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LTC1435AIS Datasheet(PDF) 6 Page - Linear Technology |
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LTC1435AIS Datasheet(HTML) 6 Page - Linear Technology |
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6 / 20 page  6 LTC1435A FUNCTIONAL DIAGRA – + + – + – + – SWITCH LOGIC DROP OUT DET 0.6V 1.19V Q S R I1 INTVCC 4k – + 8k 180k 30k + – – + EA SHUTDOWN 6V 3 µA 1.19V 1.28V – + 4.8V VIN SHUTDOWN 1.19V REF CSS CC RC RUN/SS 2 ITH 3 DFB* SENSE+ 7 8 SENSE– EXTVCC 9 + R2 VOSENSE OV 1 µA OSC PGND 10 BG INTVCC 12 SW 14 TG 16 BOOST SGND VIN 13 SFB 4 COSC 1 RSENSE + + VSEC CSEC VOUT COUT D1 CB DB INTVCC + CIN VIN COSC 5V LDO REG RUN SOFT START 1435A • FD VFB R1 I2 gm = 1m * FOLDBACK CURRENT LIMITING OPTION 5 6 11 15 (Refer to Functional Diagram) OPERATION Main Control Loop The LTC1435A 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 com- parator I1 resets the RS latch. The peak inductor current at which I1 resets the RS latch is controlled by the voltage on the ITH pin , which is the output of error amplifier EA. The VOSENSEpin, described in the Pin Functions section, allows EA to receive an output feedback voltage VFB from an ex- ternal resistive divider. When the load current increases, it causes a slight decrease in VFB relative to the 1.19V ref- erence, which in turn causes the ITHvoltagetoincreaseuntil 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 comparator I2, or the beginning of the next cycle. The top MOSFET driver is biased from floating bootstrap capacitor CB, which normally is recharged during each off cycle. However, when VIN decreases to a voltage close to VOUT, the loop may enter dropout and attempt to turn on the top MOSFET continuously. The dropout detector counts the number of oscillator cycles that the top MOSFET remains |
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