8.2.2.1.1 Output Capacitor, C_OUT

The output capacitor C_OUT directly affects the magnitude of the output ripple voltage. In general, the higher the value of C_OUT, the lower the output ripple magnitude. Multilayer ceramic capacitors with low ESR are the best choice. At the lighter loads, the low ESR ceramics offer a much lower V_OUT ripple than the higher ESR tantalums of the same value. At the higher loads, the ceramics offer a slightly lower V_OUT ripple magnitude than the tantalums of the same value. However, the dv/dt of the V_OUT ripple with the ceramics is much lower than the tantalums under all load conditions. Capacitor voltage rating must be sufficient, TI recommends 10 V or greater.

Some ceramic capacitors, especially those in small packages, exhibit a strong capacitance reduction with the increased applied voltage. The capacitance value can fall to below half of the nominal capacitance. Output capacitance that is too low increase the noise, and it can make the boost converter unstable.

8.2.2.1.2 List Of Recommended External Components

8.2.2.1.3 Input Capacitor, C_IN

The input capacitor C_IN directly affects the magnitude of the input ripple voltage and to a lesser degree the V_OUT ripple. A higher value C_IN gives a lower V_IN ripple. Capacitor voltage rating must be sufficient, TI recommends 10 V or greater.

8.2.2.1.4 Output Diode, D₁

A Schottky diode must be used for the output diode. To maintain high efficiency the average current rating of the Schottky diode must be larger than the peak inductor current (1 A). Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing efficiency in portable applications. Choose a reverse breakdown of the Schottky diode larger than the output voltage. Do not use ordinary rectifier diodes, since slow switching speeds and long recovery times cause the efficiency and the load regulation to suffer.

8.2.2.1.5 Inductor, L

The high switching frequency of the LP55281 device enables the use of the small surface mount inductor. A 4.7-µH shielded inductor is suggested for 2-MHz operation, use 10 µH at 1 MHz. The inductor should have a saturation current rating higher than the peak current it will experience during circuit operation (approximately 1 A). Less than 300-mΩ ESR is suggested for high efficiency. Open core inductors cause flux linkage with circuit components and interfere with the normal operation of the circuit. This should be avoided. For high efficiency, choose an inductor with a high frequency core material such as ferrite to reduce the core losses. To minimize radiated noise, use a toroid, pot core or shielded core inductor. TI recommends inductors LPS3015 and LPS4012 from Coilcraft and VLF4012 from TDK.