Digital variable resistor compensates temperature drift of voltage regulator

A variable resistor can compensate for a regulator's temperature drift if it incorporates a programmable temperature-indexed lookup table. In this case, the lookup table changes its resistance every 2°C over a range of –40°C to +102°C, thereby offsetting temperature-induced changes in the regulator's output. A typical voltage-regulator circuit consists of a voltage-regulator element, a feedback resistor divider, and several capacitors to provide filtering and regulation for transient and load-switching conditions (Figure 1). The ratio of the two feedback-divider resistors sets the regulator's output voltage. The regulator can generate a preset 3.3V or any user-defined output within its operating range.

For most voltage-regulator circuits, the output voltage varies slightly with temperature, ranging from 97.6% to 101.5% of the circuit's nominal voltage. These numbers are impressive, but you can improve them further. First, place a digitally controlled variable resistor (such as the DS1859) in the voltage-regulator circuit shown in Figure 1 in parallel with R2 (Figure 2). A temperature-indexed lookup table in internal nonvolatile memory controls this 50kΩ digital resistor, allowing you to set a different resistance value for each 2°C window.

                                            Figure 1. With a typical voltage regulator, you can set the regulated output level by adjusting the R1/R2 voltage divider.


Figure 2 If you connect half of the dual variable resistor in parallel with R2 to the circuit in Figure 1, you can achieve temperature compensation of the regulated output voltage.

You can program the lookup table to provide any resistance vs. temperature curve. In this case, the lookup table flattens the normal curve of the regulator over temperature. Therefore, these lookup tables provide a positive resistance slope with respect to temperature. The resistor has 256 programmable resistance settings (0 to 255 decimal), each setting is about 192Ω. In this case, the lookup table setting is 143 (temperature –40°C). These settings increase by 1 for every 4°C to 6°C change in temperature, so it reaches 152 at ambient temperature and 158 at +85°C.


图3 这些曲线把图1所示电路（黑色）和得到补偿的图2所示电路（粉红）的稳压输出随温度变化情况做了比较。

如图3所示，在整个温度范围内，稳压性能在精度方面显著提升：–45℃~+85℃内的变化幅度现在仅为±2 mV。为对比起见，请注意图1中的标准稳压电路的响应（黑色曲线）。图2中的数字电阻IC包含三种用于监视外部电压的ADC输入。作为备选品的DS1847双路可变电阻提供类似性能，而且不带有ADC监视器，成本也更低。