Datasheet LTC3770 (Analog Devices) - 10

LTC3770
OPERATION Main Control Loop
For applications with stringent constant frequency re- quirements, the LTC3770 can be synchronized with an The LTC3770 is a current mode controller for DC/DC external clock. By programming the nominal frequency step-down converters. In normal operation, the top of the LTC3770 the same as the external clock frequency, MOSFET is turned on for a fi xed interval determined by a the LTC3770 behaves as a constant frequency part against one-shot timer OST. When the top MOSFET is turned off, the the load and supply variations. bottom MOSFET is turned on until the current comparator ICMP trips, restarting the one-shot timer and initiating the Overvoltage and undervoltage comparators OV and UV next cycle. Inductor current is determined by sensing the pull the PGOOD output low if the output feedback voltage voltage between the SENSE– (PGND on G package) and exits a ±10% window around the regulation point after the SENSE+ (SW on G package) pins using a sense resistor internal 25μs power bad mask timer expires. Furthermore, or the bottom MOSFET on-resistance . The voltage on in an overvoltage condition, M1 is turned off and M2 is the ITH pin sets the comparator threshold corresponding turned on immediately and held on until the overvoltage to inductor valley current. The error amplifi er EA adjusts condition clears. this voltage by comparing the feedback signal VFB from a Foldback current limiting is provided if the output is shorted reference voltage set by the VREFIN pin. If the load current to ground. As V increases, it causes a drop in the feedback voltage relative FB drops, the buffered current threshold voltage I to the reference. The I THB is pulled down and clamped to 0.9V. This TH voltage then rises until the average reduces the inductor valley current level to one tenth of its inductor current again matches the load current. maximum value as VFB approaches 0V. Foldback current At low load currents, the inductor current can drop to zero limiting is disabled at start-up. and become negative. This is detected by current reversal Pulling the RUN pin low forces the controller into its comparator IREV which then shuts off M2, resulting in shutdown state, turning off both M1 and M2. Forcing a discontinuous operation. Both switches will remain off voltage above 1.5V will turn on the device. with the output capacitor supplying the load current until the ITH voltage rises above the zero current level (0.75V)
INTVCC Power
to initiate another cycle. Discontinuous mode operation is disabled by comparator F when the FCB pin is brought Power for the top and bottom MOSFET drivers and most of below 0.6V, forcing continuous synchronous operation. the internal controller circuitry is derived from the INTVCC pin. The top MOSFET driver is powered from a fl oating The operating frequency is determined implicitly by the top bootstrap capacitor, CB. This capacitor is recharged from MOSFET on-time and the duty cycle required to maintain INTVCC through an external Schottky diode DB when the regulation. The one-shot timer generates an on-time that is top MOSFET is turned off. If the input voltage is low and proportional to the ideal duty cycle, thus holding frequency INTVCC drops below 3.2V, undervoltage lockout circuitry approximately constant with changes in VIN. The nominal prevents the power switches from turning on. frequency can be adjusted with an external resistor RON. 3770fc 10