Datasheet LTC3638 (Analog Devices) - 8

LTC3638
OPERATION (Refer to Block Diagram)
The LTC3638 is a step-down DC/DC regulator with internal External feedback resistors (adjustable mode) can be used power switch that uses Burst Mode control, combining by connecting both VPRG1 and VPRG2 to ground. low quiescent current with high switching frequency, In adjustable mode the feedback comparator monitors which results in high efficiency across a wide range of the voltage on the V load currents. Burst Mode operation functions by us- FB pin and compares it to an internal 800mV reference. If this voltage is greater than the refer- ing short “burst” cycles to switch the inductor current ence, the comparator activates a sleep mode in which through the internal power MOSFET, followed by a sleep the power switch and current comparators are disabled, cycle where the power switch is off and the load current is reducing the V supplied by the output capacitor. During the sleep cycle, IN pin supply current to only 12µA. As the load current discharges the output capacitor, the voltage the LTC3638 draws only 12µA of supply current. At light on the V loads, the burst cycles are a small percentage of the total FB pin decreases. When this voltage falls 5mV below the 800mV reference, the feedback comparator cycle time which minimizes the average supply current, trips and enables burst cycles. greatly improving efficiency. Figure 1 shows an example of Burst Mode operation. The switching frequency is de- At the beginning of the burst cycle, the internal high side pendent on the inductor value, peak current, input voltage power switch (P-channel MOSFET) is turned on and the and output voltage. inductor current begins to ramp up. The inductor current increases until either the current exceeds the peak cur- SLEEP rent comparator threshold or the voltage on the VFB pin CYCLE SWITCHING BURST FREQUENCY exceeds 800mV, at which time the switch is turned off CYCLE and the inductor current is carried by the external catch diode. The inductor current ramps down until the switch INDUCTOR CURRENT node rises, indicating that the current in the catch diode is zero. If the voltage on the VFB pin is still less than the BURST FREQUENCY 800mV reference, the power switch is turned on again and another cycle commences. The average current during a burst cycle will normally be greater than the average load OUTPUT VOLTAGE current. For this architecture, the maximum average output ∆VOUT 3638 F01 current is equal to half of the peak current. The hysteretic nature of this control architecture results
Figure 1. Burst Mode Operation
in a switching frequency that is a function of the input voltage, output voltage, and inductor value. This behavior
Main Control Loop
provides inherent short-circuit protection. If the output is The LTC3638 uses the V shorted to ground, the inductor current will decay very PRG1 and VPRG2 control pins to connect internal feedback resistors to the V slowly during a single switching cycle. Since the high side FB pin. This enables fixed outputs of 1.8V, 3.3V or 5V without increas- switch turns on only when the inductor current is near ing component count, input supply current or exposure to zero, the LTC3638 inherently switches at a lower frequency noise on the sensitive input to the feedback comparator. during start-up or short-circuit conditions. 3638fa 8 For more information www.linear.com/LTC3638 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