5.7 A 29nW bandgap reference circuit

Bandgap references (BGRs) are widely used to generate a temperature-insensitive reference voltage determined by the silicon bandgap. the BGR generally utilizes PN diodes to generate both of proportional-to-absolute-temperature (PTAT) and complementary-to-absolute-temperature (CTAT) quantities and combines them to eliminate the temperature dependency. Though the BGR provides a robust voltage or current reference with insensitivity to process, voltage and temperature variations that is superior to CMOS-only reference circuits, it has received little attention in ultra-low-power (ULP) sensor applications. While CMOS-only reference circuits have recently demonstrated nanowatt power consumption [1], BGR approaches still have two critical factors to preventing nanowatt consumption. One is that PTAT generation assumes sufficient forward bias, VD, of the PN junction to allow e ^VD/(n·VT) to be much larger than 1 in the temperature range of interest, where n and VT (=kT/q) represent the ideality factor and the thermal voltage, respectively. In addition, the PTAT generation requires a start-up circuit to prevent the circuit from resting at the undesirable zero-bias condition. Since the start-up circuit utilizes a resistive voltage division between power rails, it consumes non-zero DC current, which must be larger than leakage current in order to ensure stable start-up operation. These two requirements for PTAT generation limit the use of BGRs in nanowatt ULP applications.