Dec 2005 Single Cell Step-Up DC/DC Converter Features 400mA Switch Current in an SC70 Package

DESIGN IDEAS
Single Cell Step-Up DC/DC Converter
Features 400mA Switch Current in an
by Dave Salerno
SC70 Package
Introduction
Small But FullFeatured Solution
Despite its diminutive SC70 package, the LTC3525 includes many
sophisticated features, such as:
100
LTC3525-3.3
90
100
3.0VIN
90
2.4VIN
80
EFFICIENCY (%)
The LTC3525 raises the bar for boost
converter performance and power
capability in an SC70 package. It
is an inductor-based synchronous
step-up (boost) DC/DC converter that
operates from input voltages as low as
1V, boosting them to 3.3V or 5V. Its
powerful internal 400mA switch allows
the LTC3525 to deliver up to 150mA
of load current with efficiency up to
94%. To further save space, it requires
only three external components (two
small ceramic capacitors and a small
inductor), so a complete solution fits
into spaces previously reserved only
for charge pump designs.
The LTC3525-3.3 and LTC3525-5
are both packaged in the 2mm × 2mm
× 1mm SC70 package, and operate
over an input range of 0.8V to 4.5V.
This flexibility makes them suitable for
compact applications powered by 1 to
3 alkaline/NiMH cells, or a single Liion battery. The 3.3V version can even
maintain regulation with input voltages exceeding the output voltage.
1.2VIN
70
60
50
40
30
20
10
0
0.01
Figure 1. Typical application
using the LTC3525-3.3/-5
0.1
1
10
100
LOAD CURRENT (mA)
1k
Figure 2. Efficiency versus
load for the LTC3525-3.3
output disconnect, inrush current
limiting, low output voltage ripple,
synchronous rectification, single
cell capability, anti-ring control and
less than 1µA shutdown current. It
also features overcurrent protection
and thermal shutdown, enabling it
to sustain an indefinite short circuit
without damage.
External component selection is
easy, since most applications require
just a 1µF ceramic input capacitor
for local decoupling, a 10µF ceramic
output filter capacitor and a 10µH inductor (although any value from 4.7 to
15µH can be used). Be sure to use only
X5R or X7R style capacitors, keeping
them close to the pins of the IC.
The LTC3525 is enabled by pulling
the SHDN pin up to any voltage between 1V and 5V, regardless of input
or output voltage.
High Efficiency Over
a Wide Range of Input
Voltages & Load Currents
The LTC3525 uses a proprietary, patent pending technique of adaptively
adjusting peak inductor current as
a function of load and input voltage.
This technique provides optimum
efficiency at light to medium loads,
while enabling it to supply heavier load
currents that are beyond the capability
of other solutions of this size.
The LTC3525’s low quiescent current of only 7µA on VOUT allows it to
maintain impressive efficiency down
to extremely light loads, as shown in
Figure 2, and over a broad range of
input voltages. By comparison, the
efficiency of a charge pump design
varies widely as the battery voltage
changes, as illustrated in the graph in
EFFICIENCY (%)
80
6.8µH*
70
60
CHARGE PUMP
50
1V to 1.6V
3
40
1
30
20
10
0
2
1µF
IOUT = 20mA
0.5
1
1.5
2
VIN (V)
2.5
3
LTC3525-3.3
VIN
SW
SHDN
VOUT
GND
GND
6
4
5
VOUT
3.3V
60mA
10µF**
6.3V
3.5
Figure 3. Comparison of efficiency versus
input voltage for LTC3525-3.3 and and an
equivalent charge pump based boost circuit
Linear Technology Magazine • December 2005
* COILCRAFT LPO3310-682MXD
** MURATA GRM219R60J106KE19D
Figure 4. Single cell to 3.3V converter delivers 60mA of load current in a 1mm profile
41
DESIGN IDEAS
L1*
10µH
VIN
Li-Ion
OFF ON
1µF
IOUT = 100mA
LTC3525-3.3
3V to 4.5V
50mV/DIV
SW
SHDN
VOUT
GND
GND
VOUT
5V
175mA
IOUT = 10mA
22µF**
6.3V
10µs/DIV
Figure 6. Output voltage ripple of the
5V converter at min and max load
* MURATA LQH32CN100K53
** MURATA GRM31CR60J226KE19L
Figure 5. Li-ion to 5V converter delivers 175mA of load current with <0.5% ripple
Single Cell to 3.3V
Converter with 1mm Profile
A single alkaline or nickel cell to 3.3V
converter, using the LTC3525-3.3, is
shown in Figure 4. This application
uses an inductor and output capacitor chosen to achieve a 1mm profile.
It delivers 60mA of load current from
a single cell, and 140mA from two
cells, while fitting into a 5mm × 7mm
footprint. The ability of the converter
to operate with input voltages below
1V allows it to use all the available
energy in the battery, and also prevents
the converter from shutting off in the
event that a load transient causes a
momentary drop in input voltage.
LTC4306, continued from page 31
that ENABLE remains at a constant
logic low while all other pins are connecting, so that the LTC4306 remains
in its default high impedance state
and ignores connection transients on
SDAIN and SCLIN during connection.
In addition, make the ALERT# connector pin shorter than the VCC pin,
so that VCC establishes solid contact
with the I/O card pull-up supply pin
and powers the pull-up resistors on
ALERT1#–ALERT4# before ALERT#
42
Li-ion/3-Cell to 5V Converter
Delivers Over 175mA with
Low Output Ripple
The LTC3525 has been designed for
very low output ripple with minimal
output capacitance. In most applications, a 10µF ceramic capacitor will
yield less than 1% peak-to-peak output
ripple. By using a 22µF capacitor, the
output ripple can be reduced to less
than 0.5% of VOUT, making it suitable
for many noise sensitive applications
that previously required a larger, more
expensive fixed frequency converter.
The circuit in Figure 5, which occupies a space of just 6mm × 6mm,
supplies 5V at 175mA or more from
a Li-ion battery (or three alkaline
or nickel cells). With a 22µF output
capacitor, the output ripple is only
22mVP-P at light load, and less than
50mVP-P at full load, as shown in
Figure 6. The efficiency peaks at 93%
and remains above 85% over three
decades of load current, as shown in
Figure 7. This solution could also be
used to provide 5V at 200mA in a 3.3V
powered system. The entire solution
fits in a 1.8mm profile.
makes contact. When disconnecting,
ENABLE breaks contact first, resetting
the LTC4306 to its default state, so
that it causes minimal disturbance
on the SDAIN and SCLIN bus as the
card disconnects.
Conclusion
The LTC4306 eases the practical
design issues associated with large
2-wire bus systems. It serves as a multiplexer to provide nested addressing.
It disconnects buses when they are
90
3.6VIN
80
EFFICIENCY (%)
Figure 3. Note that the charge pump
design requires an input voltage of at
least 1.7V to generate a regulated 3.3V
output. Comparable inductor-based
solutions require larger packages and
more external components, making
them unsuitable to applications where
board space is at a premium, or too
expensive for cost sensitive applications.
100
70
60
50
40
30
20
10
0
0.01
0.1
1
10
100
LOAD CURRENT (mA)
1k
Figure 7. Efficiency versus load
for the Li-ion to 5V converter
Conclusion
Many of today’s battery powered
portable devices, such as MP3 players, medical instruments and digital
cameras can benefit from the small
size, simplicity and extended battery life offered by the LTC3525. Its
tiny, low profile SC70 package and
minimal external part count make it
a viable, high performance alternative
to less efficient charge pump designs.
Its 400mA switch current and low
output ripple allow it to replace more
expensive fixed frequency converters
in cost-sensitive applications.
stuck low. It breaks a large capacitive
bus into smaller pieces and allows I/O
cards to be hot-swapped into and out
of live systems. It logs faults, reports to
the master, and works with the master
to resolve faults efficiently.
for
the latest information
on LTC products,
visit
www.linear.com
Linear Technology Magazine • December 2005
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