A variable resistor that integrates a programmable, temperature-indexed look-up table can compensate for the temperature drift of a voltage regulator. In this case, the look-up table can change the resistance every 2°C over a range of –40 to +102°C, thereby nulling any regulator-output changes that would otherwise occur because of temperature. A typical regulator circuit comprises a regulating element, a feedback-resistor divider, and capacitors to provide filtering and regulation against transients and load-switching conditions (Figure 1). The ratio of the two feedback-divider resistors sets the regulator-output voltage. The regulator can generate either a preset 3.3V or any user-defined output within its operating range.

For most regulator circuits, the output voltage varies slightly with temperature, from 97.6 to 101.5% of nominal in this circuit. These numbers are respectable, but you can improve them. First, incorporate a digitally controlled variable resistor, such as a DS1859, into the regulator circuit of Figure 1 by placing it in parallel with R₂ (Figure 2). A temperature-indexed look-up table in an internal nonvolatile memory controls the 50-kΩ digital resistor, allowing you to program a different resistance value for each 2°C window.

You can program the look-up table to provide any resistance-versus-temperature profile. In this example, the look-up table flattens the regulator’s normal curve over temperature. These look-up tables, therefore, provide a positive resistance slope with respect to temperature. The resistor has 256 programmable resistance settings of 0 to 255 decimal, and each one accounts for approximately 192Ω. In this example, the look-up table was programmed with a setting of 143 decimal at –40°C. The settings were incremented by one for every 4 to 6°C change in temperature, resulting in a value of 152 decimal for ambient and 158 decimal for +85°C.

As illustrated in Figure 3, the result of this regulated performance over temperature is a drastic increase in precision: The variation from –45 to +85°C is now only ±2 mV. For comparison, note the response of the standard regulator circuit in Figure 1 (the black curve). The digital-resistor IC of Figure 2 includes three ADC inputs for monitoring external voltages. An alternative, the DS1847 dual variable resistor, offers similar performance without the ADC monitors and at lower cost.