A handheld device includes a backlight driving unit and a power supply circuit for powering the backlight driving unit. The power supply circuit includes a power management unit, a battery, a first electronic switch, a second electronic switch, and a control unit. When a voltage of a second terminal of the second electronic switch is less than a reference voltage of the control unit, the control unit turns on the first electronic switch and turns off the second electronic switch. The backlight driving unit is powered by the battery through the first electronic switch. When the voltage of the second terminal of the second electronic switch is more than the reference voltage of the control unit, the control unit turns off the first electronic switch and turns on the second electronic switch. The backlight driving unit is powered by the PMU through the second electronic switch.