Datasheet LTC3879 (Analog Devices) - 10

LTC3879
OPERATION Main Control Loop
the regulator is forced to operate in continuous mode by The LTC3879 is a valley current mode controller IC for disabling reversal comparator, IREV , thereby allowing the use in DC/DC step-down converters. In normal continu- inductor current to become negative. ous operation, the top MOSFET is turned on for a fi xed The continuous mode operating frequency can be deter- interval determined by a one-shot timer, OST. When the mined by dividing the calculated duty cycle, VOUT/VIN, top MOSFET is turned off, the bottom MOSFET is turned by the fi xed on-time. The OST generates an on-time on until the current comparator, ICMP , trips, restarting proportional to the ideal duty cycle, thus holding the the one-shot timer and initiating the next cycle. Inductor frequency approximately constant with changes in VIN. valley current is measured by sensing the voltage between The nominal frequency can be adjusted with an external the PGND and SW pins using the bottom MOSFET on- resistor, RON. resistance. The voltage on the ITH pin sets the compara- Foldback current limiting is provided to protect against tor threshold corresponding to inductor valley current. low impedance shorts. If the controller is in current limit The error amplifi er EA adjusts this voltage by comparing and V the feedback signal V OUT drops to less 50% of regulation, the current limit FB from the output voltage to the set-point “folds back” to progressively lower values. To feedback reference voltage VFBREF . Increasing the load recover from foldback current limit, the excessive load or current causes a drop in the feedback voltage relative low impedance short needs to be removed. to the reference. The EA senses the feedback voltage drop and adjusts the ITH voltage higher until the average When RUN is less than 0.7V, the LTC3879 is in the low inductor current matches the load current. power shutdown state with a nominal bias current of 18μA. When RUN is greater than 0.7V and less than 1.5V, With DC current loads less than 1/2 of the peak-to-peak INTV ripple the inductor current can drop to zero or become CC and all internal circuitry are enabled while TG and BG are forced low. When RUN is taken greater than 1.5V, negative. In discontinuous operation, negative inductor switching begins and the TRACK/SS pin is released from current is detected and prevented by the current reversal ground. The output voltage follows the TRACK/SS input comparator IREV , which shuts off MB. Both switches remain multiplied by feedback factor when TRACK/SS is less than off with the output capacitor supplying the load current 0.6V. Soft-start is complete when TRACK/SS exceeds the until the EA moves the ITH voltage above the zero current internal 0.6V reference voltage and regulates normally. level (0.8V) to initiate another switching cycle. When the MODE pin is below the internal threshold reference, VMODE, 3879f 10