3 Design Example

This section will compare the design and performance of two 2-A buck converters; the TPS54218 and the TPS628512 which have been set to operate with a switching frequency of 200-kHz and 1.8-MHz respectively.

The inductors used for this example come from the same series of inductors from Coilcraft (the XGL4030) with a size of 4-mm x 4-mm x 3-mm and their inductances were selected so that the inductor current ripple is similar for both designs – this is important for a fair efficiency comparison since the RMS value of the inductor current affects the conduction losses. The inductor current ripple and the ripple ratio in a buck converter can be calculated with the following equations respectively:

A 1-uH inductor was chosen for the TPS628512 which is operating with a switching frequency of 1.8-MHz and an 8.2-uH inductor was selected for the TPS54218 operating at 200-kHz. The resulting inductor current ripple ratio at 2-A is 0.32 for the TPS628512 and 0.35 for the TPS54218, the current ripple ratios are sufficiently close for this test.

The fact that both the 1uH and the 8.2-uH inductors are the same size, means that the 8.2-uH inductor will naturally have a greater DCR. The reason for that is, in order to achieve a higher inductance, the winding of the inductor has to contain a higher number of turns – thereby increasing the length of the copper wire which increases its resistance.

For this particular design the 1 uH inductor has a typical DCR of 6.5-mOhms and the 8.2-uH inductor has a typical DCR of 55-mOhms. It can be concluded that the DCR of the inductor has increased by the same factor as the inductance, almost 8-fold.

Figure 3-1 compares the measured efficiencies of both designs at V_IN = 5-V and V_OUT = 1.8-V, showing that the TPS628512 has less losses when the output current exceeds 1-A. The TPS628512 also has a Power Save Mode, allowing the converter to operate with pulse-frequency modulation at lower currents which decreases the switching losses and increases efficiency.

This efficiency improvement goes on top of the other benefits of high switching frequency converters such as having a smaller solutions size because of requiring less output capacitance to meet the same output ripple and transient performance as the low switching frequency converters. In this case, due to the lower BOM count and the lower output capacitance, the total solution size of the TPS628512 comes in at 48mm², whereas the TPS54218 running at a lower switching frequency takes up 92 mm² of board space – almost twice as much.