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LTC3770EUH Datasheet(PDF) 10 Page - Linear Technology |
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LTC3770EUH Datasheet(HTML) 10 Page - Linear Technology |
10 / 24 page 10 LTC3770 3770f OPERATIO Main Control Loop The LTC3770 is a current mode controller for DC/DC step-down converters. 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 com- parator ICMP trips, restarting the one-shot timer and initi- ating the next cycle. Inductor current is determined by sensing the voltage between the SENSE– (PGND on G Package) and SENSE+ (SW on G Package) pins using a sense resistor or the bottom MOSFET on-resistance . The voltage on the ITH pin sets the comparator threshold corresponding to inductor valley current. The error ampli- fier EA adjusts this voltage by comparing the feedback signal VFB from a reference voltage set by the VREFIN pin. 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. At low load currents, the inductor current can drop to zero and become negative. This is detected by current reversal comparator IREV which then shuts off M2, resulting in discontinuous operation. Both switches will remain off with the output capacitor supplying the load current until the ITH voltage rises above the zero current level (0.75V) to initiate another cycle. Discontinuous mode operation is disabled by comparator F when the FCB pin is brought below 0.6V, forcing continuous synchronous operation. 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. For applications with stringent constant frequency re- quirements, the LTC3770 can be synchronized with an external clock. By programming the nominal frequency of the LTC3770 the same as the external clock frequency, the LTC3770 behaves as a constant frequency part against the load and supply variations. 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 after the internal 25µs power bad mask timer expires. Furthermore, in an overvoltage condition, M1 is turned off and M2 is turned on immediately and held on until the overvoltage condition clears. Foldback current limiting is provided if the output is shorted to ground. As VFB drops, the buffered current threshold voltage ITHB is pulled down and clamped to 0.9V. This reduces the inductor valley current level to one tenth of its maximum value as VFBapproaches0V.Foldback current limiting is disabled at start-up. Pulling the RUN pin low forces the controller into its shutdown state, turning off both M1 and M2. Forcing a voltage above 1.5V will turn on the device. 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. If the input voltage is low and INTVCC drops below 3.2V, undervoltage lockout circuitry prevents the power switches from turning on. |
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