)

6.00

(V

SAT

) Output Saturation Voltage - (V)

1.20

3.00

4.80

1000

0.60

3.60

4.20

5.40

1.80

VCC = 15V
Source Transistor

TA = -55°C

TA = 25°C

TA = 125°C

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

Copyright © 1994

Rev. 1.0a 1/01

12

P

RODUCTION DATA SHEET

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 250µA of current (typically 2x I

ST(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.

Max. Start-up Current
Specification (IST)

Typical Start-Up
Resistor Value (R

ST

)

Max. Start-Up Resistor
Power Dissipation (PR)

Design Using SG384x UC384xA LX155x

2.26W 1.13W 0.56W

62K

ΩΩ

ΩΩ

Ω 124K

ΩΩ

ΩΩ

Ω 248K

ΩΩ

ΩΩ

Ω

1000µA 500µA 250µA

(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

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.

HysterisesVoltage

(V

HYS

)

PART #

Start-up Voltage

(V

ST

)

LX1552
LX1553
LX1554
LX1555

16V

8.4V
16V

8.4V

6V

0.8V
6V

0.8V

<100%
<100%

<50%
<50%

UVLO

MAXIMUM

DUTY CYCLE

TABLE 1

R

S

GND

DC BUS

C

1

D

1

I1 > 250µA

1ST < 250µA

V

IN

REF

R

T/CT

V

O

GND

R

T

C

T

LX1554

TABLE 2

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

13

Copyright © 1994

Rev. 1.0a 1/01

PRODUCTION DATA SHEET

THEORY OF OPERATION

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.

FIGURE 25 — SIMPLIFIED SCHEMATIC OF OSCILLATOR SECTION

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 (R

T

) and (CT). A simplified schematic of the oscillator

is shown in Figure 25. The operation is as follows; Capacitor (C

T

)

is charged from the 5V reference thru resistor (R

T

) 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 (V

V

) position. This will allow the
capacitor to discharge with a current equal to the difference
between a constant discharge current (I

D

) and current through

charging resistor (I

R

), 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

FIGURE 24 — REFERENCE VOLTAGE vs. TEMPERATURE

4.95

4.99

(TA) Ambient Temperature - (°C)

(V

REF

) Reference Voltage - (V)

4.97

5.00

4.96

5.01

5.02

4.98

-75

-50 -25 0 25 50 75 100 125

5.03

VCC = 15V
I

L

= 1mA

FIGURE 26 — DUTY CYCLE VARIATION vs. DISCHARGE CURRENT

20

60

(RT) Timing Resistor - ()

100

Oscillator Duty Cycle - (%)

40

70

600 700 800 900

1000

TA = 25°C
V

P

= 2.7V
V = 1V
V

REF

= 5V

30

80

90

50

Id = 7.5mA

Id = 8.0mA

Id = 8.6mA

Id = 9.3mA

SG384x Lower Limit

LX155x Limits

SG384x Upper Limit

C

T

R

T

I

R

REF

5V

R

T/CT

ID = 8.3mA

2

1

OPEN

2.8V 1.1V

S2

V

P

V

V

S1

A1

TO OUTPUT
STAGE

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 (I

D

- IR) must be

decreased by increasing I

R

. Consequently, this increases the
sensitivity of the frequency and duty cycle to any small variations
of the internal current source (I

D

), 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.

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

Copyright © 1994

Rev. 1.0a 1/01

14

P

RODUCTION DATA SHEET

Given: frequency ≅ f; maximum duty-cycle ≅ Dm
Calculate:

1) R

T

= 277 (Ω), 0.3 ≤ Dm ≤ 0.95

Note: R

T

must always be greater than 520Ω for proper

operation of oscillator circuit.

2) C

T

= (µf)

for duty cycles above 95% use:

3) f ≈ where R

T

≥ 5kΩ

THEORY OF OPERATION

OSCILLATOR (continued)
The oscillator is designed such that many values of R

T

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.

1-Dm

Dm

(1.74) -1

1

Dm

1.81

R

TCT

(1.74) -1

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 R

T

and CT for the

a) LX1552/53, and the b) LX1554/55 ?

FIGURE 28 — MAXIMUM DUTY CYCLE vs. TIMING RESISTOR

0.1

0

40

(RT) Timing Resistor - (k)

100

Maximum Duty Cycle - (%)

20

50

80

110100

10

60

70

90

30

VCC = 15V
T

A

= 25°C

1.81 * Dm
f * R

T

100

0.1

0.1

1

1000

Oscillator Frequency - (kHz)

(RT) Timing Resistor - (k)

100

10

1

10

VCC = 15V
T

A

= 25°C

CT = 3.3nF

CT = 1nF

CT = 6.8nF

CT = 22nF

CT = 47nF

CT = 0.1µF

FIGURE 27 — OSCILLATOR FREQUENCY vs. TIMING RESISTOR

a) LX1552/53

Given: f = 100kHz
Dm = 0.45

R

T

= 267 = 669Ω

C

T

= = .012 µf

b) LX1554/55

f

OUT

= ½ f

OSC

(due to internal flip flop)

f

OSC

= 200kHz

select C

T

= 1000pf

using Figure 27 or Equation 3: R

T

= 9.1k

(1.74) -1

(1.74) -1

1

.45

.55
.45

1.81 * 0.45

100x10

3

* 669

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

15

Copyright © 1994

Rev. 1.0a 1/01

PRODUCTION DATA SHEET

THEORY OF OPERATION

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:

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:

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.

0

0.4

Error Amplifier Output Voltage - (V)

Current Sense Threshold - (V)

0.2

0.5

0.1

0.6

0.7

0.3

0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 5.0

1.0

0.8

0.9

4.0 4.5

TA = 25°C

1.1

TA = 125°C

TA = -55°C

FIGURE 29 — CURRENT SENSE THRESHOLD vs. ERROR AMPLIFIER OUTPUT

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 0.5µA, 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 (R

F

) value of 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 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 V

CC

ramp-up. Figure 20, in the characteristic curves

section, shows output sink saturation voltage vs. current at 5V.

V

Z

R

S

(1) ISP = where: ISP≡Peak switch current

V

Z

≡internal zener

0.9V ≤ V

Z

≤ 1.1V

(1) I

SP

= where: VE≡Voltage at pin 1

V

F

≡Diode - Forward voltage

0.7V at T

A

= 25°C

VE - 2V

F

3 * R

S

R

FMIN

= ≈ 10K

3(1.1) + 1.8

0.5mA

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

Copyright © 1994

Rev. 1.0a 1/01

16

P

RODUCTION DATA SHEET

TYPICAL APPLICATION CIRCUITS

FIGURE 33. — EXTERNAL DUTY CYCLE CLAMP AND MULTI-UNIT

SYNCHRONIZATION

FIGURE 32. — ADJUSTABLE BUFFERED REDUCTION OF CLAMP

LEVEL WITH SOFT-START

Precision duty cycle limiting as well as synchronizing several parts is
possible with the above circuitry.

Soft start and adjustable peak current can be done with the external
circuitry shown above.

f =

(RA + 2RB)C

1.44

f =

RA + 2R

B

R

B

V

CS

R

S

V

EAO

- 1.3

5 ( )

R1 R

2

R1+R

2

t

SOFTSTART

= -ln 1 - () C

where; V

EAO

≡ voltage at the Error Amp Output under

minimum line and maximum load conditions.

R

1

R1+R

2

R

1

R1+R

2

IPK = Where: V

CS

= 1.67 () and V

C.S.MAX

= 1V (Typ.)

Unless otherwise specified, pin numbers refer to 8-pin package.

FIGURE 30. — CURRENT SENSE SPIKE SUPPRESSION FIGURE 31. — MOSFET PARASITIC OSCILLATIONS

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.)

The RC low pass filter will eliminate the leading edge current spike
caused by parasitics of Power MOSFET.

LX155x

3

5

6

7

R

S

C

Q1

V

CC

DC BUS

I

PK

I

PK(MAX)

=

1.0V
R

S

LX155x

6

7

Q1

V

CC

DC BUS

5

R

S

R

1

MPSA63

R

1

R

2

C

1N4148

1

2

4

8

LX155x

5

3

6

7

Q

1

I

PK

V

CS

R

S

V

CC

V

IN

2

6

7

R

B

R

A

5 1

84

3

555

TIMER

4

5

8

LX155x

To other

LX155x devices

0.01

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

17

Copyright © 1994

Rev. 1.0a 1/01

PRODUCTION DATA SHEET

TYPICAL APPLICATION CIRCUITS (continued)

FIGURE 34. — SLOPE COMPENSATION

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.

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.

OSCILLATOR

V

REF

GOOD LOGIC

S

R5VREF

INTERNAL

BIAS

8(14)

4(7)

2(3)

1(1)

R

F

C

F

R

d

R

i

From V

O

R

SLOPE

2N222A

R

T

5V

UVLO

2.5V

ERROR

AMP

C

T

1V

2R

R

C.S.

COMP

PWM

LATCH

5(9)

3(5)

5(8)

CR

S

R

6(10)

7(11)

7(12)

V

CC

DC BUS

V

O

Q1

LX155x

FIGURE 35. — OPEN LOOP LABORATORY FIXTURE

2

3

4

8

7

6

5

COMP

V

FB

I

SENSE

RTC

T

V

REF

V

CC

OUTPUT

GROUND

0.1µF 0.1µF

A

LX155x

R

T

2N2222

100K

4.7K

1K

4.7K

5K

I

SENSE

ADJUST

ERROR AMP
ADJUST

C

T

1K

GROUND

OUTPUT

V

CC

V

REF

1

U

LTRA-LOW START-UP CURRENT

, C

URRENT-MODE

PWM

LX1552/3/4/5

PRODUCT DATABOOK 1996/1997

Copyright © 1994

Rev. 1.0a 1/01

18

P

RODUCTION DATA SHEET

TYPICAL APPLICATION CIRCUITS (continued)

FIGURE 36. — OFF-LINE FLYBACK REGULATOR

7

150k

Ω

100pF

V

FB

COMP

V

REF

RT/C

T

4700µF

10V

5V

2-5A

ISOLATION

BOUNDARY

3600pF
400V

1N4935

820pF

2.5k

Ω

1N4935

IRF830

27k

Ω

0.01µF

10µF
20V

1N4935

1k

Ω

470pF

0.85k

Ω

MBR735TI

4.7k

Ω

2W

250kΩ
1/2W

220µF
250V

4.7

Ω

1W

1N4004

1N40041N4004

1N4004

AC

INPUT

V

CC

OUT

CUR
SEN

GND

LX1554

20k

Ω

3.6k

Ω

10k

Ω

.0022µF0.01µF

16V

3

6

2

1

8

4

5

SPECIFICATIONS

Input line voltage: 90VAC to 130VAC
Input frequency: 50 or 60Hz
Switching frequency: 40KHz ±10%
Output power: 25W maximum
Output voltage: 5V +5%
Output current: 2 to 5A
Line regulation: 0.01%/V
Load regulation: 8%/A*
Efficiency @ 25 Watts,

V

IN

= 90VAC: 70%

V

IN

= 130VAC: 65%

Output short-circuit current: 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.