Power for the high-side and low-side MOSFET drivers and most other internal circuitry is derived from the V7V pin. The internal built-in low-dropout (LDO) linear regulator supplies 4.96 V (typical) from 5 V V_IN to the V7V voltage. The user must connect a 10-µF ceramic capacitor from the V7V pin to power ground.

If the input voltage decreases to the UVLO threshold voltage, the UVLO comparator detects the V7V pin voltage and forces the converter off.

Each high-side MOSFET driver is biased from the floating bootstrap capacitor, C_B, shown in Figure 7-8, which is normally recharged during each cycle through an internal low-side MOSFET or the body diode of a low-side MOSFET when the high-side MOSFET turns off. The boot capacitor is charged when the BST pin voltage is less than the input voltage and the BST-LX voltage is below regulation. TI recommends using a 47-nF ceramic capacitor. TI recommends using a ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher because of the stable characteristics over temperature and voltage. Each low-side MOSFET driver is powered from the V7V pin directly.

To improve dropout, the device is designed to operate at 100% duty cycle as long as the BST to LXx pin voltage is greater than the BST-LX UVLO threshold, which is typically 2.1 V. When the voltage between the BST and LXx pins drops below the BST-LX UVLO threshold, the high-side MOSFET is turned off and the low-side MOSFET is turned on allowing the boot capacitor to be recharged.

Figure 7-8 V7V Linear Dropout Regulator and Bootstrap Voltage Diagram