To calculate the value of the inductor, LIR is used. LIR is a coefficient that represents the amount of inductor ripple current relative to the maximum output current. In every cycle, inductor current is LIR times of max output current from Equation 3. The inductor ripple current is filtered by the output capacitor. Therefore, choosing high inductor ripple currents impact the selection of the output capacitor since the output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. A good rule is that LIR is normally from 0.2 to 0.5 for the majority of applications. So inductor can be received with Equation 4 based on this rule.

Equation 3.

Equation 4.

Sometimes the real maximum loading is less than 3A, such as like 1A. In this condition, it still uses maximum loading of the used part to calculate inductance. TPS563202 is a 3A part. It’s stable at 3A, of course it’s also stable with any below 3A loading, similar to 1A. And it doesn’t need to worry about output voltage ripple. Even though the inductor peak to peak current is larger compared with real 1A loading, output voltage ripple is still very small.

Here is uses an example of 12 V input voltage to 5 V output voltage based on TPS563202. Maximum loading of TPS563202 is 3A. The real loading is only 1A in one application. Based on Equation 4, LIR is set to 0.35. So inductor should be set to 4.7uH. If so, inductor peak to peak current is about 1.05A which is even larger than real 1A loading. But it’s no any problems in real application. Figure 2-3 is waveform at 1A loading, Figure 2-4 is waveform at 3A loading. From waveforms, inductor peak to peak current does be 1A, but output voltage ripple is less than 50mV at both 1A and 3A loading.

Figure 2-3 Ripple at 1A Loading

Figure 2-4 Ripple at 3A loading