DN487 - Supercapacitor-Based Power Backup Prevents Data Loss in RAID Systems

Supercapacitor-Based Power Backup Prevents Data Loss
in RAID Systems – Design Note 487
Jim Drew
Introduction
Redundant arrays of independent disks, or RAID, systems, by nature are designed to preserve data in the face
of adverse circumstances. One example is power failure,
thereby threatening data that is temporarily stored in
volatile memory. To protect this data, many systems
incorporate a battery-based power backup that supplies
short-term power—enough watt-seconds for the RAID
controller to write volatile data to nonvolatile memory.
However, advances in flash memory performance such
as DRAM density, lower power consumption and faster
write time, in addition to technology improvements in
supercapacitors such as lower ESR and higher capacitance per unit volume, have made it possible to replace
the batteries in these systems with longer lasting, higher
performance and “greener” supercapacitors. Figure 1
shows a supercapacitor-based power backup system
using the LTC®3625 supercapacitor charger, an automatic
power crossover switch using the LTC4412 PowerPath™
controller and an LTM®4616 dual output μModule® DC/
DC converter.
The LTC3625 is a high efficiency supercapacitor charger
ideal for small profile backup in RAID applications. It
comes in a 3mm × 4mm × 0.75mm 12-lead DFN package and requires few external components. It features
a programmable average charge current up to 1A,
automatic cell voltage balancing of two series-connected
supercapacitors and a low current state that draws less
than 1μA from the supercapacitors.
Backup Power Applications
An effective power backup system incorporates a supercapacitor stack that has the capacity to support a complete
data transfer. A DC/DC converter takes the output of the
supercapacitor stack and provides a constant voltage
to the data recovery electronics. Data transfer must be
completed before the voltage across the supercapacitor
stack drops to the minimum input operating voltage (VUV )
of the DC/DC converter.
To estimate the minimum capacitance of the supercapacitor stack, the effective circuit resistance (RT ) needs to be
determined. RT is the sum of the ESR of the supercapacitors, distribution losses (RDIST ) and the RDS(ON) of the
automatic crossover’s MOSFETs:
RT = ESR + RDIST + RDS(ON)
Allowing 10% of the input power to be lost in RT at VUV,
RT(MAX) may be determined:
R T(MAX) =
0.1• VUV 2
PIN
L, LT, LTC, LTM, μModule, Linear Technology and the Linear logo are registered
trademarks and PowerPath is a trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
Q2
Si4421DY
5V
VIN
294k
EN
VOUT
SW1
LTC3625
CTL
SW2
VMID
VSEL
GND
PFI
10μF
100k
22μF
Q1
Si4421DY
CTOP
360F
CBOT
360F
VOUT1
VIN2
FB1
LTM4616
GND
L1 3.3μH
L2 3.3μH
VIN1
4.78k
ITHM1
VOUT2
FB2
10k
LTC4412
VIN SENSE
GND GATE
470k
ITHM2
GND
DN4JD
GND
CTL
PROG
STAT
PFO
RPROG
78.7k
L1, L2: COILCRAFT MSS7341-332NL
CTOP, CBOT: NESSCAP ESHSR-0360C0-002R7A
Figure 1. Supercapacitor Energy Storage System for Data Backup
02/11/487
1.8V
COUT1
100μF
COUT2
100μF
w2
1.2V
The voltage required across the supercapacitor stack
(VC(UV)) at VUV:
VC(UV ) =
VUV 2 +PIN •R T
VUV
The minimum capacitance (CMIN) requirement can now
be calculated based on the required backup time (tBU) to
transfer data into the flash memory, the initial stack voltage
(VC(O)) and (VC(UV)):
CMIN =
2 •PIN • tBU
VC(O)2 – VC(UV )2
CMIN is half the capacitance of one supercapacitor. The
ESR used in the expression for calculating RT is twice
the end-of-life ESR. End of life is defined as when the
capacitance drops to 70% of its initial value or the ESR
doubles.
The Charge Profile into Matched SuperCaps graph in the
LTC3625 data sheet shows the charge profile for two
configurations of the LTC3625 charging a stack of two
10F supercapacitors to 5.3V with RPROG set to 143k. This
graph, combined with the following equation, is used to
determine the value of RPROG that would produce the
desired charge time for the actual supercapacitors in the
target application:
RPROG = 143k •
10F
C ACTUAL
5.3V – VC(UV ) tRECHARGE
•
•
VOUT – VC(UV ) tESTIMATE
VC(UV) is the minimum voltage of the supercapacitors
at which the DC/DC converter can produce the required
output. VOUT is the output voltage of the LTC3625 in the
target application (set by VSEL pin). tESTIMATE is the time
required to charge from VC(UV) to the 5.3V, as extrapolated
from the charge profile curves. tRECHARGE is the desired
recharge time in the target application.
Design Example
For example, say it takes 45 seconds to store the data
in flash memory where the input power to the DC/DC
converter is 20W, and the VUV of the DC/DC converter
is 2.7V. A tRECHARGE of ten minutes is desired. The full
charge voltage of the stack is set to 4.8V—a good compromise between extending the life of the supercapacitor
and utilizing as much of the storage capacity as possible.
The components of RT are estimated: RDIST = 10mΩ, ESR
= 20mΩ and RDS(ON) = 10mΩ.
The resulting estimated values of RT(MAX) = 36mΩ and
RT = 40mΩ are close enough for this stage of the design.
Data Sheet Download
www.linear.com
VC(UV) is estimated at 3V. CMIN is 128F. Two 360F capacitors provide an end-of-life capacitance of 126F and ESR
of 6.4mΩ. The crossover switch consists of the LTC4412
and two P-channel MOSFETs. The RDS(ON), with a gate
voltage of 2.5V, is 10.75mΩ (max). An RT of 26.15mΩ
is well within RT(MAX). The value for RPROG is estimated
at 79.3k. The nearest standard 1% resistor is 78.7k. The
data sheet suggests a 3.3μH value for both the buck and
boost inductors.
The LTC3625 contains a power-fail comparator, which is
used to monitor the input power to enable the LTC4412.
A voltage divider connected to the PFI pin sets the power
fail trigger point (VPF ) to 4.75V.
Figure 2 shows the actual backup time of the system with a
20W load. The desired backup time is 45 seconds, whereas
this system yields 76.6 seconds. The difference is due
to a lower RT than estimated and an actual VUV of 2.44V.
Figure 3 shows the actual recharge time of 685 seconds
compared to the 600 seconds used in the calculation, a
difference due to the lower actual VUV.
1.8VOUT
1.2VOUT
VSCAP
VIN
20s/DIV
DN4JD F02
Figure 2. Supercapacitor Backup Time Supporting
a 20W Load
1.8VOUT
1.2VOUT
VSCAP
VIN
200s/DIV
DN4JD F03
Figure 3. Recharge Time After Backup
Conclusion
Supercapacitors are replacing batteries to satisfy green
initiative mandates for data centers. The LTC3625 is an
efficient 1A supercapacitor charger with automatic cell
balancing that can be combined with the LTC4412 low
loss PowerPath controller to produce a backup power
system that protects data in storage applications.
For applications help,
call (978) 656-3768
Linear Technology Corporation
dn487f LT/AP 0211 226k • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2011
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
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