Recently promoted each kind of integrated type voltage dropping The DC/DC converter has adopted to the external connection leans lowly the MOSFET synchronous rectifier’s loss of voltage sampling method, does not need the high side electric current examination resistor. This kind of analysis situs saved examined resistor’s cost and the print circuit wafer space, also raised the circuit efficiency suitably. But, the MOSFET breakover resistance and the temperature have the very big relevance, it has decided the current limiting size. What fortunately is, certain new DC/DC converter (for example Maxim Corporation’s MAX1714) may from the exterior adjustment current limiting threshold value. Figure 1 electric circuit demonstrated how to use a thermistor to make the temperature compensation to electric circuit’s output current limit.
MAX1714 the IC1 6th foot’s linear current limiting (ILIM) the input range from 0.5V to 2V, the corresponding current limiting threshold value respectively is 50 mV to 200mV. When default current limiting establishment value 100 mV, the electric circuit in 25℃ the time current limiting size is 7.5A. But, Figure 2 the demonstration current limiting value’s range of variation may from -40℃ 9A to 85℃ 6A. For the design temperature compensating network, builds the model electric circuit first, changes MAX1714 with the external power supply the current limiting input voltage, thus causes the output current limit value maintains constant. In entire electric circuit’s operating temperature scope, each gap 10℃ is redundant a survey.
Must compensate IC1 the temperature change, may carry on the choice from many kinds of resistance - thermistor network analysis situs. First, needs to choose an appropriate thermistor, describes its resistance the characteristic which changes along with the temperature. Because the MAX1714 current limiting input foot is a relatively high input impedance voltage followed level, therefore this thermistor normalized resistance request reaches as high as 100 kΩ. The inexpensive thermistor’s resistance number - temperature characteristic has the obvious misalignment relations, but has one kind of relatively simple linearized plan, soon on thermistor parallel has with this thermistor same rated resistance fixed resistor (reference). In Figure in 1 network, R1 to the thermistor linearization, R2 and R3 separately establishes the current limiting voltage - temperature characteristic curve the slope and the intercept.
In order to obtain most superior R2, the R3 value, we have prepared an electronic watch, has contained the primitive current limiting voltage - temperature data, and increased a fence for network in each resistor, meanwhile has the thermistor specification table resistance and the temperature relations data. When observes electric circuit’s temperature and the voltage transformation function relations, we change in the electronic watch R2 and the R3 value, most approaches until the transformation function in the current limiting voltage - temperature data which surveys. Finally, we construct the electric circuit, and in the entire temperature range to its test, discovered that it has the quite smooth response.
In Figure 2 after the adjustment input level curvature (red pathline) is the thermistor unique. Although smoothness not very ideal, but after adjusting the curve still (black pathline) had the very big improvement compared to the original curve, satisfies the original design goal sufficiently the request. If must realize a preciser compensation, may choose the different thermistor, or uses many thermistors.
