Moteur de recherche de fiches techniques de composants électroniques |
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ML4761CS Fiches technique(PDF) 6 Page - Micro Linear Corporation |
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ML4761CS Fiches technique(HTML) 6 Page - Micro Linear Corporation |
6 / 8 page 6 ML4761 Figure 5. Typical Efficiency as a Function of VIN. 100 1.0 3.0 2.0 200 300 400 500 0 L = 10µH L = 15µH L = 27µH L = 56µH VIN (V) ML4761(VOUT = 3.3V) 100 1.0 3.0 4.0 2.0 200 300 400 500 0 L = 10µH L = 15µH L = 27µH L = 56µH ML4761(VOUT = 5.0V) VIN (V) 70% 13 2 75% 80% 85% 90% 65% 0 L = 10µH L = 15µH L = 27µH L = 56µH VIN ML4761-(VOUT = 3.3V) 95% 75% 1.0 3.0 4.0 2.0 80% 85% 90% 95% 70% 0 L = 10µH L = 15µH L = 27µH L = 57µH VIN (V) ML4761-(VOUT = 5.0V) Figure 5 shows efficiency under the conditions used to create Figure 4. It can be seen that efficiency is mostly independent of input voltage and is closely related to inductor value. This illustrates the need to keep the inductor value as high as possible to attain peak system efficiency. As the inductor value goes down to 10µH, the efficiency drops to between 70% and 75%. With 56µH, the efficiency exceeds 90% and there is little room for improvement. At values greater than 100µH, the operation of the synchronous rectifier becomes unreliable because the inductor current is so small that it is difficult for the control circuitry to detect. After the appropriate inductor value is chosen, it is necessary to find the minimum inductor current rating required. Peak inductor current is determined from the following formula: I TV L L PEAK ON MAX IN MAX MIN () () () = × (3) When comparing various inductors, it is important to keep in mind that suppliers use different criteria to determine their ratings. Many use a conservative current level, where inductance has dropped to 90% of its normal level. In any case, it is a good idea to try inductors of various current ratings with the ML4761 to determine which inductor is the best choice. Check efficiency and maximum output current, and if a current probe is available, look at the inductor current to see if it looks like the waveform shown in Figure 3. For additional information, see Application Note 29. Suitable inductors can be purchased from the following suppliers: Coilcraft (708) 639-6400 Coiltronics (407) 241-7876 Dale (605) 665-9301 Sumida (708) 956-0666 Figure 4. Output Current vs. Input Voltage. |
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