Apparatus and method for controlling a power supply

An apparatus for controlling a power supply, comprising a first power supply selecting circuit, a first low dropout regulator (LDO), a second power supply selecting circuit, a second LDO, a reference circuit, a voltage detecting circuit,

• wherein the first power supply selecting circuit is configured to select a higher voltage from a universal serial bus input (USBIN) voltage and an alternating current input (ACIN) voltage to be a first output voltage;
• wherein the first LDO is configured to decrease the first output voltage to a second output voltage, and to close a self-start circuit after receiving a reference voltage;
• wherein the second power supply selecting circuit is configured to select a higher voltage form the second output voltage and a voltage of battery (VBAT) to be a third output voltage;
• wherein the second LDO is configured to supply, when being disabled, the third output voltage as a first input voltage to the reference circuit, and to supply, when being enabled, a first input voltage to the reference circuit;
• wherein the reference circuit is configured to supply the reference voltage to the voltage detecting circuit, the first LDO and the second LDO according to the first input voltage;
• wherein the voltage detecting circuit is configured to supply an enable signal to the second LDO when determining that the USBIN voltage or the ACIN voltage reaches a threshold voltage according to the reference voltage.