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IRU1150CM Datasheet(PDF) 5 Page - International Rectifier |
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IRU1150CM Datasheet(HTML) 5 Page - International Rectifier |
5 / 6 page IRU1150 5 Rev. 1.6 03/18/02 www.irf.com The IRU1150 keeps a constant 1.25V between the VSENSE pin and the VADJ pin. By placing a resistor R1 across these two pins and connecting the VSENSE and VOUT pin together, a constant current flows through R1, adding to the Iadj current and into the R2 resistor producing a volt- age equal to the (1.25/R1) 3R2 + IADJ3R2. This voltage is then added to the 1.25V to set the output voltage. This is summarized in the above equation. Since the minimum load current requirement of the IRU1150 is 10mA, R1 is typically selected to be a 121 V resistor so that it automatically satisfies this condition. Notice that since the IADJ is typically in the range of 50 mA it only adds a small error to the output voltage and should be considered when very precise output voltage setting is required. Load Regulation Since the IRU1150 has separate pins for the output (VOUT) and the sense (VSENSE), it is ideal for providing true re- mote sensing of the output voltage at the load. This means that the voltage drops due to parasitic resistance such as PCB traces between the regulator and the load are compensated for using remote sensing. Figure 4 shows a typical application of the IRU1150 with remote sensing. Figure 4 - Schematic showing connection for best load regulation. Stability The IRU1150 requires the use of an output capacitor as part of the frequency compensation in order to make the regulator stable. Typical designs for the microprocessor applications use standard electrolytic capacitors with typical ESR in the range of 50 to 100m V and an output capacitance of 500 to 1000 mF. Fortunately as the ca- pacitance increases, the ESR decreases resulting in a fixed RC time constant. The IRU1150 takes advantage of this phenomena in making the overall regulator loop stable. For most applications a minimum of 100 mF aluminum electrolytic capacitor such as Sanyo, MVGX series, Panasonic FA series as well as the Nichicon PL series insures both stability and good transient response. Thermal Design The IRU1150 incorporates an internal thermal shutdown that protects the device when the junction temperature exceeds the allowable maximum junction temperature. Although this device can operate with junction tempera- tures in the range of 150 8C, it is recommended that the selected heat sink be chosen such that during maxi- mum continuous load operation the junction tempera- ture is kept below this number. The example below shows the steps in selecting the proper surface mount package. Assuming, the following conditions: Calculate the maximum power dissipation using the fol- lowing equation: Using table below select the proper package and the amount of copper board needed. Pkg Copper θJA(°C/W) Max Pd Max Pd Area (TA=25 °C) (TA=45 °C) TO-263 1.4"X1.4" 25 4.4W 3.6W TO-263 1.0"X1.0" 30 3.7W 3.0W TO-263 0.7"X0.7" 35 3.1W 2.6W TO-263 Pad Size 45 2.4W 2.0W SO-8 1.0"X1.0" 55 2.0W 1.63W Note: Above table is based on the maximum junction temperature of 135 8C. As shown in the above table, any of the two packages will do the job. For low cost applications the SO-8 pack- age is recommended. VOUT = 2.7V VIN = 3.3V VCTRL = 5V IOUT = 2A (DC Avg) PD = 2 3 (3.3 - 2.7) + oo pp3(5-2.7)=1.28W 2 60 PD = IOUT 3 (VIN - VOUT) + oo pp3(V CTRL - VOUT) IOUT 60 R1 R2 VIN VCTRL R L IRU1150 VSENSE Adj VOUT VCTRL Vin |
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