9.2.2.3.2.2.1 Suggested Inductors and Their Suppliers

The designer should choose the inductors that best match the system requirements. A very wide range of inductors are available as regarding physical size, height, maximum current (thermally limited, and inductance loss limited), series resistance, maximum operating frequency, losses, etc. In general, smaller physical size inductors will have higher series resistance (DCR) and implicitly lower overall efficiency is achieved. Very low-profile inductors may have even higher series resistance. The designer should try to find the best compromise between system performance and cost.

9.2.2.3.2.3.1 Output Capacitor Selection

The output capacitor of a switching converter absorbs the AC ripple current from the inductor and provides the initial response to a load transient. The ripple voltage at the output of the converter is the product of the ripple current flowing through the output capacitor and the impedance of the capacitor. The impedance of the capacitor can be dominated by capacitive, resistive, or inductive elements within the capacitor, depending on the frequency of the ripple current. Ceramic capacitors have very low ESR and remain capacitive up to high frequencies. Their inductive component can usually be neglected at the frequency ranges at which the switcher operates.

The output-filter capacitor smooths out the current flow from the inductor to the load and helps maintain a steady output voltage during transient load changes. It also reduces output voltage ripple. These capacitors must be selected with sufficient capacitance and low enough ESR to perform these functions.

Note that the output voltage ripple increases with the inductor current ripple and the Equivalent Series Resistance of the output capacitor (ESR_COUT). Also note that the actual value of the capacitor’s ESR_COUT is frequency and temperature dependent, as specified by its manufacturer. The ESR should be calculated at the applicable switching frequency and ambient temperature.

Equation 3.

Output ripple can be estimated from the vector sum of the reactive (capacitance) voltage component and the real (ESR) voltage component of the output capacitor where:

Equation 4.

where:

Equation 5.

where

• V_{OUT-RIPPLE-PP}: estimated output ripple,
• V_ROUT: estimated real output ripple,
• V_COUT: estimated reactive output ripple.

The device is designed to be used with ceramic capacitors on the outputs of the buck regulators. The recommended dielectric type of these capacitors is X5R, X7R, or of comparable material to maintain proper tolerances over voltage and temperature. The recommended value for the output capacitors is 22 μF, 6.3V with an ESR of 2 mΩ or less. The output capacitors need to be mounted as close as possible to the output/ground pins of the device.

9.2.2.3.2.3.2 Input Capacitor Selection

There are 3 buck regulators in the LM10506 device. Each of these buck regulators has its own input capacitor which should be located as close as possible to their corresponding SWx_VIN and SWx_GND pins, where x designates Buck 1, 2, or 3. The 3 buck regulators operate at 120° out of phase, which means that they switch on at equally spaced intervals, in order to reduce the input power rail ripple. It is recommended to connect all the supply/ground pins of the buck regulators, SWx_VIN to two solid internal planes located under the device. In this way, the 3 input capacitors work together and further reduce the input current ripple. A larger tantalum capacitor can also be located in the proximity of the device.

The input capacitor supplies the AC switching current drawn from the switching action of the internal power FETs. The input current of a buck converter is discontinuous, so the ripple current supplied by the input capacitor is large. The input capacitor must be rated to handle both the RMS current and the dissipated power.

The input capacitor must be rated to handle this current:

Equation 6.

The power dissipated in the input capacitor is given by:

Equation 7.

The device is designed to be used with ceramic capacitors on the inputs of the buck regulators. The recommended dielectric type of these capacitors is X5R, X7R, or of comparable material to maintain proper tolerances over voltage and temperature. The minimum recommended value for the input capacitor is 10 µF with an ESR of 10 mΩ or less. The input capacitors need to be mounted as close as possible to the power/ground input pins of the device.

The input power source supplies the average current continuously. During the PFET switch on-time, however, the demanded di/dt is higher than can be typically supplied by the input power source. This delta is supplied by the input capacitor.

A simplified “worst case” assumption is that all of the PFET current is supplied by the input capacitor. This will result in conservative estimates of input ripple voltage and capacitor RMS current.

Input ripple voltage is estimated as follows:

Equation 8.

where

• V_PPIN: estimated peak-to-peak input ripple voltage,
• I_OUT: Output Current
• C_IN: Input capacitor value
• ESR_CIN: input capacitor ESR.

This capacitor is exposed to significant RMS current, so it is important to select a capacitor with an adequate RMS current rating. Capacitor RMS current estimated as follows:

Equation 9.

where

• I_RMSCIN: estimated input capacitor RMS current.