Datenblatt-Suchmaschine für elektronische Bauteile |
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ML0308A Datenblatt(PDF) 5 Page - Minilogic Device Corporation Limited |
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ML0308A Datenblatt(HTML) 5 Page - Minilogic Device Corporation Limited |
5 / 9 page ML0308 Preliminary ______________________________________________________________________________________________________ 5/9 Rev. B, July 2005 APPLICATION INFORMATION Choosing an Inductor Several recommended inductors that work well with the ML0308. Consult each manufacturer for more detailed information and for their entire selection of related parts. Many different sizes and shapes are available. Use the equations and recommendations in the next few sections to find the correct inductance value for your design. Inductor Selection—Boost Regulator The formula below calculates the appropriate inductor value to be used for a boost regulator using the ML0308 (or at least provides a good starting point). This value provides a good tradeoff in inductor size and system performance. Pick a standard inductor close to this value. A larger value can be used to slightly increase the available output current, but limit it to around twice the value calculated below, as too large of an inductance will increase the output voltage ripple without providing much additional output current. A smaller value can be used (especially for systems with output voltages greater than 12V) to give a smaller physical size. Inductance can be calculated as: OFF LIMIT D MIN IN OUT t I V V V L + − = ) ( where VD = 0.4V (Schottky diode voltage), ILIMIT = 180mA, and tOFF = 250ns; for designs with varying VIN such as battery powered applications, use the minimum VIN value in the above equation. For most systems with output voltages below 7V, a 4.7uH inductor is the best choice, even though the equation above might specify a smaller value. This is due to the inductor current overshoot that occurs when very small inductor values are used (see Current Limit Overshoot section). For higher output voltages, the formula above will give large inductance values. For a 3V to 16V converter (typical LCD Bias application), a 22uH inductor is called for with the above equation, but a 10uH inductor could be used without excessive reduction in maximum output current. Current Limit Overshoot For a fixed duty ratio PFM control scheme of the ML0308, the power switch is turned off after 3/4 of a cycle or the 180mA current limit is reached. There is a 100ns delay between the time when the current limit is reached and when the switch actually turns off. During this delay, the inductor current exceeds the current limit by a small amount. The peak inductor current can be calculated by d DSSAT MAX IN LIMIT PEAK t L V V I I − + = ) ( Where VDSSAT = 0.25V (switch saturation voltage) and td = 100ns. The current overshoot will be most evident for systems with high input voltages and for systems where smaller inductor values are used. This overshoot can be beneficial as it helps increase the amount of available output current for smaller inductor values. This will be the peak current seen by the inductor (and the diode) during normal operation. For designs using small inductance values (especially at input voltages greater than 5V), the current limit overshoot can be quite high. Although it is internally current limited to 180mA, the power switch of the ML0308 can handle larger currents without problem, but the overall efficiency will suffer. |
Ähnliche Teilenummer - ML0308A |
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Ähnliche Beschreibung - ML0308A |
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