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LTC3127 Datasheet(PDF) 13 Page - Linear Technology |
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LTC3127 Datasheet(HTML) 13 Page - Linear Technology |
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13 / 20 page  LTC3127 3127f applicaTions inForMaTion Output and Input Capacitor Selection When selecting output capacitors for large pulsed loads, the magnitude and duration of the pulse current, together with the droop voltage specification, determine the choice of the output capacitor. Both the ESR of the capacitor and the charge stored in the capacitor each cycle contribute to the output voltage droop. The droop due to the charge is approximately: V I V I V I DROOP LOAD PULSE IN IN MAX OUT STAN _ ( ) • • = − − h DDBY OUT D T C V • • ( ) where IPULSE = pulsed load current ISTANDBY = static load current in standby mode IIN(MAX) = programmed input current limit in amps T = period of the load pulse D = load pulse’s duty cycle VDROOP = amount the output falls out of regulation in volts h = the efficiency of the converter at the input current limit point The preceding equation is a worst-case approximation assuming all the pulsing energy comes from the output capacitor. Thedroopduetothecapacitorequivalentseriesresistance (ESR) is: V I V I V I DROOP ESR PULSE IN IN MAX OUT STAND _ ( ) • • = − − h BBY ESR V • ( ) Low ESR and high capacitance are critical to main- taining low output droop. Table 2 and the Typical Applica- tions schematics show a list of several reservoir capacitor manufacturers. The total output voltage droop is given by: VDROOP = VDROOP_LOAD + VDROOP_ESR (V) HighcapacitancevaluesandlowESRcanleadtoinstability in typical internally compensated buck-boost convert- ers. Using proportional compensation, the LTC3127 is stable with low ESR output capacitor values greater than 1000µF. Multilayer ceramic capacitors are an excellent choice for input decoupling of the step-up converter as they have extremely low ESR and are available in small footprints. Input capacitors should be located as close as possible to the device. While a 10µF input capacitor is sufficient for most applications, larger values may be used to improve input decoupling without limitation. Consult the manufac- turers directly for detailed information on their selection of ceramic capacitors. Although ceramic capacitors are recommended, low ESR tantalum capacitors may be used as well. When using a large capacitance to help with pulsed load applications, the maximum load for a given duty cycle, and the minimum capacitance can be calculated by: I V I D V A C LOAD MAX IN IN MAX OUT OUT MIN ( ) ( ) ( ) ( ) = • • • h == − • • − I V I V I PULSE IN IN MAX OUT STANDBY ( ) h • D T V F DROOP • ( ) Table 2. Capacitor Vendor Information SUPPLIER PHONE WEB SITE Vishay 402-563-6866 www.vishay.com AVX 803-448-9411 www.avxcorp.com Cooper Bussmann 516-998-4100 www.cooperbussmann.com CAP-XX 843-267-0720 www.cap-xx.com Panasonic 800-394-2112 www.panasonic.com |
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