Table 2. Design Parameters

8.2.2.1 Inductor Selection

There are two main considerations when choosing an inductor; the inductor must not saturate, and the inductor current ripple is small enough to achieve the desired output voltage ripple. Different manufacturers follow different saturation current rating specifications, so attention must be given to details. Saturation current ratings are typically specified at 25°C so ratings over the ambient temperature of application should be requested from manufacturer.

Minimum value of inductance to ensure good performance is 0.3 µH at bias current (I_LIM (typical)) over the ambient temperature range. Shielded inductors radiate less noise and are preferred. There are two methods to choose the inductor saturation current rating:

8.2.2.1.1 Method 1

The saturation current must be greater than the sum of the maximum load current and the worst-case average-to-peak inductor current. This can be written as:

Equation 2.

where

• I_RIPPLE: average-to-peak inductor current
• I_{OUT_MAX}: maximum load current (750 mA)
• V_IN: maximum input voltage in application
• L minimum inductor value including worst-case tolerances (30% drop can be considered for Method 1)
• F: minimum switching frequency (5.7 MHz)
• V_OUT: output voltage

The resistance of the inductor must be less than approximately 0.1 Ω for good efficiency. Table 3 lists suggested inductors and suppliers.

8.2.2.2 Capacitor Selection

The LM3242 is designed for use with ceramic capacitors for its input and output filters. Use a 10-µF ceramic capacitor for input and a sum total of 4.7-µF ceramic capacitors for the output. They must maintain at least 50% capacitance at DC bias and temperature conditions. Ceramic capacitors types such as X5R, X7R, and B are recommended for both filters. These provide an optimal balance between small size, cost, reliability and performance for cell phones and similar applications. Table 4 lists some suggested part numbers and suppliers. DC bias characteristics of the capacitors must be considered when selecting the voltage rating and case size of the capacitor. If it is necessary to choose a 0603 (1608) size capacitor for V_IN and 0402 (1005) size capacitor for V_OUT, the operation of the LM3242 must be carefully evaluated on the system board. Use of a 2.2-µF capacitor in conjunction with multiple 0.47 µF or 1 µF capacitors in parallel may also be considered when connecting to power amplifier devices that require local decoupling.

The input filter capacitor supplies AC current drawn by the PFET switch of the LM3242 in the first part of each cycle and reduces the voltage ripple imposed on the input power source. The output filter capacitor absorbs the AC inductor current, 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. The ESR of the filter capacitors is generally a major factor in voltage ripple.