Table 2. Design Parameters

9.2.2.1 Inductor Selection

Because the selection of inductor affects the steady-state operation of the power supply, transient behavior, loop stability, and the boost converter efficiency, the inductor is one of the most important components in switching power regulator design. There are three specifications most important to the performance of the inductor: inductor value, DC resistance (DCR), and saturation current. The TPS61163 is designed to work with inductor values from 4.7 µH to 10 µH to support all applications. A 4.7-µH inductor is typically available in a smaller or lower profile package, while a 10-µH inductor produces lower inductor ripple. If the boost output current is limited by the overcurrent protection of the device, using a 10-µH inductor may maximize the output current capability of controller. A 22-µH inductor can also be used for some applications, such as 6s2p and 7s2p, but may cause stability issue when more than eight WLED diodes are connected per string. Therefore, TI recommends customers verify the inductor in their application if it is different from the values in Recommended Operating Conditions.

Inductor values can have ±20% or even ±30% tolerance with no current bias. When the inductor current approaches saturation level, its inductance can decrease 20% to 35% from the 0A value depending on how the inductor vendor defines saturation. When selecting an inductor, make sure its rated current, especially the saturation current, is larger than its peak current during the operation.

Follow Equation 4 to Equation 6 to calculate the peak current of the inductor. To calculate the current in the worst case, use the minimum input voltage, maximum output voltage and maximum load current of the application. In order to leave enough design margin, the minimum switching frequency, the inductor value with –30% tolerance, and a low power conversion efficiency, such as 80% or lower are recommended for the calculation.

In a boost regulator, the inductor DC current can be calculated as Equation 4.

Equation 4.

where

• V_OUT = boost output voltage
• I_OUT = boost output current
• V_IN = boost input voltage
• η = boost power conversion efficiency

The inductor current peak to peak ripple can be calculated as Equation 5.

Equation 5.

where

• I_PP = inductor peak-to-peak ripple
• L = inductor value
• F_S = boost switching frequency
• V_OUT = boost output voltage
• V_IN = boost input voltage

Therefore, the peak current I_P seen by the inductor is calculated with Equation 6.

Equation 6.

Select an inductor with saturation current over the calculated peak current. If the calculated peak current is larger than the switch MOSFET current limit I_LIM, use a larger inductor, such as 10 µH, and make sure its peak current is below I_LIM.

Boost converter efficiency is dependent on the resistance of its current path, the switching losses associated with the switch MOSFET and power diode and the inductor’s core loss. The TPS61163 has optimized the internal switch resistance, however, the overall efficiency is affected a lot by the DCR of the inductor, equivalent series resistance (ESR) at the switching frequency, and the core loss. Core loss is related to the core material and different inductors have different core loss. For a certain inductor, larger current ripple generates higher DCR/ESR conduction losses as well as higher core loss. Normally a datasheet of an inductor does not provide the ESR and core loss information. If needed, consult the inductor vendor for detailed information. Generally, an inductor with lower DCR/ESR is recommended for TPS61163 application. However, there is a trade-off between the inductance, DCR/ESR resistance, and footprint of the inductor; furthermore, shielded inductors typically have higher DCR than unshielded ones. Table 3 lists some recommended inductors for the TPS61163. Verify whether the recommended inductor can support the target application by using the previous calculations as well as bench validation.

9.2.2.2 Compensation Capacitor Selection

The compensation capacitor C4 (refer to Additional Application Circuits) connected from the COMP pin to GND, is used to stabilize the feedback loop of the TPS61163. A 330-nF ceramic capacitor for C4 is suitable for most applications. A 470-nF capacitor also works for some applications, and it is suggested that customers verify it in their applications.

9.2.2.3 Output Capacitor Selection

The output capacitor is mainly selected to meet the requirement for the output ripple and loop stability. A 1-µF to 2.2-µF capacitor is recommended for the loop stability consideration. This ripple voltage is related to the capacitor’s capacitance and its ESR. Due to its low ESR, V_{ripple_ESR} could be neglected for ceramic capacitors. Assuming a capacitor with zero ESR, the output ripple can be calculated with Equation 7.

Equation 7.

where

• V_ripple = peak-to-peak output ripple.

The additional part of ripple caused by the ESR is calculated using V_{ripple_ESR} = I_OUT × R_ESR and can be ignored for ceramic capacitors.

Note that capacitor degradation increases the ripple much. Select the capacitor with 50-V rated voltage to reduce the degradation at the output voltage. If the output ripple is too large, change a capacitor with less degradation effect or with higher rated voltage could be helpful.

9.2.2.4 Schottky Diode Selection

The TPS61163 demands a low forward voltage, high-speed and low capacitance schottky diode for optimum efficiency. Ensure that the diode average and peak current rating exceeds the average output current and peak inductor current. In addition, the reverse breakdown voltage of the diode must exceed the open LED protection voltage. TI recommends ONSemi MBR0540 and NSR05F40, and Vishay MSS1P4 for the TPS61163.