Datasheet LTC3823 (Analog Devices) - 10

LTC3823
OPERATION Main Control Loop
The LTC3823 is a current mode controller for DC/DC For applications with stringent constant frequency re- step-down converters. In normal operation, the top quirements, the LTC3823 can be synchronized with an MOSFET is turned on for a fi xed interval determined by external clock. By programming the nominal frequency a one-shot timer, OST. When the top MOSFET is turned of the LTC3823 the same as the external clock frequency, off, the bottom MOSFET is turned on until the current the LTC3823 behaves as a constant frequency part against comparator ICMP trips, restarting the one-shot timer and the load and supply variations. initiating the next cycle. Inductor current is determined Overvoltage and undervoltage comparators OV and UV by sensing the voltage between the SENSE– and SENSE+ pull the PGOOD output low if the output feedback voltage pins using a sense resistor or the bottom MOSFET on- exits a ±10% window around the regulation point after the resistance . The voltage on the ITH pin sets the comparator internal 20μs power bad mask timer expires. Furthermore, threshold corresponding to inductor valley current. The in an overvoltage condition, M1 is turned off and M2 is error amplifi er EA adjusts this voltage by comparing the turned on immediately and held on until the overvoltage feedback signal, VFB, to an internal reference voltage. If condition clears. the load current increases, it causes a drop in the feedback voltage relative to the reference. The ITH voltage then rises Foldback current limiting is provided if the output is shorted until the average inductor current again matches the load to ground. As VFB drops, the buffered current threshold current. voltage, ITHB, is pulled down and clamped to 0.9V. This reduces the inductor valley current level to one-tenth of its At low load currents, the inductor current can drop to zero maximum value as V and become negative. This is detected by current reversal FB approaches 0V. Foldback current limiting is disabled at start-up. comparator IREV which then shuts off M2, resulting in discontinuous operation. Both switches will remain off Pulling the RUN pin low forces the controller into its with the output capacitor supplying the load current until shutdown state, turning off both M1 and M2. Forcing a the ITH voltage rises above the zero current level (0.75V) voltage above 1.5V will turn on the device. to initiate another cycle. Discontinuous mode operation is disabled by comparator F when the FCB pin is brought
INTVCC Power
below 0.6V, forcing continuous synchronous operation. Power for the top and bottom MOSFET drivers and most of The operating frequency is determined implicitly by the the internal controller circuitry is derived from the INTVCC top MOSFET on-time and the duty cycle required to main- pin. The top MOSFET driver is powered from a fl oating tain regulation. The one-shot timer generates an on time bootstrap capacitor, CB. This capacitor is recharged from that is proportional to the ideal duty cycle, thus holding INTVCC through an external Schottky diode, DB, when the frequency approximately constant with changes in V top MOSFET is turned off. If the input voltage is low and IN. The nominal frequency can be adjusted with an external INTVCC drops below 3V, undervoltage lockout circuitry resistor, R prevents the power switches from turning on. ON. 3823fd 10