LINEAR Supercapacitor-Based Power Backup for RAID Systems - Design Note 487 by Jim Drew Owner's Manual User Guide

Supercapacitor-Based Power Backup Prevents Data Loss in RAID Systems – Design Note 487 Jim Drew Introduction supercapacitors and a low current state that draws less

Redundant arrays of independent disks, or RAID, sys- than 1μA from the supercapacitors.

tems, by nature are designed to preserve data in the face

Backup Power Applications

of adverse circumstances. One example is power failure,

An effective power backup system incorporates a superca-

thereby threatening data that is temporarily stored in

pacitor stack that has the capacity to support a complete

volatile memory. To protect this data, many systems

data transfer. A DC/DC converter takes the output of the

incorporate a battery-based power backup that supplies

supercapacitor stack and provides a constant voltage

short-term power—enough watt-seconds for the RAID

to the data recovery electronics. Data transfer must be

controller to write volatile data to nonvolatile memory.

completed before the voltage across the supercapacitor

However, advances in fl ash memory performance such

stack drops to the minimum input operating voltage (VUV)

as DRAM density, lower power consumption and faster

of the DC/DC converter.

write time, in addition to technology improvements in

supercapacitors such as lower ESR and higher capaci- To estimate the minimum capacitance of the supercapaci- tance per unit volume, have made it possible to replace tor stack, the effective circuit resistance (RT) needs to be the batteries in these systems with longer lasting, higher determined. RT is the sum of the ESR of the supercapaci- performance and “greener” supercapacitors. Figure 1 tors, distribution losses (RDIST) and the RDS(ON) of the shows a supercapacitor-based power backup system automatic crossover’s MOSFETs:

using the LTC®3625 supercapacitor charger, an automatic

RT = ESR + RDIST + RDS(ON)

power crossover switch using the LTC4412 Power Path™

controller and an LTM®4616 dual output μModule® DC/ Allowing 10% of the input power to be lost in RT at VUV, DC converter. RT(MAX) may be determined:

The LTC3625 is a high effi ciency supercapacitor charger 0.1• VUV2

RT(MAX)

ideal for small profi le backup in RAID applications. It

comes in a 3mm × 4mm × 0.75mm 12-lead DFN pack-

L, LT, LTC, LTM, μModule, Linear Technology and the Linear logo are registered

age and requires few external components. It features

trademarks and Power Path is a trademark of Linear Technology Corporation. All other a programmable average charge current up to 1A, trademarks are the property of their respective owners.

automatic cell voltage balancing of two series-connected

Q2 Si4421DY

5V VIN1 VOUT1 1.8V

COUT1

VIN2 FB1

22μF LTM4616 4.78k

Si4421DY GND ITHM1

VIN VOUT 1.2V

VOUT2

COUT2

294k EN SW1 CTOP FB2 100μF

LTC3625 L2 3.3μH 360F

PFI SW2 LTC4412

10μF CBOT

CTL VMID VIN SENSE ITHM2

100k VSEL GND GND GATE 470k

DN4JD

GND CTL STAT PFO PROG

RPROG L1, L2: COILCRAFT MSS7341-332NL 78.7k CTOP, CBOT: NESSCAP ESHSR-0360C0-002R7A

Figure 1. Supercapacitor Energy Storage System for Data Backup