Linear Technology LT1245, LT1242, LT1241, LT1244, LT1243 Datasheet

LT1241 Series

High Speed Current Mode

Pulse Width Modulators

EATU

F

■

Low Start-Up Current: < 250µA

■

50ns Current Sense Delay

■

Current Mode Operation: To 500kHz

■

Pin Compatible with UC1842 Series

■

Undervoltage Lockout with Hysteresis

■

No Cross-Conduction Current

■

Trimmed Bandgap Reference

■

1A Totem Pole Output

■

Trimmed Oscillator Frequency and Sink Current

■

Active Pull-Down on Reference and Output During

RE

S

Undervoltage Lockout

■

High Level Output Clamp: 18V

■

Current Sense Leading Edge Blanking

U

O

PPLICATI

A

■

Off-Line Converters

■

DC/DC Converters

, LTC and LT are registered trademarks of Linear Technology Corporation.

S

DUESCRIPTIO

The LT®1241 series devices are 8-pin, fixed frequency,
current mode, pulse width modulators. They are improved
plug compatible versions of the industry standard UC1842
series. These devices have both improved speed and
lower quiescent current. The LT1241 series is optimized
for off-line and DC/DC converter applications. They con­tain a temperature-compensated reference, high gain er­ror amplifier, current sensing comparator and a high
current totem pole output stage ideally suited to driving
power MOSFETs. Start-up current has been reduced to
less than 250µA. Cross-conduction current spikes in
the output stage have been eliminated, making 500kHz
operation practical. Several new features have been incor­porated. Leading edge blanking has been added to the
current sense comparator. Trims have been added to the
oscillator circuit for both frequency and sink current, and
both of these parameters are tightly specified. The output
stage is clamped to a maximum V
on state. The output and the reference output are actively
pulled low during undervoltage lockout.

of 18V in the

OUT

BLOCK

R

T/CT

COMP

FB

I

SENSE

IDAGRA

4

1

–

2

2.5V

+

3

W

1mA

5.6V

2R

OSCILLATOR

R

UV

OUTPUT

PULL-DOWN

+

–

LOCKOUT

18V

8

V

REF

7V

CC

6 OUTPUT

5 GND

1241 BD01

REFERENCE ENABLE

5V REF

MAIN BIAS

1V

–

+

REFERENCE PULL-DOWN

T

S
R

BLANKING

1.5V

1

LT1241 Series

WU

U

PACKAGE

/

O

RDER I FOR ATIO

COMP

FB

I

SENSE



R

T/CT

V

REF



V

CC


OUTPUT
GND

S8 PACKAGE

8-LEAD PLASTIC SO

N8 PACKAGE
8-LEAD PDIP

J8 PACKAGE

8-LEAD CERDIP

1
2
3
4

8
7
6
5



TOP VIEW

A

Supply Voltage ........................................................ 25V

Output Current.......................................................±1A*

Output Energy (Capacitive Load per Cycle)...............5µJ

Analog Inputs (Pins 2, 3)...............................–0.3 to 6V

Error Amplifier Output Sink Current...................... 10mA

Power Dissipation at TA ≤ 25°C ................................ 1W

Operating Junction Temperature Range

Storage Temperature Range ................. – 65°C to 150°C

Lead Temperature (Soldering, 10 sec)..................300°C

*The 1A rating for output current is based on transient switching
requirements.

E

PARAMETER CONDITIONS MIN TYP MAX UNITS
Reference Section

Output Voltage IO = 1mA, TJ = 25°C 4.925 5.000 5.075 V
Line Regulation 12V < VCC < 25V ● 320mV
Load Regulation 1mA < I
Temperature Stability 0.1 mV/°C
Total Output Variation Line, Load, Temp ● 4.87 5.13 V
Output Noise Voltage 10Hz < F < 10kHz, TJ = 25°C50µV
Long Term Stability TA = 125°C, 1000 Hrs. 5 25 mV
Output Short-Circuit Current ● –30 –90 –180 mA

Oscillator Section

Initial Accuracy RT = 10k, CT = 3.3nF, TJ = 25°C 47.5 50 52.5 kHz

Voltage Stability 12V < VCC < 25V, TJ = 25°C1%
Temperature Stability T
Amplitude TJ = 25°C (Pin 4) 1.7 V
Clock Ramp Reset Current V

Error Amplifier Section

Feedback Pin Input Voltage V
Input Bias Current VFB = 2.5V ● –2 µA
Open-Loop Voltage Gain 2 < VO < 4V ● 65 90 dB
Unity-Gain Bandwidth TJ = 25°C 0.7 1.3 2 MHz
Power Supply Rejection Ratio 12V < VCC < 25V ● 60 dB
Output Sink Current V
Output Source Current V

2

W

O

LUTEXI T

S

A

WUW

ARB

U
G

I

S

LT124XC ............................................. 0°C to 100°C

LT124XI......................................... – 40°C to 100°C

LT124XM........................................ – 55°C to 125°C

LECTRICAL C CHARA TERIST

VREF

RT = 13.0k, CT = 500pF, TJ = 25°C 228 248 268 kHz

< TJ < T

MIN

(Pin 4) = 2V, TJ = 25°C 7.9 8.2 8.5 mA

OSC

= 2.5V ● 2.42 2.50 2.58 V

PIN1

= 2.7V, V

PIN2

= 2.3V, V

PIN2

ICS

< 20mA ● –6 –25 mV

MAX

(Notes 1, 2)

= 1.1V ● 26 mA

PIN1

= 5V ● –0.5 –0.75 mA

PIN1

T

= 125°C, θJA = 100°C/W (J8)

JMAX

= 100°C, θJA = 130°C/W (N8)

T

JMAX

= 100°C, θJA = 150°C/W (S8)

T

JMAX

ORDER PART

NUMBER

LT124XCJ8
LT124XCN8
LT124XCS8
LT124XIN8
LT124XIS8
LT124XMJ8

S8 PART MARKING

124X
124XI

–0.05 %/°C

LT1241 Series

LECTRICAL C CHARA TERIST

E

PARAMETER CONDITIONS MIN TYP MAX UNITS
Error Amplifier Section

Output Voltage High Level V
Output Voltage Low Level V

Current Sense Section

Gain ● 2.85 3.00 3.15 V/V
Maximum Current Sense Input Threshold V
Power Supply Rejection Ratio ● 70 dB
Input Bias Current ● –1 10 µA
Delay to Output ● 50 100 ns
Blanking Time 100 ns
Blanking Override Voltage 1.5 V

Output Section

Output Low Level I

Output High Level I

Rise Time CL = 1nF, TJ = 25°C5080ns
Fall Time CL = 1.0nF, TJ = 25°C3060ns
Output Clamp Voltage IO = 1mA ● 18 19.5 V

Undervoltage Lockout

Start-Up Threshold

LT1241
LT1242/LT1244
LT1243/LT1245

Minimum Operating Voltage

LT1241/LT1243/LT1245 ● 7.0 7.6 8.2 V
LT1242/LT1244

Hysteresis

LT1241 1.6 2.0 V
LT1242/LT1244 5.5 6.0 V
LT1243/LT1245 0.4 0.8 V

PWM

Maximum Duty Cycle

LT1241/LT1244/LT1245 T
LT1242/LT1243 T

Minimum Duty Cycle ● 0%

Total Device

Start-Up Current ● 170 250 µA
Operating Current ● 710 mA

PIN2

PIN2

PIN3

= 20mA ● 0.25 0.4 V

OUT

= 200mA ● 0.75 2.2 V

I

OUT

= 20mA ● 12.0 V

OUT

= 200mA ● 11.75 V

I

OUT

= 25°C4648%

J

= 25°C9496%

J

ICS

= 2.3V, RL = 15k to GND ● 5 5.6 V
= 2.7V, RL = 15k to Pin 8 ● 0.2 1.1 V

< 1.1V ● 0.90 1.00 1.10 V

(Notes 1, 2)

● 9.0 9.6 10.2 V

● 15 16 17 V

● 7.8 8.4 9.0 V

● 9.0 10 11 V

The ● denotes those specifications which apply over the full operating
temperature range.

Note 1: Unless otherwise specified, V

= 15V, RT = 10k, CT = 3.3nF.

CC

Note 2: Low duty cycle pulse techniques are used during test to maintain
junction temperature close to ambient.

3

LT1241 Series

TEMPERATURE (°C)

–50

6

V

CC

(V)

7

8

9

10

11

–25 25 75 125

LT1241 • TPC03

0 50 100

MINIMUM OPERATING VOLTAGE

START-UP THRESHOLD

TEMPERATURE (°C)

–50

5

I

CC

(mA)

6

7

8

9

10

–25 25 75 125

LT1241 • TPC06

0 50 100

VCC = 15V
R

T

= 10k

C

T

= 3300pF

TEMPERATURE (°C)

–50

7.7

OSCILLATOR SINK CURRENT (mA)

8.7

–25 25 75 125

LT1241 • TPC08

0 50 100

7.8

8.0

8.2

8.4

8.6

8.5

7.9

8.1

8.3

V

PIN4

= 2V

UW

Y

PICA

11

LPER

F

O

R

AT

CCHARA TERIST

E

C

ICS

Undervoltage Lockout – Undervoltage Lockout –

Undervoltage Lockout – LT1241 LT1242, LT1244 LT1243, LT1245

17

START-UP THRESHOLD

MINIMUM OPERATING VOLTAGE

0 50 100

–25 25 75 125

TEMPERATURE (°C)

LT1241 • TPC02

(V)

CC

V

10

9

8

7

6

–50

START-UP THRESHOLD

MINIMUM OPERATING VOLTAGE

–25 25 75 125

0 50 100

TEMPERATURE (°C)

LT1241 • TPC01

(V)

CC

V

16

15

11

10

9

–50

Start-Up Current Start-Up Current Supply Current

200
180
160
140
120
100

80
60

START-UP CURRENT (µA)

40
20

0

–50

–25 25 75 125

0 50 100

TEMPERATURE (°C)

LT1241 • TPC05

START-UP CURRENT (µA)

200

START-UP

THRESHOLD

150

LT1243/5

100

50

0

0

410146 16

2 8 12 18

LT1241

VCC (V)

LT1242/4

TJ = 25°C

LT1241 • TPC04

Supply Current vs
Oscillator Frequency Oscillator Frequency Oscillator Sink Current

10

9
8

7
6
5
4

3

SUPPLY CURRENT (mA)

VCC = 15V

2

= 10k

R

T

= 15pF

C

1

L

0

10k

4

OSCILLATOR FREQUENCY (Hz)

LT1242, LT1243

LT1241, LT1244, LT1245

100k 1M

LT1241 • TPC18

60
58
56
54
52
50
48

FREQUENCY (kHz)

46
44
42
40

–25 25 75 125

–50

VCC = 5V

= 10k

R

T

= 3300pF

C

T

0 50 100

TEMPERATURE (°C)

LT1241 • TPC07

Y

OUTPUT SINK CURRENT (mA)

0

0

OUTPUT SATURATION VOLTAGE (V)

4.0

510

LT1241 • TPC15

2.0

0.5

1.0

1.5

2.5

3.0

3.5

TJ = 125°C

TJ = –55°C

TJ = 25°C

PICA

LPER

Reference Voltage Reference Short-Circuit Current Feedback Pin Input Voltage

5.05
IO = 1mA

5.04

5.03

5.02

5.01

5.00

4.99

4.98

REFERENCE VOLTAGE (V)

4.97

4.96

4.95

–50

–25 25 75 125

Error Amplifier Open-Loop Gain
and Phase Current Sense Clamp Voltage Current Sense Input Threshold

100

80

60

40

20

OPEN-LOOP VOLTAGE GAIN (dB)

0

VOL

A

–20

10

100 1k 100k 1M

High Level Output Low Level Output Voltage During Undervoltage
Saturation Voltage Saturation Voltage Lockout

4.0

3.5

3.0

2.5

2.0

1.5

1.0

OUTPUT SATURATION VOLTAGE (V)

0.5

0

0

OUTPUT SOURCE CURRENT (mA)

R

F

O

0 50 100

TEMPERATURE (°C)

LT1241 • TPC10

VCC = 15V

= 2.0V - 4.0V

V

GAIN

FREQUENCY (Hz)

O

= 100k

R

L

= 25°C

T

A

PHASE

10k 10M

LT1241 • TPC16

TJ = –55°C

TJ = 25°C

TJ = 125°C

100

LT1241 • TPC13

AT

200

UW

CCHARA TERIST

E

C

140

120

100

80

60

40

REFERENCE SHORT-CIRCUIT CURRENT (mA)

20

–50

225

180

135

90

45

0

–45

1.05

1.04

1.03

1.02

PHASE (DEG)

1.01

1.00

0.99

0.98

0.97

CURRENT SENSE CLAMP VOLTAGE (V)

0.96

0.95
–50

1.0

0.5

OUTPUT SATURATION VOLTAGE (V)

0

0

0 50 100

–25 25 75 125

TEMPERATURE (°C)

–25 25 75 125

OUTPUT SINK CURRENT (mA)

ICS

LT1241 • TPC09

0 50 100

TEMPERATURE (°C)

LT1241 • TPC12

TJ = 125°C

TJ = 25°C

100 200

TJ = –55°C

LT1241 • TPC14

LT1241 Series

2.55

2.54

2.53

2.52

2.51

2.50

2.49

2.48

2.47

FEEDBACK PIN INPUT VOLTAGE (V)

2.46

2.45
–50

1.2

1.0

0.8

0.6

0.4

0.2

CURRENT SENSE INPUT THRESHOLD (V)

0

0

Low Level Output Saturation

0 50 100

–25 25 75 125

TEMPERATURE (°C)

TJ = –55°C

=125°C

T

J

TJ = 25°C

12 45

ERROR AMP OUTPUT VOLTAGE (V)

36

LT1241 • TPC11

LT1241 • TPC17

5