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LTC1736 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1736 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 28 page  8 LTC1736 FU CTIO AL DIAGRA OPERATIO (Refer to Functional Diagram) Main Control Loop The LTC1736 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 the error amplifier EA. Pin VOSENSE, described in the Pin Functions, allows EA to receive an output feedback voltage VFB from the internal resistive divider. When the load current increases, it causes a slight decrease in VFB relative to the 0.8V reference, which in turn causes the ITH voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by current com- parator I2, or the beginning of the next cycle. The top MOSFET driver is powered from a floating bootstrap capacitor CB. This capacitor is normally re- charged 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 continuously (‘’dropout’’). A dropout counter detects this condition and forces the top MOSFET to turn off for about 500ns every tenth cycle to recharge the bootstrap capacitor. SW – + + – 0.86V 0.74V – + 0.55V 2.4V 0.8V 47pF 0.86V I1 + – I2 + – EA A BURST DISABLE FC OV B – + 4.8V IREV + – + – F FC S R Q DROP OUT DET 0.8V REF SWITCH LOGIC SD 6V RUN/SS CSS RC VFB 40k 1.2 µA RUN SOFT START + OVER- CURRENT LATCH-OFF SD ITH CC 0.17 µA OSC 4(VFB) BUFFERED ITH SLOPE COMP + – – + 3mV ICMP R2 10k R1 SGND VFB VOSENSE 2k 45k BOT TOP ON FORCE BOT 45k 30k 30k SENSE+ SENSE– SYNC 1.2V 0.8V C TOP UVL BOT INTVCC 5.2V LDO REG VIN + CINTVCC VOUT VSEC INTVCC BG PGND VIN VIN BOOST TG INTVCC CB DB D1 COSC + CIN + CSEC • • + COUT EXTVCC FCB R4 R3 COSC RSENSE 1736 FD 1 PGOOD 6 9 10 5 VIDVCC VID4 INTVCC 15 VID3 14 VID2 13 VID1 12 VID0 11 4 21 23 24 22 20 19 18 17 7 8 3 2 VID DECODER 16 gm =1.3m |
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