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LT1944EMS Datasheet(PDF) 6 Page - Linear Technology |
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LT1944EMS Datasheet(HTML) 6 Page - Linear Technology |
6 / 8 page 6 LT1944 APPLICATIO S I FOR ATIO limited to 350mA, the power switch of the LT1944 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 LT1944. Capacitor Selection Low ESR (Equivalent Series Resistance) capacitors should be used at the output to minimize the output ripple voltage. Multilayer ceramic capacitors are the best choice, as they have a very low ESR and are available in very small packages. Their small size makes them a good companion to the LT1944’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 LT1944. A 4.7 µF input capacitor is suffi- cient for most applications. Table 2 shows a list of several capacitor manufacturers. Consult the manufacturers for more detailed information and for their entire selection of related parts. Table 2. Recommended Capacitors CAPACITOR TYPE VENDOR Ceramic Taiyo Yuden (408) 573-4150 www.t-yuden.com Ceramic AVX (803) 448-9411 www.avxcorp.com Ceramic Murata (714) 852-2001 www.murata.com Setting the Output Voltage Set the output voltage for each switching regulator by choosing the appropriate values for feedback resistors R1 and R2 (see Figure 1). RR V V OUT 12 123 1 =− . Diode Selection For most LT1944 applications, the Motorola MBR0520 surface mount Schottky diode (0.5A, 20V) is an ideal choice. Schottky diodes, with their low forward voltage drop and fast switching speed, are the best match for the LT1944. For higher output voltage applications the 30V MBR0530 or 40V MBR0540 can be used. Many different manufacturers make equivalent parts, but make sure that the component is rated to handle at least 0.35A. Lowering Output Voltage Ripple Using low ESR capacitors will help minimize the output ripple voltage, but proper selection of the inductor and the output capacitor also plays a big role. The LT1944 pro- vides energy to the load in bursts by ramping up the inductor current, then delivering that current to the load. If too large of an inductor value or too small of a capacitor value is used, the output ripple voltage will increase because the capacitor will be slightly overcharged each burst cycle. To reduce the output ripple, increase the output capacitor value or add a 4.7pF feed-forward capaci- tor in the feedback network of the LT1944 (see the circuits in the Typical Applications section). Adding this small, inexpensive 4.7pF capacitor will greatly reduce the output voltage ripple. |
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