Datasheet LTC1735-1 (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | High Efficiency Synchronous Step-Down Switching Regulator |
| Pages / Page | 28 / 8 — FU CTIO AL DIAGRA. OPERATIO. (Refer to Functional Diagram). Main Control … |
| File Format / Size | PDF / 356 Kb |
| Document Language | English |
FU CTIO AL DIAGRA. OPERATIO. (Refer to Functional Diagram). Main Control Loop
Model Line for this Datasheet
Text Version of Document
LTC1735-1
U U W FU CTIO AL DIAGRA
VIN + 13 VIN INTV CIN CC COSC INTV UVL CC 0.8V 100k REF INTV PGOOD 4 C 1 SGND 8 0.17 OSC µA CC FC DB C F BOOST – – SYNC OSC 15 + 1.2V 0.8V + CB TG FORCE BOT TOP 16 DROP OUT SW – DET BOT SWITCH 14 + 0.74V OV LOGIC S + TOP ON D1 L R Q SENSE V 0.86V – R 0.55V + B + OUT – COUT 2.4V SD 2k 45k 45k VOSENSE Ω VFB g ICMP IREV m =1.3m – – 7 – BOT I EA 1 – + + – I2 + 0.8V + + INTVCC R1 R2 0.86V BURST 3mV V 12 SD IN DISABLE + FC C 5.2V INTVCC RUN INTVCC LDO 1.2µA SOFT- A REG START 4(VFB) BUFFERED + ITH BG 4.8V + OVER- 11 6V 30k 30k CURRENT – SLOPE COMP LATCHOFF R PGND C RUN/SS 2 3 ITH SENSE+ 6 5 SENSE– EXTVCC 9 10 CSS CC 1735-1 FD
U OPERATIO (Refer to Functional Diagram) Main Control Loop
: the average inductor current matches the new load cur- rent. While the top MOSFET is off, the bottom MOSFET is The LTC1735-1 uses a constant frequency, current mode turned on until either the inductor current starts to reverse, step-down architecture. During normal operation, the top 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 ITH, which is the output of error amplifier from INTVCC through an external Schottky diode when the EA. Pin VOSENSE, described in the Pin Functions, allows EA top MOSFET is turned off. As VIN decreases towards VOUT, to receive an output feedback voltage VFB from the external the converter will attempt to turn on the top MOSFET con- resistive divider. When the load current increases, it tinuously (“dropout’’). A dropout counter detects this con- causes a slight decrease in VFB relative to the 0.8V refer- dition and forces the top MOSFET to turn off for about 500ns ence, which in turn causes the ITH voltage to increase until every tenth cycle to recharge the bootstrap capacitor. 8
U U W FU CTIO AL DIAGRA
VIN + 13 VIN INTV CIN CC COSC INTV UVL CC 0.8V 100k REF INTV PGOOD 4 C 1 SGND 8 0.17 OSC µA CC FC DB C F BOOST – – SYNC OSC 15 + 1.2V 0.8V + CB TG FORCE BOT TOP 16 DROP OUT SW – DET BOT SWITCH 14 + 0.74V OV LOGIC S + TOP ON D1 L R Q SENSE V 0.86V – R 0.55V + B + OUT – COUT 2.4V SD 2k 45k 45k VOSENSE Ω VFB g ICMP IREV m =1.3m – – 7 – BOT I EA 1 – + + – I2 + 0.8V + + INTVCC R1 R2 0.86V BURST 3mV V 12 SD IN DISABLE + FC C 5.2V INTVCC RUN INTVCC LDO 1.2µA SOFT- A REG START 4(VFB) BUFFERED + ITH BG 4.8V + OVER- 11 6V 30k 30k CURRENT – SLOPE COMP LATCHOFF R PGND C RUN/SS 2 3 ITH SENSE+ 6 5 SENSE– EXTVCC 9 10 CSS CC 1735-1 FD
U OPERATIO (Refer to Functional Diagram) Main Control Loop
: the average inductor current matches the new load cur- rent. While the top MOSFET is off, the bottom MOSFET is The LTC1735-1 uses a constant frequency, current mode turned on until either the inductor current starts to reverse, step-down architecture. During normal operation, the top 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 ITH, which is the output of error amplifier from INTVCC through an external Schottky diode when the EA. Pin VOSENSE, described in the Pin Functions, allows EA top MOSFET is turned off. As VIN decreases towards VOUT, to receive an output feedback voltage VFB from the external the converter will attempt to turn on the top MOSFET con- resistive divider. When the load current increases, it tinuously (“dropout’’). A dropout counter detects this con- causes a slight decrease in VFB relative to the 0.8V refer- dition and forces the top MOSFET to turn off for about 500ns ence, which in turn causes the ITH voltage to increase until every tenth cycle to recharge the bootstrap capacitor. 8
