A boost converter, as the name implies, converts a low voltage power rail and boosts it to a higher voltage. A boost converter consists of three main components: an inductor, a switch (MOSFET), and a diode. The basic boost converter schematic is shown in the following figure.

To convert the low voltage to a high voltage, the boost converter operates in two phases. In the first phase the inductor is charged by closing the switch and forcing current through the inductor to ground. During this period the current through the inductor increases allowing the inductor to store charge.

Once the inductor current reaches a maximum threshold, the switch opens and forces the inductor to dump the stored charge through the diode and onto the output capacitor and load.

By repeating this charge and dump process, the boost converter is able to increase the output voltage.

The DRV8662, DRV2700, DRV2665, and DRV2667 use a hysteretic boost converter design to generate the high voltage needed to drive Piezos. This section describes the basic operating principle of the hysteretic boost converter.

The hysteretic boost converter uses fairly simple feedback to control the timing and frequency of the switch. The I_LIM value in the following figure is the peak current through the inductor every time the switch turns on. Once the peak current is reached, the switch opens. The peak inductor current of the DRV8662, DRV2665, and DRV2667 is set by the resistor on the REXT pin (pin 15).

The boost converter only switches when the output voltage (V_BST) is below the final target value, meaning that it will only switch when it needs to. Unlike a fixed-frequency boost converter design, the hysteretic boost converter design has a continually varying switching frequency and is load-dependent. Note that the DRV8662, DRV2665, and DRV2667 have forced switching at approximately 37 kHz.

The boost converter efficiency for the DRV8662, DRV2700, DRV2665, and DRV2667 is shown in Figure 2-1 through Figure 2-4. The measurements were taken using the DRV8662EVM.