Ultra-low Power Bandgap Reference
February 23, 2015, ISSCC, San Francisco—Aatmesh Shrivastava from PsiKick detailed the workings of a 32 nW bandgap reference that is operational from 0.5 V. The IoT will need to have very low power reference sources to help the rest of the device(s) meet realistic battery life requirements.
Reference voltage is used in many analog blocks including ADC, DAC, voltage regulators, and others. The most common configuration is a bandgap reference due to its temperature and performance characteristics. The basic design is to use the differential currents from scaled bipolar transistor emitters to develop a voltage across a sense resistor. The problem has been that the transistors use a lot of current, and the resistors take a large area to get high values of differential voltage.
The first concept is to use a doubling charge pump to drive the BJT, and ideally, the minimum Vin has to be greater than Veb/2, about 375 mV. This design is shown in the figure. 
The bandgap reference uses a current reference to bias a current-controlled ring oscillator. The ring oscillator generates the non-overlapping clock phases through a clock doubler. The two phases are part of the feedback for the reference. The output is trimmed with a 4-bit calibration capacitor. Simulations indicate a 3-sigma process variation of 2 percent at 20° with an untrimmed mean voltage of 498 mV.
Overall measured parameters are power: 32 nW, minimum Vin: 0.5 V, temperature variation 75 ppm/°, ripple 50 uV. The chip can ger better performance at higher input voltages, but suffer from linearly proportional increases in power consumption with increased input voltage.
