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IRS2540SPBF Datasheet(PDF) 10 Page - International Rectifier |
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IRS2540SPBF Datasheet(HTML) 10 Page - International Rectifier |
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10 / 14 page  IRS254(0,1)(S)PbF www.irf.com Page 10 operation, the IC does not enter UVLO unless the supply voltage falls below VCCUV--. Inductance Selection To maintain tight hysteretic current regulation the inductor and output capacitor COUT (in parallel with the LEDs) need to be large enough to maintain the supply to the load during tHO,ON and avoid significant undershooting of the load current, which in turn causes the average current to fall below the desired value. First, we are going to look at the effect of the inductor when there is no output capacitor to clearly demonstrate the impact of the inductor. In this case, the load current is identical to the inductor current. Fig. 9 shows how the inductor value impacts the frequency over a range of input voltages. As can be seen, the input voltage has a great impact on the frequency and the inductor value has the greatest impact at reducing the frequency for smaller input voltages. 175 225 275 325 375 425 30 80 130 180 Vin (V) 470uH 680uH 1mH 1.5mH Fig.9 Frequency Response for Chosen Inductances Iout = 350 mA, Vout = 16.8 V Fig. 10 shows how the variation in load current increases over a span of input voltages, as the inductance is decreased. Fig. 11 shows the variation of frequency over different output voltages and different inductance values. Finally Fig. 12 shows how the load current variation increases with lower inductance over a range of output voltages. The output capacitor can be used simultaneously to achieve the target frequency and current control accuracy. Fig. 11 shows how the capacitance reduces the frequency over a range of input voltage. A small capacitance of 4.7 µF has a large effect on reducing the frequency. Fig. 12 shows how the current regulation is also improved with the output capacitance. There is a point at which continuing to add capacitance no longer has a significant effect on the operating frequency or current regulation, as can be seen in Figs. 13 and 14. 330 340 350 360 370 380 390 400 30 80 130 180 Vin (V) 470uH 680uH 1mH 1.5mH Fig.10 Current Regulation for Chosen Inductances Iout = 350 mA, Vout = 16.8 V 200 220 240 260 280 300 320 340 360 380 400 13 18 23 28 33 Vout (V) 470uH 680uH 1mH 1.5mH Fig.11 Frequency Response for Chosen Inductances Iout = 350 mA, Vin = 50 V 325 327 329 331 333 335 337 339 341 343 345 13 18 23 28 33 Vout (V) 470uH 680uH 1mH 1.5mH Fig.12 Current Regulation for Chosen Inductances Iout = 350 mA, Vin = 50 V |
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