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LTC3854EMSE 数据表(PDF) 19 Page - Linear Technology |
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LTC3854EMSE 数据表(HTML) 19 Page - Linear Technology |
19 / 28 page LTC3854 3854fa applicaTions inForMaTion Figure 5. Kelvin Sensing RSENSE 3) Are the SENSE– and SENSE+ leads routed together with minimum PC trace spacing? The filter capacitor between SENSE+ and SENSE– should be as close as possible to the LTC3854. Ensure accurate current sensing with Kelvin connections as shown in Figure 5. Series resistance can be added to the SENSE lines to increase noise rejection. 4) Does the (+) terminal of CIN connect to the drain of the topside MOSFET(s) as closely as possible? This capacitor provides the AC current to the MOSFET(s). 5) Is the INTVCC decoupling capacitor connected closely between INTVCC and GND? This capacitor carries the MOSFET driver peak currents. 6) Keep the switching node (SW), top gate node (TG), bottom gate node (BG) and boost node (BOOST) away from sensitive small-signal nodes, especially from the voltage and current sensing feedback pins. All of these nodes have very large and fast moving signals and therefore should be kept on the “output side” (Pins 4,5,6 and 8) of the LTC3854 GND and occupy minimum PC trace area. 3854 F04 SENSE+ SENSE– CURRENT SENSE RESISTOR (RSENSE) PC Board Layout Debugging It is helpful to use a DC-50MHz current probe to monitor thecurrentintheinductorwhiletestingthecircuit.Monitor the output switching node (SW pin) to synchronize the oscilloscope to the internal oscillator and probe the actual output voltage as well. Check for proper performance over theoperatingvoltageandcurrentrangeexpectedintheap- plication.Thefrequencyofoperationshouldbemaintained over the input voltage range down to dropout. Thedutycyclepercentageshouldbemaintainedfromcycle tocycleinawell-designed,lownoisePCBimplementation. Variation in the duty cycle at a subharmonic rate can sug- gest noise pickup at the current or voltage sensing inputs or inadequate loop compensation. Overcompensation of the loop can be used to tame a poor PC layout if regula- tor bandwidth optimization is not required. A 1Ω to 10Ω boost resistor may help to improve noise immunity. This resistor is placed between the BOOST pin and the node formed by the cathode of the boost Schottky and the positive terminal of the boost capacitor. Investigate whether any problems exist only at higher out- put currents or only at higher input voltages. If problems coincide with high input voltages and low output currents, look for capacitive coupling between the BOOST, SW, TG, and possibly BG connections and the sensitive voltage and current pins. The capacitor placed across the current sensing pins needs to be placed immediately adjacent to the pins of the IC. This capacitor helps to minimize the effects of differential noise injection due to high frequency capacitive coupling. If problems are encountered with high current output loading at lower input voltages, look for inductive coupling between CIN, Schottky and the top MOSFET components to the sensitive current and voltage sensing traces. In addition, investigate common ground path voltage pickup between these components and the GND pin of the IC. |
类似零件编号 - LTC3854EMSE |
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类似说明 - LTC3854EMSE |
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