EL7801Output Capacitor where The output capacitor acts to smooth the output voltage and V D OUT = -------- in the boost configuration supplies load current directly VIN during the conduction phase of the power switch. Ripple voltage consists of two components, the first due to charging For a low ESR ceramic capacitor, output ripple is dominated and discharging of the capacitor; the second due to IR drop by the charging and discharging of the output capacitor. across the ESR of the capacitor by inductor ripple current. Care should be taken to ensure the voltage rating of the capacitor exceeds the maximum output voltage. In boost mode: I Compensation V O = -------- D × ---- + × RIPPLE I C F LPK ESR The EL7801 employs a direct summing control loop with OUT S current feedback. No error amplifier is used in the system. where: The arrangement provides fast transient response and V – makes use of the output capacitor to compensate the loop. D OUT VIN = ---------------- V The effect of the pole associated with the inductor is OUT minimized by the current feedback. The number LEDs, their and DC bias current and the value of feedback resistor alter loop stability due to their effect on feedback factor which is I (V – ) (1 – D) I O OUT VIN = × --------- LPK ------- + ------------------ heavily influenced by the small signal impedance of the 1 – D 2 × L fs LEDs. Generally, higher numbers of LEDs, lower bias levels In buck mode: and smaller values of feedback resistor will require smaller output capacitors to achieve loop stability. A combination of (V – ) × D V IN VOUT = ------------------------ D × RIPPLE ------------- + ESR low ESR electrolytic and ceramic capacitors may be used to 2 × f × s L f × s COUT reduce implementation costs. TABLE 2. BOOST MODE COMPENSATION. 2.7V OPERATIONVOUT710.51417.52124.528VFBIOUTLED’s234567850mV50mAElectrolytic 94µF 47µF DMAX DMAX Ceramic 40µF 20µF 40µF 20µF 20µF 100mV100mAElectrolytic 94µF Ceramic 60µF 60µF 40µF 40µF 40µF 200mV350mAElectrolytic 94µF 47µF 47µF 47µF ILIM ILIM ILIM Ceramic 60µF 40µF 40µF 40µF 200mV1AElectrolytic ILIM ILIM ILIM ILIM ILIM ILIM ILIM CeramicTABLE 3. BOOST MODE COMPENSATION. 5V OPERATIONVOUT710.51417.52124.528VFBIOUTLED’s234567850mV50mAElectrolytic 94µF 47µF Ceramic 40µF 20µF 40µF 20µF 20µF 20µF 20µF 100mV100mAElectrolytic 141µF 47µF Ceramic 60µF 60µF 60µF 40µF 40µF 40µF 40µF 200mV350mAElectrolytic 141µF 47µF 47µF Ceramic 60µF 60µF 40µF 60µF 40µF 40µF 40µF 200mV1AElectrolytic 94µF 47µF ILIM ILIM ILIM ILIM ILIM Ceramic 40µF 40µF 10 FN7354.0 December 22, 2005
