LT1246/LT1247 1MHz Off-Line Current Mode PWM and DC/DC Converter FEATURES DESCRIPTIO n Current Mode Operation to 1MHz The LT 1246/LT1247 are 8-pin, fixed frequency, current n 30ns Current Sense Delay mode, pulse width modulators. These devices are de- n < 250A Low Start-Up Current signed to be improved plug compatible versions of the n Current Sense Leading Edge Blanking industry standard UC1842 PWM circuit. The LT1246/ n Pin Compatible with UC1842 LT1247 are optimized for off-line and DC/DC converter n Undervoltage Lockout with Hysteresis applications. They contain a temperature compensated n No Cross-Conduction Current reference, high gain error amplifier, current sensing com- n Trimmed Bandgap Reference parator, and a high current totem pole output stage ideally n 1A Totem Pole Output suited to driving power MOSFETs. Start-up current has n Trimmed Oscillator Frequency and Sink Current been reduced to less than 250A. Cross-conduction cur- n Active Pull-Down on Reference and Output During rent spikes in the totem pole output stage have been Undervoltage Lockout eliminated, making 1MHz operation practical. Several new n 18V High Level Output Clamp features have been incorporated. Leading edge blanking has been added to the current sense comparator. This minimizes or eliminates the filter that is normally required. APPLICATI S Eliminating this filter allows the current sense loop to n Off-Line Converters operate with minimum delays. Trims have been added to n DC/DC Converters the oscillator circuit for both frequency and sink current, and both of these parameters are tightly specified. The Minimum output stage is clamped to a maximum V of 18V in the OUT Start-Up Operating Maximum on state. The output and the reference output are actively Device Threshold Voltage Duty Cycle Replaces pulled low during under-voltage lockout. LT1246 16V 10V 100% UC1842 LT1247 8.4V 7.6V 100% UC1843 , LTC and LT are registered trademarks of Linear Technology Corporation. BLOCK DAI GRA REFERENCE ENABLE UV LOCKOUT REFERENCE PULLDOWN 5V REF 8 V REF MAIN BIAS OUTPUT 7V PULLDOWN CC R /C 4 OSCILLATOR T T S 6 OUTPUT R 1 5.6V COMPENSATION 1V 1mA 2 FEEDBACK 5 GND 2R BLANKING 2.5V 18V + R + + 1.5V I 3 SENSE LT1246 BD01 1 W UO ULT1246/LT1247 O PACKAGE/ORDER I FOR ATIO ABS LUTEXAI U RATI GS Supply Voltage ....................................................... 25V ORDER PART Output Current ...................................................... 1A* TOP VIEW NUMBER Output Energy (Capacitive Load per Cycle)............. 5J COMP 1 8 V REF Analog Inputs (Pins 2, 3) .............................. 0.3 to 6V LT1246CN8 FB V 2 7 CC Error Amplifier Output Sink Current..................... 10mA LT1246CS8 I 3 6 OUTPUT SENSE Power Dissipation at T 25C ............................... 1W A LT1247CN8 R /C 4 5 GND T T Operating Junction Temperature Range LT1247CS8 N8 PACKAGE LT1246C/LT1247C ............................. 0C to 100C 8-LEAD PDIP Storage Temperature Range ................ 65C to 150C S8 PACKAGE S8 PART MARKING 8-LEAD PLASTIC SO Lead Temperature (Soldering, 10 sec)................. 300C T = 100C, = 130C/W (N8) 1246 JMAX JA T = 100C, = 150C/W (S8) JMAX JA 1247 *The 1A rating for output current is based on transient switching requirements. Consult factory for Industrial and Military grade parts. ELECTRICAL CHARACTERISTICS (Notes 1, 2) PARAMETER CONDITIONS MIN TYP MAX UNITS Reference Section Output Voltage I = 1mA, T = 25C 4.925 5.000 5.075 V O J Line Regulation 12V < V < 25V l 320 mV CC Load Regulation 1mA < I < 20mA l 6 25 mV REF Temperature Stability 0.1 mV/C Total Output Variation Line, Load, Temperature l 4.87 5.13 V Output Noise Voltage 10Hz < F < 10kHz, T = 25C50 V J Long-Term Stability T = 125C, 1000 Hrs. 5 25 mV A Output Short-Circuit Current l 30 90 180 mA Oscillator Section Initial Accuracy R = 10k, C = 3.3nF, T = 25C 47.5 50 52.5 kHz T T J R = 6.2k, C = 500pF, T = 25C 465 500 535 kHz T T J Voltage Stability 12V < V < 25V, T = 25C1% CC J Temperature Stability T < T < T 0.05 %/C MIN J MAX Amplitude Pin 4 1.7 V Clock Ramp Reset Current V (Pin 4) = 2V, T = 25C 7.9 8.2 8.5 mA OSC J Error Amplifier Section Feedback Pin Input Voltage V = 2.5V l 2.42 2.50 2.58 V PIN 1 Input Bias Current V = 2.5V l 2 A FB Open-Loop Voltage Gain 2 < V < 4V l 65 90 dB O Unity-Gain Bandwidth T = 25C 1 2 MHz J Power Supply Rejection Ratio 12V < V < 25V l 60 dB CC Output Sink Current V = 2.7V, V = 1.1V l 26 mA PIN 2 PIN 1 Output Source Current V = 2.3V, V = 5V l 0.5 0.75 mA PIN 2 PIN 1 2 U W W W U WU