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Supercapacitor-Based Power Backup Prevents Data Lossin RAID Systems – Design Note 487Jim DrewIntroductionRedundant arrays of independent disks, or RAID, systems,by nature are designed to preserve data in the faceof adverse circumstances. One example is power failure,thereby threatening data that is temporarily stored involatile memory. To protect this data, many systemsincorporate a battery-based power backup that suppliesshort-term power—enough watt-seconds for the RAIDcontroller to write volatile data to nonvolatile memory.However, advances in fl ash memory performance suchas DRAM density, lower power consumption and fasterwrite time, in addition to technology improvements insupercapacitors such as lower ESR and higher capacitanceper unit volume, have made it possible to replacethe batteries in these systems with longer lasting, higherperformance and “greener” supercapacitors. Figure 1shows a supercapacitor-based power backup systemusing the LTC ® 3625 supercapacitor charger, an automaticpower crossover switch using the LTC4412 PowerPathcontroller and an LTM ® 4616 dual output μModule ® DC/DC converter.The LTC3625 is a high effi ciency supercapacitor chargerideal for small profi le backup in RAID applications. Itcomes in a 3mm × 4mm × 0.75mm 12-lead DFN packageand requires few external components. It featuresa programmable average charge current up to 1A,automatic cell voltage balancing of two series-connected5V10μF294k100kV IN V OUTEN SW1LTC3625PFI SW2CTL V MIDV SEL GNDGNDPROGPFOL1 3.3μHL2 3.3μHC TOP360FC BOT360FLTC4412V IN SENSEGND GATECTL STATsupercapacitors and a low current state that draws lessthan 1μA from the supercapacitors.Backup Power ApplicationsAn effective power backup system incorporates a supercapacitorstack that has the capacity to support a completedata transfer. A DC/DC converter takes the output of thesupercapacitor stack and provides a constant voltageto the data recovery electronics. Data transfer must becompleted before the voltage across the supercapacitorstack drops to the minimum input operating voltage (V UV )of the DC/DC converter.To estimate the minimum capacitance of the supercapacitorstack, the effective circuit resistance (R T ) needs to bedetermined. R T is the sum of the ESR of the supercapacitors,distribution losses (R DIST ) and the R DS(ON) of theautomatic crossover’s MOSFETs:R T = ESR + R DIST + R DS(ON)Allowing 10% of the input power to be lost in R T at V UV ,R T(MAX) may be determined:R T(MAX) = 2P INL, LT, LTC, LTM, μModule, Linear Technology and the Linear logo are registeredtrademarks and PowerPath is a trademark of Linear Technology Corporation. All othertrademarks are the property of their respective owners.Q2Si4421DYQ1Si4421DY470kV IN1 V OUT1V IN2 FB122μF LTM4616GND ITHM1V OUT2FB2ITHM2GND4.78k10kDN4JDC OUT1100μFC OUT2100μF21.8V1.2VR PROG78.7kL1, L2: COILCRAFT MSS7341-332NLCTOP, CBOT: NESSCAP ESHSR-0360C0-002R7AFigure 1. Supercapacitor Energy Storage System for Data Backup02/11/487