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LTC1709-8 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1709-8 Datasheet(HTML) 9 Page - Linear Technology |
9 / 24 page 9 LTC1778/LTC1778-1 1778fb OPERATIO Main Control Loop The LTC1778 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 PGND and SW pins using the bottom MOSFET on-resistance . The voltage on the ITH pin sets the comparator threshold corresponding to in- ductor valley current. The error amplifier EA adjusts this voltage by comparing the feedback signal VFB from the output voltage with an internal 0.8V reference. 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 (0.8V) to initiate another cycle. Discontinuous mode operation is disabled by comparator F when the FCB pin is brought below 0.8V, 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. Overvoltage and undervoltage comparators OV and UV pull the PGOOD output low if the output feedback voltage exits a ±7.5% window around the regulation point. Furthermore, in an overvoltage condition, M1 is turned off and M2 is turned on 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 by clamp Q3 to a 1V level set by Q4 and Q6. This reduces the inductor valley current level to one sixth of its maximum value as VFB approaches 0V. Pulling the RUN/SS pin low forces the controller into its shutdown state, turning off both M1 and M2. Releasing the pin allows an internal 1.2µA current source to charge up an external soft-start capacitor CSS. When this voltage reaches 1.5V, the controller turns on and begins switch- ing, but with the ITH voltage clamped at approximately 0.6V below the RUN/SS voltage. As CSS continues to charge, the soft-start current limit is removed. INTVCC/EXTVCC 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. When the EXTVCC pin is grounded, an internal 5V low dropout regulator supplies the INTVCC power from VIN. If EXTVCC rises above 4.7V, the internal regulator is turned off, and an internal switch connects EXTVCC to INTVCC. This allows a high efficiency source connected to EXTVCC, such as an external 5V supply or a secondary output from the converter, to provide the INTVCC power. Voltages up to 7V can be applied to EXTVCC for additional gate drive. If the input voltage is low and INTVCC drops below 3.5V, undervoltage lockout circuitry prevents the power switches from turning on. |
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Similar Description - LTC1709-8 |
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