Datasheet LTC3642 (Analog Devices) - 8

LTC3642
OPERATION (Refer to Block Diagram)
The LTC3642 is a step-down DC/DC converter with internal The hysteretic nature of this control architecture results power switches that uses Burst Mode control, combining in a switching frequency that is a function of the input low quiescent current with high switching frequency, voltage, output voltage and inductor value. This behavior which results in high efficiency across a wide range of provides inherent short-circuit protection. If the output load currents. Burst Mode operation functions by using is shorted to ground, the inductor current will decay very short “burst” cycles to ramp the inductor current through slowly during a single switching cycle. Since the high side the internal power switches, followed by a sleep cycle switch turns on only when the inductor current is near where the power switches are off and the load current is zero, the LTC3642 inherently switches at a lower frequency supplied by the output capacitor. During the sleep cycle, during start-up or short-circuit conditions. the LTC3642 draws only 12µA of supply current. At light loads, the burst cycles are a small percentage of the total
Start-Up and Shutdown
cycle time which minimizes the average supply current, If the voltage on the RUN pin is less than 0.7V, the LTC3642 greatly improving efficiency. enters a shutdown mode in which all internal circuitry is disabled, reducing the DC supply current to 3µA. When the
Main Control Loop
voltage on the RUN pin exceeds 1.21V, normal operation of The feedback comparator monitors the voltage on the VFB the main control loop is enabled. The RUN pin comparator pin and compares it to an internal 800mV reference. If has 110mV of internal hysteresis, and therefore must fall this voltage is greater than the reference, the comparator below 1.1V to disable the main control loop. activates a sleep mode in which the power switches and The HYST pin provides an added degree of flexibility for current comparators are disabled, reducing the VIN pin the RUN pin operation. This open-drain output is pulled supply current to only 12µA. As the load current discharges to ground whenever the RUN comparator is not tripped, the output capacitor, the voltage on the VFB pin decreases. signaling that the LTC3642 is not in normal operation. In When this voltage falls 5mV below the 800mV reference, applications where the RUN pin is used to monitor the V the feedback comparator trips and enables burst cycles. IN voltage through an external resistive divider, the HYST pin At the beginning of the burst cycle, the internal high side can be used to increase the effective RUN comparator power switch (P-channel MOSFET) is turned on and the hysteresis. inductor current begins to ramp up. The inductor current An internal 1ms soft-start function limits the ramp rate of increases until either the current exceeds the peak cur- the output voltage on start-up to prevent excessive input rent comparator threshold or the voltage on the VFB pin supply droop. If a longer ramp time and consequently less exceeds 800mV, at which time the high side power switch supply droop is desired, a capacitor can be placed from the is turned off and the low side power switch (N-channel SS pin to ground. The 5µA current that is sourced out of MOSFET) turns on. The inductor current ramps down until this pin will create a smooth voltage ramp on the capacitor. the reverse current comparator trips, signaling that the If this ramp rate is slower than the internal 1ms soft-start, current is close to zero. If the voltage on the VFB pin is then the output voltage will be limited by the ramp rate still less than the 800mV reference, the high side power on the SS pin instead. The internal and external soft-start switch is turned on again and another cycle commences. functions are reset on start-up and after an undervoltage The average current during a burst cycle will normally be or overvoltage event on the input supply. greater than the average load current. For this architecture, the maximum average output current is equal to half of In order to ensure a smooth start-up transition in any the peak current. application, the internal soft-start also ramps the peak 3642fc 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