fail to perform properly due to the lack of the reference voltage during such operating conditions a bandgap voltage reference is a temperature independent voltage reference circuit widely used in integrated circuits the bandgap voltage reference produces a fixed constant voltage regardless of power supply variations temperature changes and circuit loading from a device in some examples it commonly has an output voltage around 1 25 v close to the theoretical 1 22 ev bandgap of silicon at 0 k the improved ldo described herein continues to supply power for the digital logic circuit of our whole chip when the chip power supply system is available and bandgap voltage is ready in some applications such as chips for mobile devices a digital watchdog timer function is incorporated in the circuit in a chip the digital watchdog timer is used to alarm and reset a system including multiple chips the digital watchdog timer starts to work when the main power and functions of the chip are disabled and or disconnected hence the only power supply available during such condition is from a charge holding capacitor since capacitors take large space on a chip to keep chip and device sizes smaller such capacitors are typically smaller to keep the digital watchdog timer to operate for a long time several seconds it is desired to design a low power and self sustained ldo to provide a required output voltage e g 1 8v during this operation period.