Datasheet LTC3630 (Analog Devices) - 9

LTC3630
operaTion (Refer to Block Diagram)
The LTC3630 is a synchronous step-down DC/DC con- reference, the comparator activates a sleep mode in which verter with internal power switches that uses Burst Mode the power switches and current comparators are disabled, control. The low quiescent current and high switching reducing the VIN pin supply current to only 12µA. As the frequency results in high efficiency across a wide range load current discharges the output capacitor, the voltage of load currents. Burst Mode operation functions by using on the VFB pin decreases. When this voltage falls 5mV short “burst” cycles to switch the inductor current through below the 800mV reference, the feedback comparator the internal power MOSFETs, followed by a sleep cycle trips and enables burst cycles. where the power switches are off and the load current is At the beginning of the burst cycle, the internal high side supplied by the output capacitor. During the sleep cycle, power switch (P-channel MOSFET) is turned on and the the LTC3630 draws only 12µA of supply current. At light inductor current begins to ramp up. The inductor current loads, the burst cycles are a small percentage of the total increases until either the current exceeds the peak cur- cycle time which minimizes the average supply current, rent comparator threshold or the voltage on the V greatly improving efficiency. Figure 1 shows an example FB pin exceeds 800mV, at which time the high side power switch of Burst Mode operation. The switching frequency and the is turned off and the low side power switch (N-channel number of switching cycles during Burst Mode operation MOSFET) turns on. The inductor current ramps down until are dependent on the inductor value, peak current, load the reverse current comparator trips, signaling that the current, input voltage and output voltage. current is close to zero. If the voltage on the VFB pin is SLEEP still less than the 800mV reference, the high side power CYCLE SWITCHING BURST FREQUENCY switch is turned on again and another cycle commences. CYCLE The average current during a burst cycle will normally be greater than the average load current. For this architecture, INDUCTOR the maximum average output current is equal to half of CURRENT the peak current. BURST FREQUENCY The hysteretic nature of this control architecture results in a switching frequency that is a function of the input OUTPUT voltage, output voltage, and inductor value. This behavior VOLTAGE provides inherent short-circuit protection. If the output is ∆VOUT 3630 F01 shorted to ground, the inductor current will decay very slowly during a single switching cycle. Since the high side
Figure 1. Burst Mode Operation
switch turns on only when the inductor current is near zero, the LTC3630 inherently switches at a lower frequency
Main Control Loop
during start-up or short-circuit conditions. The LTC3630 uses the VPRG1 and VPRG2 control pins to connect internal feedback resistors to the V
Start-Up and Shutdown
FB pin. This enables fixed outputs of 1.8V, 3.3V or 5V without increas- If the voltage on the RUN pin is less than 0.7V, the LTC3630 ing component count, input supply current or exposure to enters a shutdown mode in which all internal circuitry is noise on the sensitive input to the feedback comparator. disabled, reducing the DC supply current to 5µA. When the External feedback resistors (adjustable mode) can still voltage on the RUN pin exceeds 1.21V, normal operation be used by connecting both VPRG1 and VPRG2 to ground. of the main control loop is enabled. The RUN pin com- In adjustable mode the feedback comparator monitors parator has 110mV of internal hysteresis, and therefore the voltage on the V must fall below 1.1V to stop switching and disable the FB pin and compares it to an inter- nal 800mV reference. If this voltage is greater than the main control loop. 3630fd For more information www.linear.com/LTC3630 9 Document Outline Features Applications Description 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 Typical Application Related Parts