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LTC3718 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3718 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 LTC3718 3718fa Main Control Loop The LTC3718 is a current mode controller for DC/DC step-down converters designed to operate from low input voltages. It incorporates a boost converter with a buck regulator. Buck Regulator Operation In normal operation, the top MOSFET is turned on for a fixed interval determined by a one-shot timer OST. When the top MOSFET is turned off, the bottom MOSFET is turned on until the current comparator ICMP trips, restart- ing the one-shot timer and initiating the next cycle. Induc- tor current is determined by sensing the voltage between the SENSE+ and SENSE– pins using the bottom MOSFET on-resistance . The voltage on the ITH pin sets the com- parator threshold corresponding to inductor valley cur- rent. The error amplifier EA adjusts this voltage by com- paring the feedback signal VFB1 from the output voltage with an internal reference generated from one half of the voltage on VREF. If the load current increases, it causes a drop in the feedback voltage relative to the reference. The ITH voltage then rises until the average inductor current again matches the load current. The operating frequency is determined implicitly by the top MOSFET on-time and the duty cycle required to maintain regulation. The one-shot timer generates an on- time that is proportional to the ideal duty cycle, thus holding frequency approximately constant with changes in VIN. The nominal frequency can be adjusted with an external resistor RON. Overvoltage and undervoltage comparators OV and UV pull the PGOOD output low if the output feedback voltage exits a ±10% window around the regulation point. Furthermore, in an overvoltage condition, M1 is turned off and M2 is turned on and held on until the overvoltage condition clears. Pulling the RUN/SS pin low forces the controller into its shutdown state, turning off both M1 and M2. Releasing the pin allows an internal 1.2 µAcurrentsourcetochargeupan external soft-start capacitor CSS.Whenthisvoltagereaches 1.5V, the controller turns on and begins switching, but with the ITH voltage clamped at approximately 0.6V below the RUN/SS voltage. As CSS continues to charge, the soft- start current limit is removed. OPERATIO INTVCC Power Power for the top and bottom MOSFET drivers and most of the internal controller circuitry is derived from the INTVCC pin. The top MOSFET driver is powered from a floating bootstrap capacitor CB. This capacitor is re- charged from INTVCC through an external Schottky diode DB when the top MOSFET is turned off. Boost Regulator Operation The 5V power source for INTVCC can be provided by a current mode, internally compensated fixed frequency step-up switching regulator that has been incorporated into the LTC3718. Operation can be best understood by referring to the Functional Diagrams. Q1 and Q2 form a bandgap refer- ence core whose loop is closed around the output of the regulator. The voltage drop across R5 and R6 is low enough such that Q1 and Q2 do not saturate, even when VIN2 is 1V. When there is no load, VFB2 rises slightly above 1.23V, causing VC (the error amplifier’s output) to de- crease. Comparator A2’s output stays high, keeping switch Q3 in the off state. As increased output loading causes the VFB2 voltage to decrease, A1’s output increases. Switch current is regulated directly on a cycle-by-cycle basis by the VC node. The flip-flop is set at the beginning of each switch cycle, turning on the switch. When the summation of a signal representing switch current and a ramp gen- erator (introduced to avoid subharmonic oscillations at duty factors greater than 50%) exceeds the VC signal, comparator A2 changes state, resetting the flip-flop and turning off the switch. More power is delivered to the output as switch current is increased. The output voltage, attenuated by external resistor divider R7 and R8, appears at the VFB2 pin, closing the overall loop. Frequency com- pensation is provided internally by RC and CC. Transient response can be optimized by the addition of a phase lead capacitor CPL in parallel with R7 in applications where large value or low ESR output capacitors are used. As the load current is decreased, the switch turns on for a shorter period each cycle. If the load current is further decreased, the boost converter will skip cycles to main- tain output voltage regulation. If the VFB2 pin voltage is increased significantly above 1.23V, the boost converter will enter a low power state. |
Similar Part No. - LTC3718_15 |
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Similar Description - LTC3718_15 |
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