LT1944 UUWUAPPLICATIO S I FOR ATIOChoosing an Inductor voltages below 7V, a 4.7µH inductor is the best choice, even though the equation above might specify a smaller Several recommended inductors that work well with the value. This is due to the inductor current overshoot that LT1944 are listed in Table 1, although there are many other occurs when very small inductor values are used (see manufacturers and devices that can be used. Consult each Current Limit Overshoot section). manufacturer for more detailed information and for their entire selection of related parts. Many different sizes and For higher output voltages, the formula above will give shapes are available. Use the equations and recommenda- large inductance values. For a 2V to 20V converter (typical tions in the next few sections to find the correct inductance LCD Bias application), a 21µH inductor is called for with value for your design. the above equation, but a 10µH inductor could be used Table 1. Recommended Inductors without excessive reduction in maximum output current. PARTVALUE ( µ H)MAX DCR ( Ω )VENDORInductor Selection—SEPIC Regulator LQH3C4R7 4.7 0.26 Murata LQH3C100 10 0.30 (714) 852-2001 The formula below calculates the approximate inductor LQH3C220 22 0.92 www.murata.com value to be used for a SEPIC regulator using the LT1944. CD43-4R7 4.7 0.11 Sumida As for the boost inductor selection, a larger or smaller CD43-100 10 0.18 (847) 956-0666 value can be used. CDRH4D18-4R7 4.7 0.16 www.sumida.com CDRH4D18-100 10 0.20 DO1608-472 4.7 0.09 Coilcraft V V L OUT D = + 2 t DO1608-103 10 0.16 (847) 639-6400 OFF I DO1608-223 22 0.37 www.coilcraft.com LIM Inductor Selection—Boost RegulatorCurrent Limit Overshoot The formula below calculates the appropriate inductor For the constant off-time control scheme of the LT1944, value to be used for a boost regulator using the LT1944 (or the power switch is turned off only after the 350mA current at least provides a good starting point). This value pro- limit is reached. There is a 100ns delay between the time vides a good tradeoff in inductor size and system perfor- when the current limit is reached and when the switch mance. Pick a standard inductor close to this value. A actually turns off. During this delay, the inductor current larger value can be used to slightly increase the available exceeds the current limit by a small amount. The peak output current, but limit it to around twice the value inductor current can be calculated by: calculated below, as too large of an inductance will in- crease the output voltage ripple without providing much V ( ) − V IN MAX SAT = + additional output current. A smaller value can be used I I ns PEAK LIM 100 L (especially for systems with output voltages greater than 12V) to give a smaller physical size. Inductance can be Where VSAT = 0.25V (switch saturation voltage). The calculated as: current overshoot will be most evident for systems with high input voltages and for systems where smaller induc- V − V + V OUT IN MIN D ( ) tor values are used. This overshoot can be beneficial as it L = tOFF helps increase the amount of available output current for ILIM smaller inductor values. This will be the peak current seen where VD = 0.4V (Schottky diode voltage), ILIM = 350mA by the inductor (and the diode) during normal operation. and tOFF = 400ns; for designs with varying VIN such as For designs using small inductance values (especially at battery powered applications, use the minimum VIN value input voltages greater than 5V), the current limit over- in the above equation. For most systems with output shoot can be quite high. Although it is internally current 5
