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LM2673SX-ADJ データシート(PDF) 11 Page - Texas Instruments |
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LM2673SX-ADJ データシート(HTML) 11 Page - Texas Instruments |
11 / 35 page LM2673 www.ti.com SNVS030N – APRIL 2000 – REVISED APRIL 2013 The RMS current and working voltage (WV) ratings of the output capacitor are also important considerations. In a typical step-down switching regulator, the inductor ripple current (set to be no more than 30% of the maximum load current by the inductor selection) is the current that flows through the output capacitor. The capacitor RMS current rating must be greater than this ripple current. The voltage rating of the output capacitor should be greater than 1.3 times the maximum output voltage of the power supply. If operation of the system at elevated temperatures is required, the capacitor voltage rating may be de-rated to less than the nominal room temperature rating. Careful inspection of the manufacturer's specification for de-rating of working voltage with temperature is important. INPUT CAPACITOR Fast changing currents in high current switching regulators place a significant dynamic load on the unregulated power source. An input capacitor helps to provide additional current to the power supply as well as smooth out input voltage variations. Like the output capacitor, the key specifications for the input capacitor are RMS current rating and working voltage. The RMS current flowing through the input capacitor is equal to one-half of the maximum dc load current so the capacitor should be rated to handle this. Paralleling multiple capacitors proportionally increases the current rating of the total capacitance. The voltage rating should also be selected to be 1.3 times the maximum input voltage. Depending on the unregulated input power source, under light load conditions the maximum input voltage could be significantly higher than normal operation and should be considered when selecting an input capacitor. The input capacitor should be placed very close to the input pin of the LM2673. Due to relative high current operation with fast transient changes, the series inductance of input connecting wires or PCB traces can create ringing signals at the input terminal which could possibly propagate to the output or other parts of the circuitry. It may be necessary in some designs to add a small valued (0.1 μF to 0.47μF) ceramic type capacitor in parallel with the input capacitor to prevent or minimize any ringing. CATCH DIODE When the power switch in the LM2673 turns OFF, the current through the inductor continues to flow. The path for this current is through the diode connected between the switch output and ground. This forward biased diode clamps the switch output to a voltage less than ground. This negative voltage must be greater than −1V so a low voltage drop (particularly at high current levels) Schottky diode is recommended. Total efficiency of the entire power supply is significantly impacted by the power lost in the output catch diode. The average current through the catch diode is dependent on the switch duty cycle (D) and is equal to the load current times (1-D). Use of a diode rated for much higher current than is required by the actual application helps to minimize the voltage drop and power loss in the diode. During the switch ON time the diode will be reversed biased by the input voltage. The reverse voltage rating of the diode should be at least 1.3 times greater than the maximum input voltage. BOOST CAPACITOR The boost capacitor creates a voltage used to overdrive the gate of the internal power MOSFET. This improves efficiency by minimizing the on resistance of the switch and associated power loss. For all applications it is recommended to use a 0.01 μF/50V ceramic capacitor. RADJ, ADJUSTABLE CURRENT LIMIT A key feature of the LM2673 is the ability to control the peak switch current. Without this feature the peak switch current would be internally set to 5A or higher to accommodate 3A load current designs. This requires that both the inductor (which could saturate with excessively high currents) and the catch diode be able to safely handle up to 5A which would be conducted under load fault conditions. If an application only requires a load current of 2A or so the peak switch current can be set to a limit just over the maximum load current with the addition of a single programming resistor. This allows the use of less powerful and more cost effective inductors and diodes. Copyright © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2673 |
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