LTC1735 UUWFU CTIO AL DIAGRA VIN + VIN CIN 0.8V UVL COSC REF PGOOD INTV C SGND 0.17 OSC µA CC FCB FC DB C F BOOST – – SYNC OSC VSEC + 1.2V 0.8V + CB TG – FORCE BOT TOP 0.74V + DROP + OUT SW LTC1735F CSEC DET BOT ONLY SWITCH OV LOGIC S + TOP ON D1 Q 0.86V – R 0.55V + B – 2.4V SD 2k 45k 45k V V OSENSE OUT Ω VFB gm =1.3m ICMP IREV – – – BOT + I EA 1 – + + – I2 + COUT 0.8V + + INTVCC R1 R2 0.86V BURST 3mV V SD IN DISABLE + FC C 5.2V INTVCC RUN INTVCC LDO 1.2µA SOFT- A REG START 4(VFB) BUFFERED + ITH BG 4.7V + OVER- 6V 30k 30k CURRENT – SLOPE COMP LATCHOFF R PGND C RUN/SS ITH SENSE+ SENSE– EXTVCC CSS CC RSENSE 1735 FD UOPERATIO(Refer to Functional Diagram)Main Control Loop increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom The LTC1735 uses a constant frequency, current mode MOSFET is turned on until either the inductor current step-down architecture. During normal operation, the top starts to reverse, as indicated by current comparator I MOSFET is turned on each cycle when the oscillator sets 2, or the beginning of the next cycle. the RS latch and turned off when the main current comparator I1 resets the RS latch. The peak inductor The top MOSFET driver is powered from a floating boot- current at which I1 resets the RS latch is controlled by the strap capacitor CB. This capacitor is normally recharged voltage on Pin 3 (ITH), which is the output of error from INTVCC through an external diode when the top amplifier EA. Pin␣ 6 (VOSENSE), described in the pin func- MOSFET is turned off. As VIN decreases towards VOUT, the tions, allows EA to receive an output feedback voltage VFB converter will attempt to turn on the top MOSFET continu- from an external resistive divider. When the load current ously (“dropout’’). A dropout counter detects this condi- increases, it causes a slight decrease in VFB relative to the tion and forces the top MOSFET to turn off for about 500ns 0.8V reference, which in turn causes the ITH voltage to every tenth cycle to recharge the bootstrap capacitor. 1735fc 9
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