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LTC3127EDDPBF Datasheet(PDF) 9 Page - Linear Technology |
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LTC3127EDDPBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LTC3127 3127f The LTC3127 is an average input current controlled buck- boostDC/DCconverterofferedinbothathermallyenhanced 3mm × 3mm DFN package and a thermally enhanced 12- lead MSOP package. The buck-boost converter utilizes a proprietary switching algorithm which allows its output voltage to be regulated above, below or equal to the input voltage. The low RDS(ON), low gate charge synchronous switches efficiently provide high frequency PWM control. High efficiency is achieved at light loads when Burst Mode operation is commanded. PWM Mode Operation The LTC3127 uses fixed frequency, average input cur- rent PWM control. The MODE pin can be used to select automatic Burst Mode operation (MODE connected to VIN) or to disable Burst Mode operation and select forced continuousconductionoperationforlownoiseapplications (MODE grounded). A proprietary switching algorithm allows the converter to switch between buck, buck-boost and boost modes without discontinuity in inductor current or loop charac- teristics.Theswitchtopologyforthebuck-boostconverter is shown in Figure 1. When the input voltage is significantly greater than the output voltage, the buck-boost converter operates in buck mode. Switch D turns on continuously and switch C remains off. Switches A and B are pulse width modulated to produce the required duty cycle to support the output regulation voltage. As the input voltage decreases, switch A remains on for a larger portion of the switching cycle. When the duty cycle reaches approximately 85%, the switch pair AC begins turning on for a small fraction of the switching period. As the input voltage decreases further, L D PGND PGND LTC3127 A SW1 SW2 B C 3127 F01 VOUT VIN Figure 1. Buck-Boost Switch Topology the AC switch pair remains on for longer durations and the duration of the BD phase decreases proportionally. As the input voltage drops below the output voltage, the AC phase will eventually increase to the point that there is no longer any BD switching. At this point, switch A remains on continuously while switch pair CD is pulse width modu- lated to obtain the desired output voltage. At this point, the converter is operating solely in boost mode. This switching algorithm provides a seamless transition between operating modes and eliminates discontinuities in average inductor current, inductor current ripple, and loop transfer function throughout all three operational modes. These advantages result in increased efficiency and stability in comparison to the traditional 4-switch buck-boost converter. In forced PWM mode operation, the inductor is forced to have continuous conduction. This allows for a constant switching frequency and better noise performance. Error Amplifier and Compensation The buck-boost converter utilizes two control loops. The outer voltage loop determines the amount of current re- quired to regulate the output voltage. The voltage loop is externally compensated and can be configured with either integral compensation or proportional control. The inner currentloopisinternallycompensatedandforcestheinput current to equal the commanded current. When VC is compensated via proportional control, the dominant pole of the output capacitor is used to ensure stability with a minimum of 1000µF of capacitance on the output when a 499k resistor is used. There is no maximum capacitance limitation with proportional compensation. operaTion |
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