Datasheet

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LT1241 Series

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 con-

tain a temperature-compensated reference, high gain er-

ror 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 incor-

porated. 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

OUT

of 18V in the

on state. The output and the reference output are actively

pulled low during undervoltage lockout.

High Speed Current Mode

Pulse Width Modulators

■

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

Undervoltage Lockout

■

High Level Output Clamp: 18V

■

Current Sense Leading Edge Blanking

ESCRIPTIO

■

Off-Line Converters

■

DC/DC Converters

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

–

COMP

2.5V

1.5V

–

SENSE

BLANKING

1mA

5.6V

OSCILLATOR

18V

5 GND

6 OUTPUT

REF

5V REF

MAIN BIAS

REFERENCE PULL-DOWN

OUTPUT

PULL-DOWN

REFERENCE ENABLE

1241 BD01

UV

LOCKOUT

IDAGRA

Summary of content (16 pages)

PAGE 1
LT1241 Series High Speed Current Mode Pulse Width Modulators U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 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.
PAGE 2
LT1241 Series U U RATI GS PACKAGE/ORDER I FOR ATIO W W W 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 ................................
PAGE 3
LT1241 Series ELECTRICAL CHARACTERISTICS PARAMETER (Notes 1, 2) CONDITIONS MIN TYP MAX UNITS Error Amplifier Section Output Voltage High Level VPIN2 = 2.3V, RL = 15k to GND ● Output Voltage Low Level VPIN2 = 2.7V, RL = 15k to Pin 8 ● 5 5.6 V 0.2 1.1 V Current Sense Section Gain Maximum Current Sense Input Threshold VPIN3 < 1.1V ● 2.85 3.00 3.15 V/V ● 0.90 1.00 1.
PAGE 4
LT1241 Series U W TYPICAL PERFOR A CE CHARACTERISTICS Undervoltage Lockout – LT1242, LT1244 Undervoltage Lockout – LT1241 Undervoltage Lockout – LT1243, LT1245 17 11 11 START-UP THRESHOLD START-UP THRESHOLD VCC (V) VCC (V) 9 8 16 10 15 9 VCC (V) 10 11 MINIMUM OPERATING VOLTAGE START-UP THRESHOLD 8 MINIMUM OPERATING VOLTAGE MINIMUM OPERATING VOLTAGE 10 7 6 –50 –25 25 75 0 50 TEMPERATURE (°C) 100 7 9 –50 –25 125 0 50 25 75 TEMPERATURE (°C) Start-Up Current Start-Up Current LT
PAGE 5
LT1241 Series U W TYPICAL PERFOR A CE CHARACTERISTICS Reference Voltage Reference Short-Circuit Current 5.03 5.02 5.01 5.00 4.99 4.98 4.97 4.96 4.95 –50 –25 0 50 25 75 TEMPERATURE (°C) 100 125 2.55 120 100 80 60 40 0 50 25 75 TEMPERATURE (°C) PHASE 90 20 45 0 0 1k 100k 10k FREQUENCY (Hz) 1M 100 1.03 1.02 1.01 1.00 0.99 0.98 0.97 0.96 0 50 25 75 TEMPERATURE (°C) 100 75 TJ = 25°C TJ = 125°C 1.0 0.5 0 125 0.8 TJ = –55°C 0.6 TJ =125°C 0.4 TJ = 25°C 0.
PAGE 6
LT1241 Series U W TYPICAL PERFOR A CE CHARACTERISTICS Output Deadtime vs Oscillator Frequency – LT1242, LT1244 Output Deadtime vs Oscillator Frequency – LT1241, LT1243,LT1245 Timing Resistor vs Oscillator Frequency 75 60 100 100pF 200pF 50 70 40 30 500pF 20 10nF 5nF 2nF 1nF 65 500pF 60 2nF 10 5nF CT =10nF 55 10 50 0 0 100 OSCILLATOR FREQUENCY (kHz) 0 1000 100 OSCILLATOR FREQUENCY (kHz) 1000 1 10k OUTPUT VOLTAGE 5V/DIV Current Sense Delay CURRENT SENSE INPUT 1V/DIV OUTPUT CROSSC
PAGE 7
LT1241 Series UO U U PI FU CTI S COMP (Pin 1): Compensation Pin. This pin is the output of the Error Amplifier and is made available for loop compensation. It can also be used to adjust the maximum value of the current sense clamp voltage to less than 1V. This pin can source a minimum of 0.5mA (0.8mA typ) and sink a minimum of 2mA (4mA typ) FB (Pin 2) Voltage Feedback Pin. This pin is the inverting input of the error amplifier.
PAGE 8
LT1241 Series W U U UO APPLICATI S I FOR ATIO for various values of RT and CT appear in the Typical Performance Characteristics section. Frequency and deadtime can also be calculated using the following formulas: Oscillator Rise Time: t r = 0.583 • RC Oscillator Discharge Time: t d = Error Amplifier 3.46 • RC (0.0164) R − 11.
PAGE 9
LT1241 Series U W U UO APPLICATI S I FOR ATIO Current Sense Comparator and PWM Latch LT1241 series devices are current mode controllers. Under normal operating conditions the output (Pin 6) is turned on at the start of every oscillator cycle, coincident with the rising edge of the oscillator waveform. The output is then turned off when the current reaches a threshold level proportional to the error voltage at the output of the error amplifier.
PAGE 10
LT1241 Series U W U UO APPLICATI S I FOR ATIO Undervoltage Lockout The LT1241 series devices incorporate an undervoltage lockout comparator which prevents the internal reference circuitry and the output from starting up until the supply voltage reaches the start-up threshold voltage. The quiescent current, below the start-up threshold, has been reduced to less than 250µA (170µA typ.) to minimize the power loss due to the bleed resistor used for start-up in off-line converters.
PAGE 11
LT1241 Series UO TYPICAL APPLICATI S Soft Start External Clock Synchronization VREF VREF 8 5V REF 8 RT R RT/CT 4 EXTERNAL SYNC INPUT OSCILLATOR + 5.6V – 2R + D1 47Ω 1V 1mA FB 2 C CT 0.01µF 5V REF COMP 1 – R + + 2.5V 1.5V ISENSE D1 IS REQUIRED IF THE SYNC AMPLITUDE IS LARGE ENOUGH TO PULL THE BOTTOM OF CT MORE THAN LT1241 • TA01 300mV BELOW GROUND.
PAGE 12
LT1241 Series UO TYPICAL APPLICATI S 300kHz Off-Line Power Supply HOT 1 R5 1M 1/2W 90VAC TO 240VAC 3 C2 0.1µF 250V MP3-X2 2 NEU D5 C3 0.1µF 250V MP3-X2 T1 BALEN C4 4700pF 250V Y-CAP – + 2KBPO8M 4 1212-R6103 COILTRONICS C6 4700pF 250V Y-CAP C5 4700pF 250V Y-CAP AC GND RT1 MCID404 2KBPOO5M D3 MUR420 L1 5 1/2 TURN AIRCORE 20V 1.
PAGE 13
LT1241 Series UO TYPICAL APPLICATI S Slope Compensation at ISENSE Pin 5V REF MAIN BIAS VREF 8 RT REFERENCE ENABLE UV LOCKOUT REFERENCE PULL-DOWN VCC OUTPUT PULL-DOWN RT/CT 4 OSCILLATOR CT COMP 1 5.6V – 7 T OUTPUT 6 S R 1V 1mA FB 2 18V 2R + – VIN GND 5 BLANKING + R + 1.5V 2.5V – ISENSE RS 3 LT1241 • TA04 U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. J8 Package 8-Lead CERDIP (Narrow 0.
PAGE 14
LT1241 Series U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.400* (10.160) MAX 8 7 6 5 1 2 3 4 0.255 ± 0.015* (6.477 ± 0.381) 0.300 – 0.325 (7.620 – 8.255) 0.009 – 0.015 (0.229 – 0.381) ( +0.025 0.325 –0.015 8.255 +0.635 –0.381 ) 0.045 – 0.065 (1.143 – 1.651) 0.065 (1.651) TYP 0.005 (0.127) MIN 0.100 ± 0.010 (2.540 ± 0.254) *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
PAGE 15
LT1241 Series U PACKAGE DESCRIPTIO Dimensions in inches (millimeters) unless otherwise noted. S8 Package 8-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.189 – 0.197* (4.801 – 5.004) 8 7 6 5 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) 1 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 2 3 0.053 – 0.069 (1.346 – 1.752) 0°– 8° TYP 0.016 – 0.050 0.406 – 1.270 0.014 – 0.019 (0.355 – 0.483) *DIMENSION DOES NOT INCLUDE MOLD FLASH.
PAGE 16
LT1241 Series U TYPICAL APPLICATION Slope Compensation at Error Amp 5V REF MAIN BIAS VREF 8 RT RT/CT CT RSLOPE REFERENCE PULL-DOWN VCC OUTPUT PULL-DOWN OSCILLATOR 4 TO VOUT UV LOCKOUT REFERENCE ENABLE COMP 1 5.6V OUTPUT 6 T S R 1V 1mA Rf 2 – FB + 7 18V 2R – GND 5 BLANKING + R + 2.5V 1.