LINEAR TECHNOLOGY LTC3100 Technical data
L DESIGN IDEAS
LOAD CURRENT (mA)
30
EFFICIENCY (%)
50
80
0.01 10 100 1000
0
10
0.1 1
100
70
90
40
60
20
VIN = 1.2V
3.3V OUTPUT
VIN = 2.4V
VIN = 5V USB
SWBST V
INBKVBST
LTC3100
FBBST
1.07M
324k
301k
4.7µF
20k
4.7µF
4.7µF
V
LDO
V
INBST
FBLDO
SWBK
PGBK
PGBST
V
BATT
0.9V TO
3.3V
USB
INPUT
FBBK
200k
100k
4.7µF
MODE
RUNBST
RUNLDO
RUNBK GND
10µH
1.8V AT 50mA
VLDO
V
OUT
64.9k
3.3µH
MBR0520
2.2µF
3.3V AT: 100mA FOR V
BATT
= 1.2V
300mA FOR V
BATT
= 2.4V
250mA FOR USB INPUT
Multi-Rail DC/DC Converter in
a 3mm × 3mm QFN Takes
Inputs as Low as 0.7V
Introduction
Modern handheld instrumentation,
portable medical devices and consumer electronics demand a multitude
of power rails for internal processors,
memory, audio and color displays.
Popular battery technologies for these
devices include single or multiple
cell alkaline, NiMH or Li-Poly/Li-Ion
batteries. Operation from a USB port
or a wall adapter is another common
trait. Replaceable alkaline batteries
are particularly attractive for remote
locations and portable medical devices
where power or time is not available
to recharge batteries. The challenge
is to create a compact and efficient
power solution for these wide VIN
range, multi-output applications.
The LTC3100 multichannel DC/DC
converter makes it easier to meet this
challenge.
The LTC3100 is a high efficiency,
1.5MHz multichannel DC/DC converter in a compact 3mm × 3mm ×
0.75mm QFN package. It features a
synchronous step-up (boost) DC/DC
converter, a synchronous step-down
(buck) DC/DC converter and a 100mA
low dropout linear regulator (LDO).
The boost and buck converters can
operate independently from different sources, from the same source,
or cascaded to create a buck-boost
converter. Internal loop compensation
simplifies the design and minimizes
external component count and solution size. Each converter has a
Power Good indicator that is useful
for voltage sequencing. The boost
converter offers up to 95% efficiency
and features true output disconnect,
a 700mA minimum current limit and
can start with input voltages as low
as 700mV, making it ideal for single
alkaline or NiMH cell applications.
The buck converter offers up to 94%
efficiency and can deliver 250mA or
more from input voltages between 1.8V
32
32
Figure 1. Multiple-input source 3.3V and 1.8V converter
and 5.25V. Both converters feature
automatic Burst Mode® operation for
high efficiency at light loads. For low
noise applications, Burst Mode operation can be disabled by grounding the
MODE pin. The 100mA LDO, whose
input is internally connected to the
boost output, can be used to produce
a third, low noise output. It can also
be used for voltage sequencing of the
boost output voltage.
Figure 2. Converter efficiency
for the circuit of Figure 1
by Dave Salerno
The circuit shown in Figure 1 takes
advantage of the LTC3100’s ability
to operate from independent input
sources to generate multiple outputs
regardless of which power source is
available. In this example, the boost
converter produces a regulated 3.3V
output from a single- or dual-cell input
voltage. The buck converter runs from
a 5V USB or wall adapter input and
produces 3.3V as well, with its output
externally tied to that of the boost.
By leaving the MODE pin open,
the automatic Burst Mode operation
feature of the LTC3100 is enabled,
maximizing battery life at light load.
When USB or wall adapter power is
available, the buck converter is automatically enabled and generates a
3.3V output that is set 3.8% higher
than that of the boost converter. This
puts the boost converter in sleep mode,
reducing the load on the batteries to
just a few micro-amps. The result is
a 3.3V output that seamlessly transitions from battery power to USB
continued on page 37
Linear Technology Magazine • June 2008
DESIGN IDEAS L
V
CAP
20V/DIV
400µs/DIVVIN = 3.6V
FIGURE 1
APPLICATION CIRCUIT
I
L
200mA/DIV
V
SW
20V/DIV
LEDs DISCONNECTED
AT THIS INSTANT
1ms/DIV
V
CORE
, 1V/DIV
V
BST
, 1V/DIV
V
I/O
, 1V/DIV
SWBST V
INBKVBST
LTC3100
FBBST
3.3µH
3.5V
1M
523k
10µF
C
IN
2.2µF
V
LDO
V
INBST
FBLDO
SWBK
PGBK
PGBST
115k
25.5k
V
BATT
0.9V TO 1.5V
2.2µF
FBBK
1M
1M
10µF
MODE
RUNBST
RUNLDO
RUNBK GND
FF EN_BURST
OFF ON
3.3µH
3.3V AT 50mA
V_I/O
120mA AT V
BATT
= 0.9V
220mA AT V
BATT
= 1.2V
V_CORE = 1.2V
BOOST_GOOD
BUCK_GOOD
1M 1M
+
shows an application where a single
LED is driven from a 5V supply.
Output Disconnect
The LT3593 has an internal disconnect
switch that is used to sense the LED
current during normal operation. This
internal switch also serves to provide
output disconnect during shutdown so
that the LEDs are truly disconnected
from the output of the regulator.
Fault Protection
The LT3593 protects against both
open and shorted LED faults. In the
case of an open LED fault, the output
voltage V
V
reaches 38V, an open fault is
CAP
triggered and the part goes into a low
frequency mode clamping the output
to 38V and minimizing input current.
LTC3100, continued from page 32
power while maximizing battery life.
The diode on the USB input prevents
any reverse current from the 3.3V
output (while operating on batteries) back to the USB input when it is
open or grounded. Figure 2 shows the
converter efficiency versus load with
various input sources, illustrating the
high efficiency over a wide load range.
The LDO in the LTC3100 (with its input
internally tied to the Boost output)
provides a second regulated output,
in this case programmed to 1.8V.
continues to rise. Once
CAP
A waveform showing the LT3593’s
response to an open LED fault can be
seen in Figure 4.
In a shorted LED fault, the LED pin
can be shorted to ground, running
excessive current from V
. To protect
CAP
from such a fault, the LT3593 limits
the maximum current out of the LED
pin to approximately 45mA.
Conclusion
The LT3593 is a step-up LED driver
that can drive up to ten white LEDs
from a single lithium-ion cell. It can
easily be programmed through a single
pin interface and combines many
desirable features as well as fault
protection against open or shorted
LEDs.
The feature-rich LT3593 is available
in the 6-lead (2mm × 2mm) DFN as
Because the buck converter input
can come from the boost output, the
LTC3100 can function as an ultra-low
voltage buck-boost converter, providing a regulated 1.2V output from a
single alkaline or NiMH cell. This is
shown in Figure 3, where the LTC3100
generates two regulated outputs from
a single cell input (whose voltage may
be above or below 1.2V) by boosting VIN
up to 3.5V and then regulating down
to 1.2V and 3.3V using the buck and
the LDO. In this example, the Power
Good outputs and the LDO are used
well as the 6-lead SOT-23. These two
small, low profile packages, together
with internal compensation and an
output disconnect device, are ideal
for a complete small board area LED
driver solution, especially in portable
device display backlighting applications.
L
Figure 4. Open LED fault protection
to provide voltage sequencing, so that
the 1.2V core supply comes up before
the 3.3V I/O supply, as shown in the
scope photo of Figure 4. The LDO also
provides additional noise filtering and
ripple rejection for the 3.3V output,
guaranteeing a low noise output for
sensitive analog circuitry, even when
the converters are in Burst Mode
operation.
Conclusion
The LTC3100 is a high efficiency, multichannel converter that can operate
from a wide range of voltage sources.
Independent input voltages for each
converter, Power Good outputs and an
LDO make the LTC3100 a small, highly
integrated and flexible solution for
many demanding applications.
L
Linear Technology Magazine • June 2008
Figure 3. Single-cell dual output converter with voltage sequencing
Figure 4. Voltage sequencing of the output
voltages for the circuit of Figure 3
3737
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