Datasheet LTC3778 (Analog Devices) - 9

LTC3778
U OPERATIO Main Control Loop
Furthermore, in an overvoltage condition, M1 is turned off and M2 is turned on and held on until the overvoltage The LTC3778 is a current mode controller for DC/DC condition clears. step-down converters. In normal operation, the top MOSFET is turned on for a fixed interval determined by a Foldback current limiting is provided if the output is one-shot timer OST. When the top MOSFET is turned off, shorted to ground. As VFB drops, the buffered current the bottom MOSFET is turned on until the current com- threshold voltage ITHB is pulled down by clamp Q3 to a 1V parator ICMP trips, restarting the one-shot timer and initi- level set by Q4 and Q6. This reduces the inductor valley ating the next cycle. Inductor current is determined by current level to one sixth of its maximum value as VFB sensing the voltage between the SENSE– and SENSE+ approaches 0V. pins. The voltage on the ITH pin sets the comparator Pulling the RUN/SS pin low forces the controller into its threshold corresponding to inductor valley current. The shutdown state, turning off both M1 and M2. Releasing error amplifier EA adjusts this voltage by comparing the the pin allows an internal 1.2µA current source to charge feedback signal VFB from the output voltage with an up an external soft-start capacitor C internal 0.6V reference. If the load current increases, it SS. When this voltage reaches 1.5V, the controller turns on and begins switch- causes a drop in the feedback voltage relative to the ing, but with the I reference. The I TH voltage clamped at approximately TH voltage then rises until the average 0.6V below the RUN/SS voltage. As C inductor current again matches the load current. SS continues to charge, the soft-start current limit is removed. At low load currents, the inductor current can drop to zero and become negative. This is detected by current reversal
EXTVCC/INTVCC/DRVCC Power
comparator IREV which then shuts off M2, resulting in Power for the top and bottom MOSFET drivers is derived discontinuous operation. Both switches will remain off from DRVCC and most of the internal controller circuitry with the output capacitor supplying the load current until is powered from the INTVCC pin. The top MOSFET driver the ITH voltage rises above the zero current level (0.8V) to is powered from a floating bootstrap capacitor CB. This initiate another cycle. Discontinuous mode operation is capacitor is recharged from DRVCC through an external disabled by comparator F when the FCB pin is brought Schottky diode DB when the top MOSFET is turned off. below 0.6V, forcing continuous synchronous operation. When the EXTVCC pin is grounded, an internal 5V low The operating frequency is determined implicitly by the dropout regulator supplies the INTVCC power from VIN. If top MOSFET on-time and the duty cycle required to EXTVCC rises above 4.7V, the internal regulator is turned maintain regulation. The one-shot timer generates an on- off, and an internal switch connects EXTVCC to INTVCC. time that is proportional to the ideal duty cycle, thus This allows a high efficiency source connected to EXTVCC, holding frequency approximately constant with changes such as an external 5V supply or a secondary output from in V the converter, to provide the INTV IN and VOUT. The nominal frequency can be adjusted CC power. Voltages up with an external resistor R to 7V can be applied to DRV ON. CC for additional gate drive. If the input voltage is low and INTV Overvoltage and undervoltage comparators OV and UV CC drops below 3.5V, undervoltage lockout circuitry prevents the power pull the PGOOD output low if the output feedback voltage switches from turning on. exits a ±10% window around the regulation point. 3778f 9