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ISL78100 Datasheet(PDF) 14 Page - Intersil Corporation |
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ISL78100 Datasheet(HTML) 14 Page - Intersil Corporation |
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14 / 19 page  14 FN6626.0 December 17, 2007 Rectifier Diode A high speed rectifier diode is necessary to prevent excessive voltage overshoot, especially in the boost configuration. Low forward voltage and reverse leakage current will minimize losses, making Schottky diodes the preferred choice. Similarly to the inductor, a diode with a suitable current rating to handle current limit in the configuration must be used. Output Capacitor The output capacitor acts to smooth the output voltage and in the boost configuration supplies load current directly during the conduction phase of the power switch. Ripple voltage consists of two components, the first due to charging and discharging of the capacitor; the second due to IR drop across the ESR of the capacitor by inductor ripple current. In boost mode: where: and In buck mode: where: For a low ESR ceramic capacitor, output ripple is dominated by the charging and discharging of the output capacitor. Care should be taken to ensure the voltage rating of the capacitor exceeds the maximum output voltage. Compensation The ISL78100 employs a direct summing control loop with current feedback. No error amplifier is used in the system. The arrangement provides fast transient response and makes use of the output capacitor to compensate the loop. The effect of the pole associated with the inductor is minimized by the current feedback. The number of LEDs, their DC bias current and the value of feedback resistor alter loop stability due to their effect on feedback factor, which is heavily influenced by the small signal impedance of the LEDs. Generally, higher numbers of LEDs, lower bias levels and smaller values of feedback resistor will require smaller output capacitors to achieve loop stability. A combination of low ESR electrolytic and ceramic capacitors may be used to reduce implementation costs. V RIPPLE I O C OUT ---------------- D F S ------- × I LPK ESR × + = (EQ. 4) D V OUT V IN – V OUT -------------------------------- = (EQ. 5) I LPK I O 1D – ------------- V OUT V IN – () 2L × ------------------------------------ + 1D – () f s ------------------ × = (EQ. 6) V RIPPLE V IN V OUT – () D × 2f s × L × ----------------------------------------------- D f s C OUT × --------------------------- ESR + ⎝⎠ ⎛⎞ × = (EQ. 7) D V OUT V IN ---------------- = (EQ. 8) TABLE 2. BOOST MODE COMPENSATION. 2.7V OPERATION VFB IOUT VOUT (V) 7 10.5 14 17.5 21 24.5 28 LED’s 2 34 567 8 50mV 50mA Electrolytic 94µF 47µF DMAX DMAX Ceramic 40µF 20µF 40µF 20µF 20µF 100mV 100mA Electrolytic 94µF Ceramic 60µF 60µF 40µF 40µF 40µF 200mV 350mA Electrolytic 94µF 47µF 47µF 47µF ILIM ILIM ILIM Ceramic 60µF 40µF 40µF 40µF 200mV 1A Electrolytic ILIM ILIM ILIM ILIM ILIM ILIM ILIM Ceramic TABLE 3. BOOST MODE COMPENSATION 6V OPERATION VFB IOUT VOUT (V) 7 10.5 14 17.5 21 24.5 28 LED’s 2 34 567 8 50mV 50mA Electrolytic 94µF 47µF Ceramic 40µF 20µF 40µF 20µF 20µF 20µF 20µF 100mV 100mA Electrolytic 141µF 47µF Ceramic 60µF 60µF 60µF 40µF 40µF 40µF 40µF 200mV 350mA Electrolytic 141µF 47µF 47µF Ceramic 60µF 60µF 40µF 60µF 40µF 40µF 40µF 200mV 1A Electrolytic 94µF 47µF ILIM ILIM ILIM ILIM ILIM Ceramic 40µF 40µF ISL78100 |
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