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Linear Technology Magazine V18N3 - September 2008
LINEAR TECHNOLOGY
SEPTEMBER 2008
VOLUME XVIII NUMBER 3
IN THIS ISSUE…
Replace Batteries in
Power Ride-Through
Applications with
Robust Supercaps
and 3mm × 3mm
Capacitor Charger
by Jim Drew
Introduction
Supercapacitors (or ultracapacitors)
are inding their way into an increasing
number of applications for short-
term energy storage and applications
that require intermittent high energy
pulses. One such application is a
power ride-through circuit, in which
a backup energy source cuts in and
powers the load if the main power
supply fails for a short time. This type
of application has been dominated
by batteries in the past, but electric
double layer capacitors (EDLCs) are
fast making inroads as their price-
per-farad, size and effective series
resistance per capacitance (ESR/C)
continue to fall.
In a power ride-through applica-
tion, series-stacked capacitors must
be charged and cell-voltage balanced.
Supercaps are switched into the power
path when needed and the power to
the load is controlled by a DC/DC
converter. The LTC3225 supercapaci-
tor charger has a number of features
that make it a good choice for power
ride-through applications. It comes
in a small, 10-lead 3mm × 3mm DFN
package and features programmable
One advantage
supercapacitors have over
batteries is their long life.
A capacitor’s cycle life is
quoted as greater than
500,000 cycles; batteries
are specified for only a few
hundred cycles. This makes
the supercapacitor an ideal
“set and forget” device,
requiring little or
no maintenance.
charging current, automatic cell volt-
age balancing, low drain current on the
supercapacitors and a patent pending,
low noise, constant current charger.
Supercapacitor
Characteristics
Supercapacitors come in a variety of
sizes, for example a 10F/2.7V super-
cap is available in a 10mm × 30mm
2-terminal radial can with an ESR of
continued on page
L , LT, LTC, LTM, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology
Corporation. Adaptive Power, Bat-Track, BodeCAD, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView,
µModule, Micropower SwitcherCAD, Multimode Dimming, No Latency ΔΣ , No Latency Delta-Sigma, No R SENSE , Operational
Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, TimerBlox, True
Color PWM, UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks
of the companies that manufacture the products.
LINEAR TECHNOLOGY
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L LINEAR IN THE NEWS
Linear in the News…
Linear Technology Analog Channel
Starting this month, you can tune in to the Linear Tech-
nology Analog Channel on the web. The channel kicks off
with a series of video design ideas covering a broad range
of topics from some of the industry’s premier analog gu-
rus. To see the videos, visit the Linear Technology website
at www.linear.com/LTchannel or the EDN website at
videos will be added periodically, so check in often, or sign
up to receive the Linear Insider email to be notiied when
new videos are available at www.linear.com/mylinear .
Check out the following videos, now available online.
drive the thermocouple negative terminal and a low offset
ampliier with enough gain to use the entire input span
of a 12- or 16-bit ADC.
Linear Technology’s LTC2492 greatly simpliies thermo-
couple instrument design. A simple ilter and protection
circuit is all that is required to build a rugged, ready-to-use
meter. Some software tricks take care of cold junction com-
pensation and the thermocouple’s non-linear output.
Investment in Environment Award
Linear Technology’s LTC4151 High Voltage I 2 C Current
and Voltage Monitor has been selected as a inalist for an
E-Legacy Investment in Environment Award by Electronic
Product Design in the UK. This award highlights electronics
companies that are devveloping environmentally respon-
sible products.
In a world where power conservation is increasingly
important, designers need to give greater consideration
to the overall impact and cost of operation of their end
product. Accurately monitoring power consumption pro-
vides information to understand and manage the power
requirements in a high voltage system to avoid wasted
resources.
“Direct Paralleling, High Power Density LDO”
with Robert Dobkin, Vice President, Engineering
and Chief Technical Officer
The LT3080 is a new architecture for linear regulators. It
provides better regulation, a simple output adjustment with
a single resistor where the output can be adjusted down
to zero. Also, this architecture allows easy paralleling of
regulators for “no heat sink” operation in all-surface-mount
applications. The LT3080 video shows circuit operation and
applications for paralleling, spreading the heat, general
purpose power supplies and current sources.
“High Voltage, Low Noise, DC/DC Converters:
A Kilovolt with 100µV of Noise”
with Jim Williams, Staff Scientist
Photomultipliers (PMT), avalanche photodiodes (APD),
ultrasonic transducers, capacitance microphones, radia-
tion detectors and similar devices require high voltage,
low current bias. Additionally, the high voltage must be
pristinely free of noise; well under a millivolt is a common
requirement with a few hundred microvolts sometimes
necessary. The video details circuits featuring outputs
from 200V to 1000V with output noise below 100µV
measured in a 100MHz bandwidth. Special techniques
enable this performance, most notably power stages opti-
mized to minimize high frequency harmonic content. An
additional aid to achieving low noise is that load currents
rarely exceed 5mA. This freedom permits output ilter-
ing methods that are usually impractical. A lab-based
circuit noise measurement demonstration concludes the
presentation.
The LTC4151 breaks the mold of traditional current
sense solutions by combining the high voltage capability
of a Hot Swap TM controller with the accuracy of a 12-bit
ADC. The LTC4151 provides a true power measurement
between 7V and 80V, rather than just a current reading
or voltage reading on its own. This is extremely valuable
data for high voltage applications, as it alerts designers to
overloading and power loss in their system. The LTC4151
is ideal for a wide range of applications. By measuring
up to 80V, the LTC4151 can accurately measure indus-
trial, telecom and automotive signals at 48V, and survive
transient surges up to 80V. This power monitor can also
measure 72V systems with ±10% tolerances (79.2V). By
measuring as low as 7V, the LTC4151 can accurately
monitor 10V and 12V industrial systems. L
“A Thermocouple Meter Reference Design
Using the LTC2492 Delta Sigma ADC”
with Mark Thoren, Applications Engineering Manager,
Mixed Signal Products
Thermocouples are perhaps the most common tempera-
ture sensor in use. And while they are extremely simple
and rugged, the output is very small—tens of microvolts
per degree Celsius. Traditionally, thermocouple measure-
ment circuits use a cold junction compensation circuit to
2
Linear Technology Magazine September 2008
2
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DESIGN FEATURES L
LTC225, continued from page
25m Ω while a 350F/2.5V supercapaci-
tor with an ESR of 1.6m Ω is available
in a D-cell battery form factor. One
advantage supercapacitors offer over
batteries is their long life. A capacitor’s
cycle life is quoted as greater than
500,000 cycles; batteries are speciied
for only a few hundred cycles. This
makes the supercapacitor an ideal
“set and forget” device, requiring little
or no maintenance.
Two parameters of the supercapaci-
tor that are critical to an application are
cell voltage and initial leakage current.
Initial leakage current is a misnomer
in that the initial leakage current is
really dielectric absorption current
which disappears after some time. The
manufacturers of supercapacitors rate
their leakage current after 100 hours of
applied voltage while the initial leakage
current in those irst 100 hours may
be as much as 50 times the speciied
leakage current.
The voltage across the capacitor
has a signiicant effect on its oper-
ating life. When used in series, the
supercapacitors must have balanced
cell voltages to prevent over-charging
of one of the series capacitors. Pas-
sive cell balancing, where a resistor
is placed across the capacitor, is a
popular and simple technique. The
disadvantage of this technique is that
the capacitor discharges through the
balancing resistor when the charging
circuit is disabled. The rule of thumb
for this scheme is to set the balancing
resistor to 50 times the worst case
leakage current, estimated at 2µA/
Farad. Given these parameters, a 10F,
2.5V supercapacitor would require a
2.5k balancing resistor. This resistor
would drain 1mA of current from the
supercapacitor when the charging
circuit is disabled.
An alternative is to use a non-dis-
sipative active cell balancing circuit,
such as the LTC3225, to maintain cell
voltage. The LTC3225 presents less
than 4µA of load to the supercapacitor
when in shutdown mode and less than
1µA when input power is removed. The
LTC3225 features a programmable
charging current of up to 150mA,
charging two series supercapacitors
to either 4.8V or 5.3V while balancing
the voltage on the capacitors.
and the circuit distribution resis-
tances.
= +
Assuming 10% of the input power is
lost in the effective circuit resistance
when the DC/DC converter is at the
minimum operating voltage, the worst
case R T is
DIST
R
= 0 1 2
. •
V
P
UV
IN
T MAX
( )
The voltage required across the
supercapacitor at the undervoltage
lockout threshold of the DC/DC con-
verter is;
V
=
V P R
V
2
+
•
C UV
( )
The required effective capacitance
can then be calculated based on the
required ride-through time (T RT ), and
the initial voltage on the capacitor
(V C(0) ) and V C(UV) .
Power Ride-Through
Applications
To provide a constant voltage to the
load, a DC/DC converter is required
between the load and the superca-
pacitor. As the voltage across the
supercapacitor decreases, the current
drawn by the DC/DC converter in-
creases to maintain constant power to
the load. The DC/DC converter drops
out of regulation when its input volt-
age reaches the minimum operating
voltage (V UV ).
To estimate the requirements for
the supercapacitor, the effective circuit
resistance (R T ) needs to be determined.
R T is the sum of the capacitors’ ESRs
C
=
P T
V V
2
0 2
• •
IN RT
EFF
−
2
C
( ) ( )
C UV
The effective capacitance of a series
connected bank of capacitors is the ef-
fective capacitance of a single capacitor
divided by the number of capacitors
while the total ESR is the sum of all
the series ESRs.
The ESR of a supercapacitor
decreases with higher frequency.
Manufacturers usually specify the ESR
Si4421DY
continued on page 2
1.8V
5.0V
D
S
V IN1
V OUT1
G
V IN2
FB1
100 µ F
LTM4616
4.87k
Si4421DY
22 µ F
ITHM1
V OUT2
GND
1.2V
D
S
V IN
C OUT
FB2
100 µ F
100 µ F
+
C+
10F
G
10k
V IN
SENSE
1 µ F
LTC3225
ITHM2
C X
LTC4412
C–
SHDN
+
GND
GATE
470k
10F
GND
2.2 µ F
V SEL
PROG
GND
CTL
STAT
12K
+
= NESSCAP ESHR-0010C-002R7 OR ILLINOIS CAPACITOR 106DCN2R7Q
Figure 1. A 5V power ride-through application
Linear Technology Magazine September 2008
3
R ESR R
T
UV IN T
UV
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L DESIGN FEATURES
New Family of Integrated Power
Controllers Combine Fast Battery
Charging, PowerPath Control and
Efficient DC/DC Converters in
Less Than 20mm 2
by Sam Nork
Introduction
The quickest way to build an eficient
power system for a battery-powered
portable application is to use an
IC that combines all power control
functions into a single chip, namely a
Power Management Integrated Circuit
(PMIC). PMICs seamlessly manage
power low from various power sources
(wall adapters, USB and batteries) to
power loads (device systems and the
charging battery), while maintaining
current limits where required (such
as that speciied for USB). To this
end, PMICs typically feature built-in
PowerPath™ control, DC/DC conver-
USB/WALL
4.35V TO 5.5V
USB COMPLIANT
STEP-DOWN
REGULATOR
TO OTHER
LOADS
CC/CV
BATTERY
CHARGER
0V
OPTIONAL
CURRENT
CONTROL
CHARGE
+
LTC3555/LTC3555-X
T
Li-Ion
ALWAYS ON LDO
3.3V/25mA
RTC/LOW
POWER LOGIC
0.8V TO 3.6V/400mA
TRIPLE
HIGH EFFICIENCY
STEP-DOWN
SWITCHING
REGULATORS
1
MEMORY
ENABLE
CONTROLS
5
0.8V TO 3.6V/400mA
2
3
I/O
0.8V TO 3.6V/1A
RST
2
CORE
PROCESSOR
I 2 C PORT
I 2 C
Figure 1. High efficiency PowerPath manager and triple step-down regulator
Table 1. Power management ICs with Li-ion/polymer battery chargers
PowerPath
Topology
Integrated Converters and Load Current Capabilities
Part Number
Interface
Buck
Buck-Boost
Boost
LDO
Package
LTC3555/-1/-3
Switching
I 2 C
1A, 400mA × 2
25mA
4mm × 5mm
QFN-28
LTC3556
Switching
I 2 C
400mA × 2
1A
25mA
4mm × 5mm
QFN-28
LTC3566
Switching
1A
25mA
4mm × 4mm
QFN-24
LTC3567
Switching
I 2 C
1A
25mA
4mm × 4mm
QFN-24
LTC3586*
Switching
400mA × 2
1A
0.8A
20mA
4mm × 6mm
QFN-38
LTC3557/-1
Linear
600mA, 400mA × 2
25mA
4mm × 4mm
QFN-28
LTC3455
Linear
600mA, 400mA
Controller
4mm × 4mm
QFN-24
LTC3558
400mA
400mA
3mm × 3mm
QFN-20
3mm × 3mm
QFN-16
*For an application of the LTC3586 see “Complete Power Solution for Digital Cameras and Other Complex Compact Portable Applications” in this issue
LTC3559/-1
400mA × 2
4
Linear Technology Magazine September 2008
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DESIGN FEATURES L
sion and battery charging functions.
PMICs can be applied in everything
from consumer electronics such as
MP3 players and Bluetooth headsets
to specialized portable medical and
industrial equipment.
Table 1 shows the wide variety of
integrated charger and DC/DC com-
binations now available from Linear
Technology. The latest additions to
the family, the LTC3555, LTC3556,
LTC3566, LTC3567 and LTC3586, are
primarily targeted toward relatively
high power Li-Ion applications and
contain blocks capable of high efi-
ciency at high current levels. (To see
an application of the LTC3586, see
“Complete Power Solution for Digital
Cameras and Other Complex Compact
Portable Applications” in the Design
Ideas section of this issue.)
The most noteworthy feature of the
new parts is the use of a proprietary
switching PowerPath design, which
improves eficiency over linear power
path or battery fed solutions.
700
BATTERY CHARGE CURRENT
on” capability if the battery is dead or
missing (as long as the load current
is less than the input current limit).
However, neither a linear charger nor
linear power manager is well-suited
for high current charging due to poor
eficiency under certain conditions.
USB is now a common source of
power, but charging/powering from
the USB host is complicated by the
host’s 2.5W limit. To take advantage of
the limited USB power, all components
in the power path must be as eficient
as possible.
A key attribute in these new PMICs
is a battery-tracking (Bat-Track™)
synchronous buck design with logic
programmable input current limit to
ensure USB compatibility. When USB
or adapter power is available, the
LTC35xx power manager generates a
V OUT supply equal to V BAT + 300mV.
The 300mV difference voltage is suf-
icient to keep the battery charger
just out of dropout and deliver the
programmed charge current at high
eficiency. As with linear power manag-
ers, the load current is provided irst,
and current that is left over is directed
to the battery. Input current limit is
controlled via an external resistor to
set absolute current and two logic
pins to control the ratio (e.g. 100mA,
500mA, 1A and Suspend).
Charging eficiency of over 80%
with a completely discharged battery
is achievable vs 60% or so for a linear
charger. Or said another way, the
switching power path dissipates only
50% of the power dissipated by a linear
600
EXTRA CURRENT
FOR FASTER CHARGING
500
500mA USB CURRENT LIMIT
400
300
200
100
V BUS 5V
5X MODE
BATTERY CHARGER PROGRAMMED FOR 1A
0
2.8
3
3.2
3.4
3.6
3.8
4
4.2
BATTERY VOLTAGE V
Figure 2. Switching power manager charge
current vs battery voltage with a 500mA input
current limit. Peak charge current = 700mA.
see the cover article in the June 2008
issue of Linear Technology magazine
titled “Speed Up Li-ion Battery Charg-
ing and Reduce Heat with a Switching
PowerPath Manager.”)
For instance, portable products
with large capacity batteries (1Ahr
plus) face a direct tradeoff between
charge time and charger power dis-
sipation—especially when a linear
charging method is used. At relatively
low charge currents, a linear charger
dissipates a modest amount of power,
but at currents required to quickly
charge high capacity batteries, a linear
charger can dissipate 2W or more.
A switching PowerPath topology is
an improvement over the commonly
used linear PowerPath topology, and
both are an improvement over battery
fed applications. A linear PowerPath
powers the application directly from
an external source rather than from
the battery itself and provides “instant
Switching PowerPath Control
Efficiently Harnesses
Available External Power
To speed up charging, some of Linear’s
new PMICs employ a unique current
limited synchronous buck switch-
ing charger architecture that uses
more power from the USB or adapter
than other topologies. This is a big
improvement over battery fed and
linear PowerPath control schemes.
(For a more detailed description of
the switching PowerPath architecture,
USB/WALL
4.5V TO 5.5V
USB COMPLIANT
STEP-DOWN
REGULATOR
TO OTHER
LOADS
100
I OUT3 50mA
90
CC/CV
BATTERY
CHARGER
80
I OUT3 200mA
0V
OPTIONAL
I OUT3 1000mA
70
60
CHARGE
+
Li-Ion
50
T
LTC3556
40
ALWAYS ON LDO
3.3V/25mA
RTC/LOW
POWER LOGIC
30
20
10
0
0.8V TO 3.6V/400mA
DUAL HIGH EFFICIENCY
BUCS
1
MEMORY
CORE
P
V OUT3 3.3V
T A 27C
ENALL
0.8V TO 3.6V/400mA
2
SEQ
HIGH EFFICIENCY
BUC-BOOST
2.7
3.1
3.5
3.9
4.3
4.7
3
2.5V to 3.3V/1A
V IN3 V
HDD/IO
3
I 2 C
I 2 C PORT
PGOODALL
3556 TA01
Figure 3. 1A buck-boost efficiency vs V IN (LTC3556, LTC3566/7, LTC3586)
Linear Technology Magazine September 2008
5
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