Datasheet LTC3632 (Analog Devices) - 8

LTC3632
OPERATION (Refer to Block Diagram)
greater than the average load current. For this architecture, The LTC3632 is a step-down DC/DC converter with internal the maximum average output current is equal to half of power switches that uses Burst Mode control, combin- the peak current. ing low quiescent current with high switching frequency, The hysteretic nature of this control architecture results which results in high efficiency across a wide range of in a switching frequency that is a function of the input load currents. Burst Mode operation functions by using voltage, output voltage and inductor value. This behavior short “burst” cycles to ramp the inductor current through provides inherent short-circuit protection. If the output the internal power switches, followed by a sleep cycle is shorted to ground, the inductor current will decay very where the power switches are off and the load current is slowly during a single switching cycle. Since the high side supplied by the output capacitor. During the sleep cycle, switch turns on only when the inductor current is near the LTC3632 draws only 12µA of supply current. At light zero, the LTC3632 inherently switches at a lower frequency loads, the burst cycles are a small percentage of the total during start-up or short-circuit conditions. cycle time which minimizes the average supply current, greatly improving efficiency.
Start-Up and Shutdown Main Control Loop
If the voltage on the RUN pin is less than 0.7V, the LTC3632 enters a shutdown mode in which all internal circuitry is The feedback comparator monitors the voltage on the VFB disabled, reducing the DC supply current to 3µA. When the pin and compares it to an internal 800mV reference. If voltage on the RUN pin exceeds 1.2V, normal operation of this voltage is greater than the reference, the comparator the main control loop is enabled. The RUN pin comparator activates a sleep mode in which the power switches and has 110mV of internal hysteresis, and therefore must fall current comparators are disabled, reducing the VIN pin below 1.1V to disable the main control loop. supply current to only 12µA. As the load current discharges the output capacitor, the voltage on the VFB pin decreases. The HYST pin provides an added degree of flexibility for When this voltage falls 5mV below the 800mV reference, the RUN pin operation. This open-drain output is pulled the feedback comparator trips and enables burst cycles. to ground whenever the RUN comparator is not tripped, signaling that the LTC3632 is not in normal operation. In At the beginning of the burst cycle, the internal high side applications where the RUN pin is used to monitor the power switch (P-channel MOSFET) is turned on and the V inductor current begins to ramp up. The inductor current IN voltage through an external resistive divider, the HYST pin can be used to increase the effective RUN comparator increases until either the current exceeds the peak cur- hysteresis. rent comparator threshold or the voltage on the VFB pin exceeds 800mV, at which time the high side power switch An internal 0.75ms soft-start function limits the ramp rate is turned off and the low side power switch (N-channel of the output voltage on start-up to prevent excessive input MOSFET) turns on. The inductor current ramps down until supply droop. If a longer ramp time and consequently less the reverse-current comparator trips, signaling that the supply droop is desired, a capacitor can be placed from the current is close to zero. If the voltage on the VFB pin is SS pin to ground. The 5µA current that is sourced out of this still less than the 800mV reference, the high side power pin will create a smooth voltage ramp on the capacitor. If switch is turned on again and another cycle commences. this ramp rate is slower than the internal 0.75ms soft-start, The average current during a burst cycle will normally be then the output voltage will be limited by the ramp rate 3632fc 8 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM OPERATION APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS