8.2.2.2 Inductor Selection

For correct operation of TPS61240-Q1 device, an inductor must be connected between pin V_IN and pin L. A boost converter requires two main passive components for storing energy during the conversion. A boost inductor and a storage capacitor at the output are required. To select the boost inductor, it is recommended to keep the possible peak inductor current below the current limit threshold of the power switch in the chosen configuration. The highest peak current through the inductor and the switch depends on the output load, the input (V_IN), and the output voltage (V_OUT). Estimation of the maximum average inductor current can be done using Equation 2.

Equation 2.

where

• η is the efficiency of the switching regulator

For example, for an output current of 200 mA at 5 V V_OUT, with efficiency of 85%, at least 392 mA of average current flows through the inductor at a minimum input voltage of 3 V.

The second parameter for choosing the inductor is the desired current ripple in the inductor. Normally, it is advisable to work with a ripple of less than 20% of the average inductor current. A smaller ripple (or larger inductor value) reduces the magnetic hysteresis losses in the inductor, as well as output voltage ripple and EMI. But with larger inductor, regulation time during load transients rises. In addition, a larger inductor increases the total system size and cost. With these parameters, it is possible to calculate the value of the minimum inductance by using Equation 3.

Equation 3.

where

• f is the switching frequency
• ΔI_L is the ripple current in the inductor

With V_IN = 4.2 V, V_OUT = 5 V, assuming inductor ripple current = 30% of minimum current limit of 0.5 A, the resulting inductor value = 1.28 μH. In typical applications, a 1.0 μH inductance is recommended. The device has been optimized to operate with inductance values between 1.0 μH and 2.2 μH. It is recommended that inductance values of at least 1.0 μH is used, even if Equation 3 yields something lower. Care has to be taken that load transients and losses in the circuit can lead to higher currents as estimated in Equation 3. Also, the losses in the inductor caused by magnetic hysteresis losses and copper losses are a major parameter for total circuit efficiency.

With the chosen inductance value, the peak current for the inductor in steady state operation can be calculated. Equation 4 shows how to calculate the peak current I.

Equation 4.

This would be the critical value for the current rating for selecting the inductor. It also needs to be taken into account that load transients and error conditions may cause higher inductor currents. Inductor with part number, LQM21PN1R0MC0 is one example of an inductor that can be used with this device. Customers need to verify and validate whether it is suitable for their application.