Table 2. Design Parameters

9.2.2.1 Inductor Selection

Because the selection of inductor affects steady-state operation of the power supply, transient behavior, loop stability, and boost converter efficiency, the inductor is one of the most important components in switching power regulator design. There are three specifications most important to performance of the inductor: inductor value, DC resistance (DCR), and saturation current. The TPS61162D 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 the controller. A 22-µH inductor can also be used for some applications, such as 6s2p and 7s2p, but may cause stability issues when more than eight WLED diodes are connected per string. Therefore, customers must 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 0-A value depending on how the inductor vendor defines saturation. When selecting an inductor, user must confirm 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 worst-case current, 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 (1 MHz for TPS61162D), 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 detected 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 core loss of the inductor. The TPS61162D 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. Inductor data sheets do not typically provide the ESR and core loss information; if needed, consult the inductor vendor for detailed information. Generally, TI recommends an inductor with lower DCR/ESR for the TPS61162D application. However, there is a trade-off between the inductance of the inductor, DCR/ESR resistance, and the inductor footprint; furthermore, shielded inductors typically have higher DCR than unshielded ones. Table 3 lists some recommended inductors for the TPS61162D. Verify whether the recommended inductor can support the target application using Equation 4, Equation 5, and Equation 6 as well as bench validation.

9.2.2.2 Schottky Diode Selection

The TPS61162D 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 TPS61162D.

9.2.2.3 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 TPS61162D. A 330-nF ceramic capacitor for C4 is suitable for most applications. A 470-nF is also acceptable for some applications, and customers are suggested to verify it in their applications.

9.2.2.4 Output Capacitor Selection

Selection of the output capacitor is primarily to meet the requirement for the output ripple and loop stability. The output ripple voltage is related to the capacitance and the ESR of the capacitor. A 1-µF to 2.2-µF ceramic type X5R or X7R capacitor is recommended. Ceramic capacitors have low ESR so the contribution of the ESR component to the output ripple is negligible. 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.