The charger IC using a voltage up and down topology allows VBATT to be higher, lower, or equal to the VIN of the device, which means that the battery can continue to charge through any power supply voltage until it reaches its target voltage. In this way, fast charging becomes possible under a wider range of conditions, although it does require a larger charger IC packaging size. Usually, if connected to an input power supply, the boost/boost charger IC can operate in three working modes: boost mode, buck mode, and boost/boost mode. The VIN in boost mode is lower than VBATT, the VIN in boost mode is higher than VBATT, and the VIN in boost mode is almost equal to VBATT. The voltage boost charger IC can provide high power throughout the entire PD voltage range, while also being backward compatible with 5V traditional USB. This type of charger IC is most commonly used in USB PD applications where multiple batteries are connected in series, such as laptops, smartphones, and mobile power supplies.