Datasheet LTC3878 (Analog Devices) - 9

LTC3878 operaTion
LTC1778 Compatibility
current is detected and prevented by the current reversal The LTC3878 is compatible with the LTC1778 in applica- comparator IREV , which shuts off MB. Both switches tions which do not use the EXTV remain off with the output capacitor supplying the load CC function. The LTC3878 offers improved gate drive and reduced dead time, which current until the EA moves the ITH voltage above the zero allows higher efficiency than the LTC1778. On the LTC1778 current level (0.8V) to initiate another switching cycle. Pin 9 is EXTV When the FCB (forced continuous bar) pin is below the CC, but on the LTC3878 it is a no connect. The other notable difference is that the shutdown latch- internal FCB threshold reference, VFCB, the regulator is off timer is removed. The LTC3878 should be a drop in, forced to operate in continuous mode by disabling reversal pin-for-pin replacement in most applications that do not comparator, IREV , thereby allowing the inductor current to use EXTV become negative. CC. The LTC3878 should be tested and verified in each application without assuming compatibility. Con- The continuous mode operating frequency can be deter- tact a Linear applications expert to answer any questions mined by dividing the calculated duty cycle, VOUT/VIN, regarding LTC3878/LTC1778 compatibility. by the fixed on-time. The OST generates an on-time proportional to the ideal duty cycle, thus holding the
Main Control Loop
frequency approximately constant with changes in VIN. The LTC3878 is a valley current mode controller IC for The nominal frequency can be adjusted with an external use in DC/DC step-down converters. In normal continu- resistor, RON. ous operation, the top MOSFET is turned on for a fixed Foldback current limiting is provided to protect against interval determined by a one-shot timer, OST. When the low impedance shorts. If the controller is in current limit top MOSFET is turned off, the bottom MOSFET is turned and VOUT drops to less 50% of regulation, the current limit on until the current comparator, ICMP , trips, restarting set-point “folds back” to progressively lower values. To the one-shot timer and initiating the next cycle. Inductor recover from foldback current limit, the excessive load or val ey current is measured by sensing the voltage between low impedance short needs to be removed. the PGND and SW pins using the bottom MOSFET on- resistance. The voltage on the I Pulling the RUN/SS pin low forces the controller into its TH pin sets the compara- tor threshold corresponding to inductor valley current. shutdown state, turning off both MT and MB. Releasing The error amplifier EA adjusts this voltage by comparing the pin allows an internal 1.2µA current source to charge the feedback signal V up an external soft-start capacitor, C FB from the output voltage to the SS. When the RUN/ feedback reference voltage V SS pin is less than 0.7V, the device is in the low power FBREF . Increasing the load current causes a drop in the feedback voltage relative shutdown condition with a nominal bias current of 18µA. to the reference. The EA senses the feedback voltage When RUN/SS is greater than 0.7V and less than 1.5V, drop and adjusts the I INTV TH voltage higher until the average CC and all internal circuitry are enabled while MT and inductor current matches the load current. MB are forced off. Current-limited soft-start begins when RUN/SS exceeds 1.5V. Normal operation at full current With DC current loads less than 1/2 of the peak-to-peak limit is achieved at approximately 3V on RUN/SS. Foldback ripple the inductor current can drop to zero or become current limit is defeated during soft-start. negative. In discontinuous operation, negative inductor 3878fa Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration order information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts