201303.pdf 36567KB Mar 22 2013 09:11:22 PM

Supercapacitor-Based Power Backup System Protects Volatile
Data in Handhelds when Power Is Lost
Design Note 498
Jim Drew
Introduction
Handheld electronic devices play a key role in our everyday
lives. Because dependability is paramount, handhelds are
carefully engineered with lightweight power sources for
reliable use under normal conditions. But no amount of
careful engineering can prevent the mistreatment they
will undergo at the hands of humans. For example, what
happens when a factory worker drops a bar code scanner,
causing the battery to pop out? Such events are
electronically unpredictable, and important data stored
in volatile memory would be lost without some form of
safety net—namely a short-term power holdup system
that stores suffi cient energy to supply standby power until
the battery can be replaced or the data can be stored in
permanent memory.
Supercapacitors are compact, robust, reliable and can
support the power requirements of a backup system for
short-term power-loss events. Like batteries, they require
careful charging and power regulation at the output. The
LTC ® 3226 is a 2-cell series supercapacitor charger with a
PowerPath controller that simplifies the design of backup
systems. Specifically, it includes a charge pump supercapacitor
charger with programmable output voltage and
automatic cell voltage balancing, a low dropout regulator and
a power-fail comparator for switching between normal and
backup modes. Low input noise, low quiescent current and
a compact footprint make the LTC3226 ideal for compact,
handheld, battery-powered applications. The device comes
in a 3mm × 3mm 16-lead QFN package.
Backup Power Application
Figure 1 shows a power holdup system that incorporates
a supercapacitor stack with the capacity to provide
standby power of 165mW for about 45 seconds in the
absence of battery power. An LDO converts the output
of the supercapacitor stack to a constant voltage supply
during backup mode.
L, LT, LTC, LTM, 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.
Q1
FDC604P
V IN
13
V IN
LTC3226EUD
GATE
5
1
V OUT
V OUT
V SB
C1
4.7μF
6.3V
X5R
0603
R1
191k
R2
121k
C2
2.2μF
10V
X5R
0603
R3
100k
3
PFI
15
C +
12
C –
8
EN_CHG
9
PROG
R8
348k
GND
17
CPO
16
14
V MID
10
CPO_FB
4
LDO_FB
6
RST_FB
7
RST
2
PFO
11
CAPGOOD
D2
D1N4148
C3
0.1μF
16V
X7R
C4
0.1μF
16V
X7R
+
+
*C5
3F
2.7V
*C6
3F
2.7V
R4
3.83M
R5
1.21M
R6
255k
R7
80.6k
R9
475k
C7
47μF
6.3V
X5R
1206
20%
R10
1.07M
*NESS CAP ESHSR-0003C0-002R7
3
R11
154k
+
4
LT6703HV-3
2
1
dn498 F02
R12
475k
Figure 1. A Typical Power Backup System Using Supercapacitors
01/12/498