Op-amps and startup circuits for CMOS bandgap references with near 1-V supply

The design of bandgap-based voltage references in digital CMOS raises several design difficulties, as the supply voltage is lower than the silicon bandgap in electron volts, i.e., 1.2 V. A current-mode architecture is used in order to address the main issues posed by the low supply, but the implementation of the operational amplifier and of dedicated startup circuits deserves some attention. Even if nonstandard devices such as depletion-mode MOS transistors may be helpful to manage the supply scaling, they are seldom available and poorly characterized. Therefore, they must be avoided in a robust design featuring a high portability. This paper proposes some circuit solutions suitable for very low-supply-voltage operation and addresses the main issues of achieving the correct bias point at the power on. A few bandgap references were implemented in digital 0.35- and 0.18-mum technologies featuring a nominal output voltage of about 500 mV and minimum supplies from 1.5 to 0.9 V.