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1LT1246 LT1247 1MHz Off Line


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Niveau: Supérieur, Doctorat, Bac+8
1LT1246/LT1247 1MHz Off-Line Current Mode PWM and DC/DC Converter + – + – 1 2FEEDBACK COMPENSATION 2.5V 2R R 1V 1.5V – + 3ISENSE BLANKING 1mA 5.6V 4RT/CT OSCILLATOR R S 18V 7 VCC UV LOCKOUT 5 GND 6 OUTPUT 8 VREF 5V REF MAIN BIAS REFERENCE PULLDOWN OUTPUT PULLDOWN REFERENCE ENABLE LT1246 • BD01 D U ESCRIPTIOSFEATURE U SA O PPLICATI n Current Mode Operation to 1MHz n 30ns Current Sense Delay n < 250µA Low Start-Up Current n Current Sense Leading Edge Blanking n Pin Compatible with UC1842 n Undervoltage Lockout with Hysteresis n No Cross-Conduction Current n Trimmed Bandgap Reference n 1A Totem Pole Output n Trimmed Oscillator Frequency and Sink Current n Active Pull-Down on Reference and Output During Undervoltage Lockout n 18V High Level Output Clamp n Off-Line Converters n DC/DC Converters The LT®1246/LT1247 are 8-pin, fixed frequency, current mode, pulse width modulators. These devices are de- signed to be improved plug compatible versions of the industry standard UC1842 PWM circuit. The LT1246/ LT1247 are optimized for off-line and DC/DC converter applications.

  • lt1246

  • output

  • current sense

  • start-up current

  • undervoltage lockout

  • current reference

  • start-up threshold

  • current



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FEATURESnCurrent Mode Operation to 1MHzn30ns Current Sense Delayn< 250mA Low Start-Up CurrentnCurrent Sense Leading Edge BlankingnPin Compatible with UC1842nUndervoltage Lockout with HysteresisnNo Cross-Conduction CurrentnTrimmed Bandgap Referencen1A Totem Pole OutputnTrimmed Oscillator Frequency and Sink CurrentnActive Pull-Down on Reference and Output DuringUndervoltage Lockoutn18V High Level Output ClampAPPLICATISnOff-Line ConvertersnDC/DC ConvertersMinimumDeviceTShtraersth-UolpdOVpoelrtaatgiengDMutayx iCmyuclmeReplacesLLTT1122446781.64VV71.06VV110000%%UUCC11884423BLOCKDIAGRART/CT4COMPENSATION1FEEDBACK2V5.2+ISENSE3OSCILLATORV6.5V1Am12RR+LT1246/LT12471MHz Off-LineCurrent Mode PWMand DC/DC ConverterDESCRIPTIO®Tmhoe dLe,T p1u2ls4e6 /LwTid1t2h4 7m aordeu l8a-tpoirns, . fiTxheed sfer edqeuveicnecsy , acruer rdeen-tisnigdnuestdr yt o stbaen idmarpdr oUveCd1 8p4lu2 g PcWomMp actiirbcluei t.v eTrshieo nLsT o1f2 t4h6e/LT1247 are optimized for off-line and DC/DC converterapplications. They contain a temperature compensatedreference, high gain error amplifier, current sensing com-parator, and a high current totem pole output stage ideallysuited to driving power MOSFETs. Start-up current hasbeen reduced to less than 250mA. Cross-conduction cur-rent spikes in the totem pole output stage have beeneliminated, making 1MHz operation practical. Several newfeatures have been incorporated. Leading edge blankinghas been added to the current sense comparator. Thisminimizes or eliminates the filter that is normally required.Eliminating this filter allows the current sense loop tooperate with minimum delays. Trims have been added tothe oscillator circuit for both frequency and sink current,and both of these parameters are tightly specified. Theoutput stage is clamped to a maximum VOUT of 18V in theon state. The output and the reference output are activelypulled low during under-voltage lockout., LTC and LT are registered trademarks of Linear Technology Corporation.REFERENCE ENABLEUV LOCKOUT5V REF REFERENCE PULLDOWN8VMAIN BIASOUTPUT REFPULLDOWN7VCCS6OUTPUTRDNG5BLANKING18V+V5.1LT1246 • BD011