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LTC1735IGN-1 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1735IGN-1 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 28 page  8 LTC1735-1 OPERATIO (Refer to Functional Diagram) FU CTIO AL DIAGRA SW – + + – 0.86V – + 0.55V 2.4V 0.8V 0.86V I1 + – I2 + – EA A BURST DISABLE FC OV gm =1.3m B – + 4.8V IREV + – + – F FC INTVCC S R Q DROP OUT DET 0.8V REF SWITCH LOGIC SD 6V R1 RUN/SS CSS RC VOSENSE VFB 1.2 µA RUN SOFT- START + OVER- CURRENT LATCHOFF SD ITH CC 0.17 µA OSC 4(VFB) BUFFERED ITH SLOPE COMP + – – + 3mV ICMP R2 2k 45k BOT TOP ON FORCE BOT 45k 100k INTVCC 30k 30k SENSE+ SENSE– SYNC 1.2V + – 0.74V 0.8V C TOP UVL BOT INTVCC 5.2V LDO REG VIN + CINTVCC + INTVCC BG PGND VIN VIN BOOST TG INTVCC CB DB D1 L VOUT RSENSE COUT COSC + CIN EXTVCC SGND COSC 1735-1 FD 1 PGOOD 4 8 13 15 16 14 12 11 10 9 5 6 3 2 7 Main Control Loop: The LTC1735-1 uses a constant frequency, current mode step-down architecture. During normal operation, the top MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the main current comparator I1 resets the RS latch. The peak inductor current at which I1 resets the RS latch is controlled by the voltage on Pin ITH, which is the output of error amplifier EA. Pin VOSENSE, described in the Pin Functions, allows EA to receive an output feedback voltage VFB from the external resistive divider. When the load current increases, it causes a slight decrease in VFB relative to the 0.8V refer- ence, which in turn causes the ITH voltage to increase until the average inductor current matches the new load cur- rent. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by current comparator I2, or the beginning of the next cycle. The top MOSFET driver is powered from a floating boot- strap capacitor CB. This capacitor is normally recharged from INTVCC through an external Schottky diode when the top MOSFET is turned off. As VIN decreases towards VOUT, the converter will attempt to turn on the top MOSFET con- tinuously (“dropout’’). A dropout counter detects this con- dition and forces the top MOSFET to turn off for about 500ns every tenth cycle to recharge the bootstrap capacitor. |
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