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LT1945 Datasheet(PDF) 6 Page - Linear Technology |
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LT1945 Datasheet(HTML) 6 Page - Linear Technology |
6 / 8 page LT1945 6 1945fa Current Limit Overshoot For the constant off-time control scheme of the LT1945, the power switch is turned off only after the 350mA 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: II VV L ns PEAK LIM IN MAX SAT =+ − ⎛ ⎝⎜ ⎞ ⎠⎟ () 100 Where VSAT=0.25V(switchsaturationvoltage).Thecurrent overshoot will be most evident for regulators with high input voltages and smaller inductor values. 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 normaloperation.Fordesignsusingsmallinductancevalues (especially at input voltages greater than 5V), the current limit overshoot can be quite high. Although it is internally current limited to 350mA, the power switch of the LT1945 can handle larger currents without problem, but the overall efficiency will suffer. Best results will be obtained when IPEAK is kept below 700mA for the LT1945. Capacitor Selection Low ESR (Equivalent Series Resistance) capacitors should be used at the output to minimize the output ripple voltage. X5R or X7R multilayer ceramic capacitors are the best choice, as they have a very low ESR and are available in very small packages. Y5V ceramics are not recommended. Their small size makes them a good companion to the LT1945’s MS10 package. Solid tantalum capacitors (like the AVX TPS, Sprague 593D families) or OS-CON capacitors can be used, but they will occupy more board area than a ceramic and will have a higher ESR. Always use a capacitor with a sufficient voltage rating. Ceramic capacitors also make a good choice for the input decoupling capacitor, which should be placed as close as possible to the LT1945. A 4.7μF input capacitor is sufficient for most applications. Table 2 shows a list of severalcapacitormanufacturers.Consultthemanufacturers for more detailed information and for their entire selection APPLICATIONS INFORMATION Inductor Selection—Inverting Regulator The formula below calculates the appropriate inductor value to be used for an inverting regulator using the LT1945 (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 (both inductors should be the same 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: L VV I t OUT D LIM OFF = + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 2 where VD = 0.4V (Schottky diode voltage), ILIM = 350mA and tOFF = 400ns. For higher output voltages, the formula above will give large inductance values. For a 2V to 20V converter (typical LCD bias application), a 47μH inductor is called for with the above equation, but a 10μH or 22μH inductor could be used without excessive reduction in maximum output current. Inductor Selection—Inverting Charge Pump Regulator For the inverting regulator, the voltage seen by the internal power switch is equal to the sum of the absolute value of the input and output voltages, so that generating high output voltages from a high input voltage source will often exceed the 36V maximum switch rating. For instance, a 12V to –30V converter using the inverting topology would generate 42V on the SW pin, exceeding its maximum rating. For this ap- plication, an inverting charge pump is the best topology. The formula below calculates the approximate inductor value to be used for an inverting charge pump regulator using the LT1945. As for the boost inductor selection, a larger or smaller value can be used. For designs with varying VIN such as battery powered applications, use the minimum VIN value in the equation below. L VV V I t OUT IN MIN D LIM OFF = −+ () |
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