Datasheet LTC3631 (Analog Devices) - 8

LTC3631
operaTion (Refer to Block Diagram)
The LTC3631 is a step-down DC/DC converter with internal greater than the average load current. For this architecture, power switches that uses Burst Mode control, combin- the maximum average output current is equal to half of ing low quiescent current with high switching frequency, the peak current. which results in high efficiency across a wide range of load currents. Burst Mode operation functions by using The hysteretic nature of this control architecture results short “burst” cycles to ramp the inductor current through in a switching frequency that is a function of the input the internal power switches, followed by a sleep cycle voltage, output voltage and inductor value. This behavior where the power switches are off and the load current is provides inherent short-circuit protection. If the output supplied by the output capacitor. During the sleep cycle, is shorted to ground, the inductor current will decay very the LTC3631 draws only 12µA of supply current. At light slowly during a single switching cycle. Since the high side loads, the burst cycles are a small percentage of the total switch turns on only when the inductor current is near cycle time which minimizes the average supply current, zero, the LTC3631 inherently switches at a lower frequency greatly improving efficiency. during start-up or short-circuit conditions.
Main Control Loop Start-Up and Shutdown
The feedback comparator monitors the voltage on the V If the voltage on the RUN pin is less than 0.7V, the LTC3631 FB pin and compares it to an internal 800mV reference. If enters a shutdown mode in which all internal circuitry is this voltage is greater than the reference, the comparator disabled, reducing the DC supply current to 3µA. When the activates a sleep mode in which the power switches and voltage on the RUN pin exceeds 1.21V, normal operation of current comparators are disabled, reducing the V the main control loop is enabled. The RUN pin comparator IN pin supply current to only 12µA. As the load current discharges has 110mV of internal hysteresis, and therefore must fall the output capacitor, the voltage on the V below 1.1V to disable the main control loop. FB pin decreases. When this voltage falls 5mV below the 800mV reference, The HYST pin provides an added degree of flexibility for the feedback comparator trips and enables burst cycles. the RUN pin operation. This open-drain output is pulled At the beginning of the burst cycle, the internal high side to ground whenever the RUN comparator is not tripped, power switch (P-channel MOSFET) is turned on and the signaling that the LTC3631 is not in normal operation. In inductor current begins to ramp up. The inductor current applications where the RUN pin is used to monitor the increases until either the current exceeds the peak cur- VIN voltage through an external resistive divider, the HYST rent comparator threshold or the voltage on the V pin can be used to increase the effective RUN comparator FB pin exceeds 800mV, at which time the high side power switch hysteresis. is turned off, and the low side power switch (N-channel An internal 1ms soft-start function limits the ramp rate of MOSFET) turns on. The inductor current ramps down until the output voltage on start-up to prevent excessive input the reverse current comparator trips, signaling that the supply droop. If a longer ramp time and consequently less current is close to zero. If the voltage on the VFB pin is supply droop is desired, a capacitor can be placed from the still less than the 800mV reference, the high side power SS pin to ground. The 5µA current that is sourced out of switch is turned on again and another cycle commences. this pin will create a smooth voltage ramp on the capacitor. The average current during a burst cycle will normally be If this ramp rate is slower than the internal 1ms soft-start, 3631fe 8 For more information www.linear.com/LTC3631 Document Outline Features Description Applications Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Related Parts