Datasheet LT1945 (Analog Devices) - 6

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