Datasheet LT1617, LT1617-1 (Analog Devices) - 6

LT1617/LT1617-1
U U W U APPLICATIO S I FOR ATIO
tor values are used. This overshoot can be beneficial as it
Diode Selection
helps increase the amount of available output current for For most LT1617 applications, the Motorola MBR0520 smaller inductor values. This will be the peak current seen surface mount Schottky diode (0.5A, 20V) is an ideal by the inductor (and the diode) during normal operation. choice. Schottky diodes, with their low forward voltage For designs using small inductance values (especially at drop and fast switching speed, are the best match for the input voltages greater than 5V), the current limit over- LT1617. For higher output voltage applications the 30V shoot can be quite high. Although it is internally current MBR0530 can be used. Many different manufacturers limited to 350mA, the power switch of the LT1617 can make equivalent parts, but make sure that the component handle larger currents without problem, but the overall is rated to handle at least 0.5A. For LT1617-1 applications, efficiency will suffer. Best results will be obtained when a Phillips BAT54 or a Central Semiconductor CMDSH-3 IPEAK is kept below 700mA for the LT1617 and below works well. 400mA for the LT1617-1.
Lowering Output Voltage Ripple Capacitor Selection
Using low ESR capacitors will help minimize the output Low ESR (Equivalent Series Resistance) capacitors should ripple voltage, but proper selection of the inductor and the be used at the output to minimize the output ripple voltage. output capacitor also plays a big role. The LT1617 pro- Multilayer ceramic capacitors are the best choice, as they vides energy to the load in bursts by ramping up the have a very low ESR and are available in very small inductor current, then delivering that current to the load. packages. Their small size makes them a good companion If too large of an inductor value or too small of a capacitor to the LT1617’s SOT-23 package. Solid tantalum capaci- value is used, the output ripple voltage will increase tors (like the AVX TPS, Sprague 593D families) or OS-CON because the capacitor will be slightly overcharged each capacitors can be used, but they will occupy more board burst cycle. To reduce the output ripple, increase the area than a ceramic and will have a larger ESR. Always use output capacitor value or add a 100pF feed-forward ca- a capacitor with a sufficient voltage rating. pacitor in the feedback network of the LT1617 (see the Ceramic capacitors also make a good choice for the input circuits in the Typical Applications section). Adding this decoupling capacitor, which should be placed as close as small, inexpensive 100pF capacitor will greatly reduce the possible to the LT1617. A 4.7µF input capacitor is suffi- output voltage ripple. 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 6