Datasheet LTC1778, LTC1778-1 (Analog Devices) - 9

LTC1778/LTC1778-1
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 LTC1778 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 PGND and SW pins using approaches 0V. the bottom MOSFET on-resistance . The voltage on the ITH Pulling the RUN/SS pin low forces the controller into its pin sets the comparator threshold corresponding to in- shutdown state, turning off both M1 and M2. Releasing ductor valley current. The error amplifier EA adjusts this the pin allows an internal 1.2µA current source to charge voltage by comparing the feedback signal VFB from the up an external soft-start capacitor C output voltage with an internal 0.8V reference. If the load SS. When this voltage reaches 1.5V, the controller turns on and begins switch- current increases, it causes a drop in the feedback voltage ing, but with the I relative to the reference. The I TH voltage clamped at approximately TH voltage then rises until the 0.6V below the RUN/SS voltage. As C average 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
INTVCC/EXTVCC Power
comparator IREV which then shuts off M2, resulting in Power for the top and bottom MOSFET drivers and most discontinuous operation. Both switches will remain off of the internal controller circuitry is derived from the with the output capacitor supplying the load current until INTVCC pin. The top MOSFET driver is powered from a the ITH voltage rises above the zero current level (0.8V) to floating bootstrap capacitor CB. This capacitor is re- initiate another cycle. Discontinuous mode operation is charged from INTVCC through an external Schottky diode disabled by comparator F when the FCB pin is brought DB when the top MOSFET is turned off. When the EXTVCC below 0.8V, forcing continuous synchronous operation. pin is grounded, an internal 5V low dropout regulator The operating frequency is determined implicitly by the supplies the INTVCC power from VIN. If EXTVCC rises top MOSFET on-time and the duty cycle required to above 4.7V, the internal regulator is turned off, and an maintain regulation. The one-shot timer generates an on- internal switch connects EXTVCC to INTVCC. This allows time that is proportional to the ideal duty cycle, thus a high efficiency source connected to EXTVCC, such as an holding frequency approximately constant with changes external 5V supply or a secondary output from the in V converter, to provide the INTV IN. The nominal frequency can be adjusted with an CC power. Voltages up to external resistor R 7V can be applied to EXTV 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 ±7.5% window around the regulation point. 1778fb 9