Patent ID: 7882383

Claim:
A system on a chip (SOC) comprises: a bus structure; a processing module coupled to the bus structure; read only memory (ROM) coupled to the bus structure; random access memory (RAM) coupled to the bus structure; a display interface coupled to the bus structure; an external memory interface coupled to the bus structure; a digital to analog conversion (DAC) module coupled to the bus structure, wherein the digital to analog converter produces an analog audio signal; an analog to digital conversion (ADC) module coupled to the bus structure; a headphone amplifier circuit coupled to amplify the analog audio signal; a battery integrated circuit (IC) pin coupled to receive a battery voltage; a real time clock module coupled to provide timing functions; a crystal oscillation circuit coupled to produce an oscillation; and a voltage supply circuit coupled to produce a supply voltage for at least a portion of the real time clock module and the crystal oscillation circuit, wherein the voltage supply circuit includes: a reference circuit coupled to produce a reference voltage based on the supply voltage; a transistor coupled to the battery IC pin, wherein the transistor produces the supply voltage based on a regulation signal and the battery voltage; an amplifier coupled to produce the regulation signal based on the reference voltage and a feedback representation of the supply voltage; and a start-up circuit coupled to enable the voltage supply circuit at battery connection, wherein the start-up circuit comprises: a second transistor coupled to the battery IC pin and the transistor; and a comparison module coupled to compare the reference voltage with a threshold, wherein, when the reference voltage compares unfavorably with the threshold, the comparison module enables the second transistor to couple the supply voltage to the battery IC pin.