To DC / DC converter current characteristics linearized thermistor network

Recently introduced a variety of integrated step-down DC / DC converters have been taken on the external low side MOSFET synchronous rectifier voltage down-sampling method, without the high-side current-sense resistor. This topology saves the cost of the detection resistor and the printed circuit board space, but also appropriate to improve the circuit efficiency. However, MOSFET on-resistance and temperature have great relevance, which determines the limiting size. Fortunately, some of the new DC / DC converter (such as Maxim's MAX1714) can be adjusted from outside the current limit threshold. Figure 1 shows how the circuit a thermistor circuit output current limit to temperature compensation.

To DC / DC converter current characteristics linearized thermistor network

MAX1714 IC1 6 a linear foot limit (ILIM) input range from 0.5V to 2V, the corresponding threshold limit values were 50 mV to 200mV. Limit set in the default value 100 mV, the circuit at 25 ℃ for limiting the size of 7.5A. However, Figure 2 shows the change in current limit range from -40 ℃ to 85 ℃ 9A to the 6A. For the design of temperature compensation network, the first Dachu model circuit, with an external power supply to change the MAX1714 current limit input voltage to maintain a constant output current limit. In the circuit of the operating temperature range, each measurement interval of 10 ℃ repeat.

IC1 to compensate the temperature can be from a variety of resistance - thermal resistance to choose the network topology. First, the need to select a suitable thermistor, depicts the resistance change characteristics with temperature. As the MAX1714 current limit input pin is a relatively high input impedance voltage follower stage, so the required nominal resistance thermistor as high as 100 kΩ. Low-cost thermistor resistance - temperature characteristics are obvious nonlinear relationship, but there is a relatively simple linear program, about a thermistor in parallel with the thermistor with the same nominal resistance fixed resistor (references). Network in Figure 1, R1 of the thermistor linearization, R2 and R3 are respectively set the current limit voltage - temperature characteristics of the slope and intercept.

In order to obtain optimal R2, R3 value, we prepared a spreadsheet that contains the original limit voltage - temperature data, and each resistor network added a column, along with thermistor specification sheet resistance and temperature of relational data. The observation of temperature and voltage conversion circuit functional relationship, we change the electronic table R2 and R3 values, until the conversion feature closest to the measured limiting voltage - temperature data. Finally, we build a good circuit, and in the whole temperature range of its test and found it very flat response.

Figure 2 The output characteristics of the curvature correction (red trace) is a unique thermistor. Although flatness is not very ideal, but still higher than the curve after correction of the original curve (black trace) has a great improvement, sufficient to meet the original design objectives. If you want to achieve more precise compensation, you can choose a different thermistor, or the use of multiple thermistor.

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