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LT3503 Datasheet(PDF) 11 Page - Linear Technology |
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LT3503 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT3503 3503f APPLICATIO S I FOR ATIO Ceramic capacitors have very low equivalent series resis- tance (ESR) and provide the best ripple performance. A good value is: COUT = 24/VOUT where COUT is in µF. Use X5R or X7R types and keep in mind that a ceramic capacitor biased with VOUT will have less than its nominal capacitance. This choice will provide low output ripple and good transient response. Transient performance can be improved with a high value capacitor, but a phase lead capacitor across the feedback resistor R1 may be required to get the full benefit (see the Compen- sation section). Using a small output capacitor results in an increased loop crossover frequency and increased sensitivity to noise. A 22pF capacitor connected between VOUT and the FB pin is required to filter noise at the FB pin and ensure stability. High performance electrolytic capacitors can be used for the output capacitor. Low ESR is important, so choose one that is intended for use in switching regulators. The ESR should be specified by the supplier and should be 0.1 Ω or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, be- cause the capacitor must be large to achieve low ESR. Table 2 lists several capacitor vendors. Figure 5 shows the transient response of the LT3503 with a few output capacitor choices. The output is 3.3V. The load current is stepped from 0.5A to 1.1A and back to 0.5A, and the oscilloscope traces show the output voltage. The upper photo shows the recommended value. The second photo shows the improved response (less voltage drop) resulting from a phase lead capacitor. The last photo shows the response to a high performance electrolytic capacitor. Transient performance is improved due to the large output capacitance. BOOST Pin Considerations Capacitor C3 and diode D2 are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.1 µF capacitor and fast switching diode (such as the 1N4148 or 1N914) will work well. Figure 6 shows two ways to arrange the boost circuit. The BOOST pin must be at least 2.3V above the SW pin for best efficiency. For outputs of 3.3V and above, the standard circuit (Figure 6a) is best. For outputs between 3V and 3.3V, use a 0.22 µF capacitor. For outputs between 2.5V and 3V, use a 0.47 µF capacitor and a small Schottky diode (such as the BAT-54). For lower output voltages tie a Schottky diode to the input (Figure 6b). The circuit in Figure 6a is more efficient because the BOOST pin current comes from a lower voltage source. You must also be sure that the maximum voltage rating of the BOOST pin is not exceeded. Table 2. Capacitor Vendors Vendor Phone URL Part Series Comments Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, EEF Series Tantalum Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495 Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, POSCAP Tantalum Murata (404) 436-1300 www.murata.com Ceramic AVX www.avxcorp.com Ceramic, Tantalum TPS Series Taiyo Yuden (864) 963-6300 www.taiyo-yuden.com Ceramic |
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