Resistorless switched-capacitor bandgap voltage reference with low sensitivity to process variations

A novel switched-capacitor bandgap voltage reference circuit is presented that dispenses entirely with the use of resistors. Since capacitors are used instead of resistors, variability problems from devices mismatch and process spread are reduced, compared with traditional topologies. The proportional-to-absolute-temperature drift and the complementary-to-absolute-temperature drift voltages are both generated by the same PNP vertical bipolar transistor obtained in bulk CMOS, minimising mismatch problems even further. A switched-capacitor circuit stores, amplifies and sums these voltages, generating the bandgap voltage after five clock cycles. The operational amplifier offset voltage is also cancelled by the switching scheme proposed. The current sources which are used to generate different junction current densities are averaged in the switching process, reducing their mismatch impact on the circuit performance. A systematic comparison to a traditional bandgap topology with resistors is presented, both designed in the CMOS 180 nm process, and demonstrates the better performance of the new topology with respect to the former. Monte Carlo simulations show significantly lower spread resulting from devices mismatch in the output reference voltage (V REF) and also in its temperature coefficient.