8.2.2.1.2.1 Recommended Method for Inductor Selection

The best way to ensure the inductor does not saturate is to choose an inductor that has saturation current rating greater than the maximum device current limit, as specified in Electrical Characteristics – General. In this case, the device prevents inductor saturation by going into current limit before the saturation level is reached.

8.2.2.1.2.2 Alternate Method for Inductor Selection

If the recommended approach cannot be used, care must be taken to ensure that the saturation current is greater than the peak inductor current as calculated in Equation 3.

Equation 3.

where

• I_SAT is the inductor saturation current at operating temperature
• I_LPEAK is the peak inductor current during worst case conditions
• I_OUTMAX is the maximum average inductor current
• I_RIPPLE is the peak-to-peak inductor current
• V_OUT is the output voltage
• V_IN is the input voltage
• L is the inductor value in Henries at I_OUTMAX
• F is the switching frequency, Hertz
• D is the estimated duty factor
• EFF is the estimated power supply efficiency

I_SAT may not be exceeded during any operation, including transients, start-up, high temperature, worst-case conditions, and so forth.

8.2.2.1.3.1 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.

Figure 27. Basic Schematic of Feedback Components

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 must be calculated at the applicable switching frequency and ambient temperature with Equation 4.

Equation 4.

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 with Equation 5 and Equation 6.

Equation 5.

where

Equation 6.

where

• V_{OUT-RIPPLE-PP} is the estimated output ripple
• V_ROUT is the estimated real output ripple
• V_COUT is the 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, 6.3 V with an ESR of 2 mΩ or less. The output capacitors need to be mounted as close as possible to the output or ground pins of the device.

8.2.2.1.3.2 Input Capacitor Selection

There are 3 buck regulators in the LM10504 device. Each of these buck regulators has its own input capacitor which must 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, to reduce the input power rail ripple. TI recommends connecting all the supply and 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 with Equation 7.

Equation 7.

The power dissipated in the input capacitor is given by Equation 8.

Equation 8.

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 and ground input pins of the device.

The input power source supplies the average current continuously. However, during the PFET switch on-time, 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 results in conservative estimates of input ripple voltage and capacitor RMS current.

Input ripple voltage is estimated with Equation 9.

Equation 9.

where

• V_PPIN is the estimated peak-to-peak input ripple voltage
• I_OUT is the output current
• C_IN is the input capacitor value
• ESR_CIN is the 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 with Equation 10.

Equation 10.

where

• I_RMSCIN is the estimated input capacitor RMS current