Table 2. Design Parameters

9.2.2.1 Inductor Selection

DC bias current characteristics of inductors must be considered. Different manufacturers follow different saturation current rating specifications, so attention must be given to details. DC bias curves should be requested from the manufacturer as part of the inductor selection process.

Minimum value of inductance to specify good performance is 0.5 µH at 1.5 A (I_LIM typical) bias current over the ambient temp range. DC resistance of the inductor must be less than 0.1 Ω for good efficiency at high-current condition. The inductor AC loss (resistance) also affects conversion efficiency. Higher Q factor at switching frequency usually gives better efficiency at light load to middle load.

Table 3 lists suggested inductors and suppliers.

9.2.2.2 Input Capacitor Selection

A ceramic input capacitor of 4.7 µF, 6.3 V/10 V is sufficient for most applications. Place the input capacitor as close as possible to the VIN pin and GND pin of the device. A larger value or higher voltage rating may be used to improve input voltage filtering. Use X7R, X5R or B types; do not use Y5V or F. DC bias characteristics of ceramic capacitors must be considered when selecting case sizes like 0402. Minimum input capacitance to ensure good performance is 2.2 µF at maximum input voltage DC bias including tolerances and over ambient temperature range.

The input filter capacitor supplies current to the PFET (high-side) switch in the first half of each cycle and reduces voltage ripple imposed on the input power source. A ceramic capacitor's low ESR provides the best noise filtering of the input voltage spikes due to this rapidly changing current. Select an input filter capacitor with sufficient ripple current rating. The input current ripple can be calculated as:

Equation 1.

9.2.2.3 Output Capacitor Selection

Use a 4.7-μF, 6.3-V ceramic capacitor, X7R, X5R or B types; do not use Y5V or F. DC bias voltage characteristics of ceramic capacitors must be considered. DC bias characteristics vary from manufacturer to manufacturer, and DC bias curves should be requested from the manufacturer as part of the capacitor selection process. The output filter capacitor smooths out current flow from the inductor to the load, helps maintain a steady output voltage during transient load changes, and reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and sufficiently low equivalent series resistance (ESR) to perform these functions. Minimum output capacitance to specify good performance is 2.2 µF at the output voltage DC bias including tolerances and over ambient temperature range.

The output voltage ripple is caused by the charging and discharging of the output capacitor and also due to its R_ESR and can be calculated as:

Voltage peak-to-peak ripple due to capacitance is shown in Equation 2:

Equation 2.

Voltage peak-to-peak ripple due to ESR Equation 3:

Because these two components are out of phase the RMS value can be used to get an approximate value of peak-to-peak ripple.

Voltage peak-to-peak ripple, root mean squared equals:

Equation 4.

Note that the output voltage ripple is dependent on the current ripple and the ESR of the output capacitor (R_ESR). The R_ESR is frequency dependent (as well as temperature dependent); make sure the value used for calculations is at the switching frequency of the part.

Table 4 lists suggested capacitors and suppliers.