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LM2678S-3.3 Fiches technique(PDF) 11 Page - National Semiconductor (TI) |
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LM2678S-3.3 Fiches technique(HTML) 11 Page - National Semiconductor (TI) |
11 / 23 page Application Hints (Continued) In some cases multiple capacitors are required either to re- duce the ESR of the output capacitor, to minimize output ripple (a ripple voltage of 1% of Vout or less is the assumed performance condition), or to increase the output capaci- tance to reduce the closed loop unity gain bandwidth (to less than 40KHz). When parallel combinations of capacitors are required it has been assumed that each capacitor is the ex- act same part type. The RMS current and working voltage (WV) ratings of the output capacitor are also important considerations. In a typi- cal 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 out- put 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 tem- perature rating. Careful inspection of the manufacturer’s specification for de-rating of working voltage with tempera- ture 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 capaci- tance. The voltage rating should also be selected to be 1.3 times the maximum input voltage. Depending on the unregu- lated 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 LM2678. 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 out- put or other parts of the circuitry. It may be necessary in some designs to add a small valued (0.1µF to 0.47µF) ce- ramic type capacitor in parallel with the input capacitor to prevent or minimize any ringing. CATCH DIODE When the power switch in the LM2678 turns OFF, the current through the inductor continues to flow. The path for this cur- rent is through the diode connected between the switch out- put 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 cur- rent 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 0.01µF/50V ceramic capacitor. SIMPLE DESIGN PROCEDURE Using the nomographs and tables in this data sheet (or use the available design software at http://www.national.com) a complete step-down regulator can be designed in a few simple steps. Step 1: Define the power supply operating conditions: Required output voltage Maximum DC input voltage Maximum output load current Step 2: Set the output voltage by selecting a fixed output LM2678 (3.3V, 5V or 12V applications) or determine the re- quired feedback resistors for use with the adjustable LM2678−ADJ Step 3: Determine the inductor required by using one of the four nomographs, Figure 3 through Figure 6. Table 1 pro- vides a specific manufacturer and part number for the induc- tor. Step 4: Using Table 3 (fixed output voltage) or Table 6 (ad- justable output voltage), determine the output capacitance required for stable operation. Table 2 provides the specific capacitor type from the manufacturer of choice. Step 5: Determine an input capacitor from Table 4 for fixed output voltage applications. Use Table 2 to find the specific capacitor type. For adjustable output circuits select a capaci- tor from Table 2 with a sufficient working voltage (WV) rating greater than Vin max, and an rms current rating greater than one-half the maximum load current (2 or more capacitors in parallel may be required). Step 6: Select a diode from Table 5. The current rating of the diode must be greater than I load max and the Reverse Volt- age rating must be greater than Vin max. Step 7: Include a 0.01µF/50V capacitor for Cboost in the de- sign. FIXED OUTPUT VOLTAGE DESIGN EXAMPLE A system logic power supply bus of 3.3V is to be generated from a wall adapter which provides an unregulated DC volt- age of 13V to 16V. The maximum load current is 4A. Through-hole components are preferred. Step 1: Operating conditions are: Vout = 3.3V Vin max = 16V Iload max = 4A Step 2: Select an LM2678T-3.3. The output voltage will have a tolerance of ±2% at room temperature and ±3% over the full operating temperature range. Step 3: Use the nomograph for the 3.3V device , Figure 3. The intersection of the 16V horizontal line (V in max) and the 4A vertical line (I load max) indicates that L46, a 15µH induc- tor, is required. From Table 1, L46 in a through-hole component is available from Renco with part number RL-1283-15-43. www.national.com 11 |
Numéro de pièce similaire - LM2678S-3.3 |
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Description similaire - LM2678S-3.3 |
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