Design of Compensation Regulator Temperature Drift Based on DS1859 and MAX604

The voltage regulator provides a continuous and stable voltage to the subsequent circuits. Some applications can accept relatively large voltage fluctuations, while some applications are very demanding on voltage fluctuations. These precision circuits need the voltage to remain constant.

This article will compare test data from a standard configuration regulator and the same regulator with a DS1859 dual temperature-controlled resistor. The DS1859 uses one of the variable resistors and a temperature-controlled lookup table (LUT) for temperature compensation, and the test results clearly show the improvement in system performance using the DS1859 temperature-indexed LUT. Simpler chips, such as the DS1847 dual temperature-controlled nonvolatile variable resistor, also have a temperature-indexed LUT and perform just as well. In addition, the DS1859 and DS1847 can provide closed-loop control without the need for a microcontroller.

Uncompensated Regulators

A typical voltage regulator circuit consists of a pass element, a feedback resistor divider, and a capacitor that provides some filtering and load regulation during transient or switching load conditions. The feedback resistor network sets the regulator output voltage based on the ratio of the two resistors. In this article, the MAX604 is selected as the regulator, which can output a preset 3.3V voltage or any voltage within the operating range. The MAX604 uses a voltage divider to determine the output voltage by controlling the SET pin voltage divider. In most voltage regulator circuits, the output voltage will vary slightly with temperature. The MAX604 varies from 97.6% to 101.5% of the nominal output voltage, which can be confirmed by experimental data obtained from the circuit in Figure 1. At -45°C, the output value is approximately 98% of the nominal value; at +85°C, the output value is 101% of the nominal value. The output of most regulators has similar temperature characteristics, and the following discusses ways to improve this indicator.

Figure 1 MAX604 adjustable output configuration

Compensation using a temperature-indexed lookup table

Figure 2 places the DS1859 in a voltage regulation circuit, with the digital potentiometer connected in parallel with R2 in Figure 1. The DS1859 digital potentiometer is controlled by a temperature index lookup table in the internal nonvolatile memory. A resistor calibration value is provided every 2°C. This circuit uses a 50kΩ DS1859.

Figure 2. MAX604 temperature compensation circuit.

The lookup table can be configured to any resistance-temperature curve according to user needs. In this example, our goal is to obtain a stable regulator temperature curve. Therefore, the DS1859 lookup table is set to approximately proportional relationship between digital resistance and temperature. The resistance has 256 set values ​​(decimal 0 to 255), each level is about 192Ω. The initial value of the digital resistance is level 152. At -40°C, it is level 143, and at +85°C, it is level 158. For every 2°C change in temperature, the digital resistance changes the compensation value accordingly, and changes by one level every 4°C to 6°C with ambient temperature.

The voltage regulation accuracy over the full temperature range is greatly improved. As shown in Figure 3, there is only a ±2mV change in the temperature range of -45°C to +85°C.

Figure 3. Comparison of uncompensated and compensated data.

in conclusion

The temperature index lookup table can be used to easily improve the temperature characteristics of the system and eliminate the impact of temperature changes. Because the digital resistors are controlled by independent lookup tables stored in non-volatile memory, each resistor can be designed completely according to the user's application. In addition to providing flexible settings for two channels, the DS1859 also provides three ADC inputs for monitoring external voltages.