The schematic diagram below show us a circuit of a battery-powered voltage reference that draws only 17uA of supply current. Two AA cells can power this reference over 18 months at this level. Drift of the reference is only at 5.5 uV/°C over the industrial temperature range at an output voltage of 1.23 V @25°C. The load regulation is 85 uV/mA and the line regulation is 120 uV/V.
The bandgap technique is based of the design reference. With scaling resistors R1 and R2, we can produces unequal currents in Q1 and 2. Temperature proportional voltage across R3 is created by the resulting VBE mismatch which, in turn, produces a larger temperature-proportional voltage across R4 and R5. This voltage appears at the output added to the VBE of Q1, which has an opposite temperature coefficient. To produce minimum drift over temperature, we can adjust the output to 1.23 V at 25′C. Bandgap references can have start-up problems. The OP90 is beyond its positive input range limit and has an undefined output state if there is no current in R1 and R2. Shorting Pin 5 (an offset adjust pin) to ground, forces the output high under these conditions and ensures reliable start-up without significantly degrading the OP90’s offset drift. [Circuit’s schematic diagram source: analog.com]