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ML4790 Scheda tecnica(PDF) 6 Page - Micro Linear Corporation |
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ML4790 Scheda tecnica(HTML) 6 Page - Micro Linear Corporation |
6 / 10 page 6 ML4790 Figure 7 shows efficiency under the conditions used to create Figure 6. 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 10 µH, the efficiency drops to between 70% and 75%. With 47 µH, the efficiency reaches approximately 90% and there is little room for improvement. At values greater than 47 µH, the operation of the synchronous rectifier becomes unreliable at low input voltages because the inductor current is so small that it is difficult for the control circuitry to detect as shown for the 5.5V output. 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 () () () = × (2) It is important to note that for reliable operation, make sure that IL(PEAK) does not exceed the 1A maximum switch current rating. In the two cell application previously described, a maximum input voltage of 3V would give a peak current of 880mA. 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 ML4790 to determine which inductor is the best choice. Check Figure 6. Output Current versus Input Voltage. 200 180 160 140 120 100 80 60 40 20 0 1.0 2.0 3.0 4.0 5.0 VIN (V) VOUT = 5.5V L = 10 µH L = 22 µH L = 47 µH 35 30 25 20 15 10 5 0 1.0 1.2 1.4 1.6 1.8 2.0 VOUT = 2.5V VIN (V) L = 10 µH L = 22 µH L = 47 µH 140 120 100 80 60 40 20 0 1.0 1.5 2.0 2.5 3.0 VIN (V) VOUT = 3.5V L = 10 µH L = 22 µH L = 47 µH 180 160 140 120 100 80 60 40 20 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 L = 10 µH L = 22 µH L = 47 µH VOUT = 4.5V VIN (V) |
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