Emerson once said: "Never imitate...Every great man is unique," and unfortunately, so is every reference circuit, at least as far as its output voltage and process technology are concerned. Bandgap references may be relatively robust and accurate but not immune to process variations and mismatch offsets whose adverse effects on accuracy varies across devices, wafers, lots, and technology nodes, making each device unique. As a result, trimming (that is, tweaking) the output voltage is necessary to produce predictable reference values, . Arbitrarily trimming a circuit to any voltage target, however, can be detrimental because the temperature drift at that level may be prohibitively large. Typically, only one voltage target produces the least temperature drift, and this target is specific to a circuit, its layout, and the processing technology with which it was fabricated (if circuit, layout, and/or process change, the optimal trim target also changes). This article discusses how this optimal trim target can be ascertained.
A reference circuit generates a predictable temperature-independent voltage by summing voltages and/or currents with opposite temperature-drift coefficients. In the case of a bandgap reference, thermal voltage Vt is the linearly dependent term that increases with temperature, in other words, proportional-to-absolute-temperature (PTAT), and base-emitter voltage VBE the mostly linear component that decreases with temperature, complementary-to-absolute-temperature (CTAT). Vt is normally derived by extracting the voltage difference of two base-emitter voltages with dissimilar current densities (that is, two transistors with dissimilar emitter areas but equal currents flowing through them as shown in the inset of Figure 1), which is a manifestation of the well-known Gilbert principle.
Figure: 1. Formulation of a temperature-independent bandgap reference voltage