LINEAR TECHNOLOGY LTC3625 Technical data

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

Design Note 487

Jim Drew

Introduction

Redundant arrays of independent disks, or RAID, sys­te ms , b y n at ur e a re d es ig ne d t o p res er ve d at a in th e f ace
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 fl ash 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
power crossover switch using the LTC4412 PowerPath™
controller and an LTM
DC converter.

The LTC3625 is a high effi ciency supercapacitor charger
ideal for small profi le 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

5V

®

3 6 2 5 s u p e r c a p a c i t o r c h a r g e r , a n a u t o m a t i c

®

4616 dual output μModule® DC/

10μF

294k

100k

V

EN

PFI
CTL

V

GND

IN

SEL

V

OUT

SW1

LTC3625

SW2

V

MID

GND

PFO

PROG

R

PROG

78.7k

L1 3.3μH

L2 3.3μH

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

C

TOP

360F

C

BOT

360F

LTC4412

V

IN

GND

CTL

supercapacitors and a low current state that draws less
than 1μA from the supercapacitors.

Backup Power Applications

An ef fective power backup sys tem incorporates a superc a­pacitor 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 inpu t operating voltage ( V
of the DC/DC converter.

To estimate the minimum capacitance of the supercapaci­tor stack, the effective circuit resistance (R
determined. R
tors, distribution losses (R

is the sum of the ESR of the supercapaci-

T

) and the R

DIST

) needs to be

T

of the

DS(ON)

automatic crossover’s MOSFETs:

= ESR + R

R

T

Allowing 10% of the input power to be lost in R
R

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.

Si4421DY

Si4421DY

SENSE

GATE

STAT

may be determined:

T(MAX)

R

T(MAX)

=

Q2

Q1

470k

DIST

0.1• V
P

22μF

IN

+ R

UV

V

V

GND

DS(ON)

2

IN1

IN2

LTM4616

V

OUT1

FB1

ITHM1

V

OUT2

FB2

ITHM2

GND

at VUV,

T

1.8V

C

OUT1

100μF

4.78k

1.2V

C

OUT2

100μF
w2

10k

DN4JD

UV

)

02/11/487

Figure 1. Supercapacitor Energy Storage System for Data Backup

The voltage required across the supercapacitor stack

C(UV)

V

C(UV)

) at VUV:

V

UV

=

2

+ PIN•R
V

UV

T

) requirement can now

MIN

) to

BU

(V

The minimum capacitance (C
be calculated based on the required backup time (t
t r a n s f e r d a t a i n t o t h e fl ash memor y, the initial st ack voltage

) and (V

(V

C(O)

C

=

MIN

C

MIN

V

is half the capacitance of one supercapacitor. The

ESR used in the expression for calculating R

2•P

C(O)

C(UV)

IN•tBU

2

–V

):

C(UV)

2

is twice

T

the end-of-life ESR. End of life is defi ned as when the
capacitance drops to 70% of its initial value or the ESR
doubles.

The Charge Profi le into Matched SuperCaps graph in the
LTC3625 data sheet shows the charge profi le for two
confi gurations of the LTC3625 charging a stack of two
10F supercapacitors to 5.3V with R

set to 143k. This

PROG

graph, combined with the following equation, is used to
determine the value of R

that would produce the

PROG

desired charge time for the actual supercapacitors in the
target application:

R

= 143k •

PROG

V

is the minimum voltage of the supercapacitors

C(UV)

10F

C

ACTUAL

5.3V – V

•
V

OUT–VC(UV)

C(UV)

t

RECHARGE

•
t

ESTIMATE

at which the DC/DC converter can produce the required
output. V
target application (set by V
r e q u i r e d t o c h a r g e f r o m V
from the charge profi le curves. t

is the output voltage of the LTC3625 in the

OUT

pin). t

SEL

C(UV)

ESTIMATE

to the 5.3V, as extrapolated

RECHARGE

is the time

is the desired

recharge time in the target application.

Design Example

For example, say it takes 45 seconds to store the data
in fl ash memory where the input power to the DC/DC
converter is 20W, and the V
is 2.7V. A t

RECHARGE

of ten minutes is desired. The full

of the DC/DC converter

UV

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 R
= 20mΩ and R

are estimated: R

T

= 10mΩ.

DS(ON)

The resulting estimated values of R

= 40mΩ are close enough for this stage of the design.

R

T

Data Sheet Download

www.linear.com

DIST

T(MAX)

= 10mΩ, ESR

= 36mΩ and

V

is estimated at 3V. C

C(UV)

is 128F. Two 360F capaci-

MIN

tors 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 R
voltage of 2.5V, is 10.75mΩ (max). An R
is well within R

. The value for R

T(MAX)

DS(ON)

T

PROG

, with a gate
of 26.15mΩ
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 (V

) to 4.75V.

PF

Figure 2 shows the ac tual backup time of the system wi th a
20W load. The desired b ackup time is 45 seconds, whereas
this system yields 76.6 seconds. The difference is due
to a lower R

than estimated and an actual VUV of 2.44V.

T

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 V

1.8V

OUT

1.2V

OUT

V

SCAP

V

IN

20s/DIV

Figure 2. Supercapacitor Backup Time Supporting
a 20W Load

1.8V

OUT

1.2V

OUT

V

SCAP

V

IN

200s/DIV

Figure 3. Recharge Time After Backup

UV

.

DN4JD F02

DN4JD F03

Conclusion

Supercapacitors are replacing batteries to satisfy green
initiative mandates for data centers. The LTC3625 is an
effi cient 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 [phone]

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone]

●

FAX: [phone] ● www.linear.com

dn487f LT/AP 0211 226k • PRINTED IN THE USA

© LINEAR TECHNOLOGY CORPORATION 2011