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LT3493IDCB Datasheet(PDF) 11 Page - Linear Technology |
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LT3493IDCB Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT3493 11 3493fb APPLICATIONS INFORMATION capacitor must be large to achieve low ESR. Table 2 lists several capacitor vendors. Figure 4 shows the transient response of the LT3493 with several output capacitor choices. The output is 3.3V. The load current is stepped from 250mA to 1A and back to 250mA, and the oscilloscope traces show the output volt- age. The upper photo shows the recommended value. The second photo shows the improved response (less voltage drop) resulting from a larger output capacitor and 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 0.1μF capacitor and fast switching diode (such as the 1N4148 or 1N914) will work well. Figure 5 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 5a) is best. For outputs between 3V and 3.3V, use a 0.22μF capacitor. For outputs between 2.5V and 3V, use a 0.47μF capacitor and a small Schottky diode (such as the BAT- 54). For lower output voltages the boost diode can be tied to the input (Figure 5b). The circuit in Figure 5a 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. The minimum operating voltage of an LT3493 applica- tion is limited by the undervoltage lockout (3.6V) and by the maximum duty cycle as outlined above. For proper start-up, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT3493 is turned on with its SHDN pin when the output is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on the input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 6 shows a plot of minimum load to start and to run as a function of input voltage. In many cases the discharged output capacitor will present a load to the switcher which will allow it to start. The plots show the worst-case situation where VIN is ramping verly slowly. For lower start-up voltage, the boost diode can be tied to VIN; however this restricts the input range to one-half of the absolute maximum rating of the BOOST pin. Table 2. Capacitor Vendors VENDOR PHONE URL PART SERIES COMMENTS Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, Tantalum EEF Series Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495 Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, Tantalum POSCAP 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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