This gives a nominal peak-to-peak ripple current of 450mA. To allow some margin, you should select a value of 10µH. Selecting a 600mA ripple current, the required inductance is: L = V l.dt/di = (1.8 × 0.5/200 × 10 3)/0.6 V l = - V o = -1.8V when the switch is off. V l = V i - V o = 1.8V when the switch is on Where V o is the output voltage and V i is the input voltage. 2, for example, let's assume a switching frequency of 200 kHz, input voltage range of 3.3V☐.3V and output of 1.8V at 1.5A with a minimum load of 300mA.įor an input voltage of 3.6V, the duty cycle will be: When selecting an inductor for a buck converter, as with all switching regulators, you'll need to define or calculate the following parameters:įor the buck converter shown in Fig. Selecting an inductor ripple current less than two times the minimum load ensures continuous mode operation. However, operating in discontinuous mode is not recommended as it makes for a more complex converter design. This means that the inductor current can flow continuously or can fall to zero during the switching cycle (discontinuous). You can operate buck converter inductors in continuous or discontinuous mode.
This also implies that the current in the inductor will be continuous for output currents greater that 5% to 15% of full load. To get a good compromise between inductor and capacitor size, you should choose a ripple current value of 10% to 30% of maximum load current. This capacitor must have a sufficiently high ripple current rating, or it overheats and dries out. Selecting the correct ripple current also affects the size of the inductor and the output capacitor. It is normally in the order of 10-500mVpk-pk.
This ripple voltage needs to be sufficiently low - so not to affect the operation of the circuit the regulator is supplying.
This will produce a ripple voltage due to the capacitor equivalent series resistance (ESR), which appears at the output of the buck converter. Because the ac component is high frequency, it will flow through the output capacitor, which offers a low HF impedance. 1, the inductor current is made up of ac and dc components. From this, we can see that the value of ripple current is dependent on the value of the inductance.įor a buck converter, choosing the correct value of inductance is important to obtain acceptable inductor and output capacitor sizes and sufficiently low output voltage ripple.Īs you can see from Fig. Where V l is the voltage across the inductor, di is the ripple current, and dt is the duration for which the voltage is applied.
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