15V Buck-Boost Converters with Ultralow 1.3µA Quiescent Current are Tailored to Micropower Applications and the Internet of Things

design features
15V Buck-Boost Converters with Ultralow 1.3µA Quiescent
Current are Tailored to Micropower Applications and the
Internet of Things
Dave Salerno
The proliferation of wireless sensors supporting the “Internet of Things” has increased
the need for small, efficient power converters tailored to untethered low power devices.
The new LTC3129 and LTC3129-1 are designed to satisfy this need. The LTC3129
and LTC3129-1 are monolithic buck-boost DC/DC converters with an input voltage
range of 2.42V to 15V. The LTC3129 has an output voltage range of 1.4V to 15.75V,
while the LTC3129-1 offers eight pin-selectable fixed output voltages between 1.8V
and 15V. Both parts can supply a minimum output current of 200mA in buck mode.
Low power sensors can take advantage
the LTC3129’s and LTC3129-1’s zero current
when disabled (on both VIN and VOUT),
and a quiescent current on VIN of just
1.3µ A when power saving Burst Mode®
operation is selected, making them ideal
for µ Power and energy harvesting applications, where high efficiency at extremely
light loads is crucial. Their buck-boost
architecture makes them well suited
to a wide variety of power sources.
PWM mode, an accurate RUN pin threshold to allow the UVLO threshold to be
programmed, a power good output and
an MPPC (maximum power point control)
function for optimizing power transfer
when operating from photovoltaic cells.
The compact 3mm × 3mm QFN package and the high level of integration ease
the LTC3129/LTC3129-1’s placement into
space-constrained applications. Only a
few external components and an inductor, which can be as small as 2mm × 3mm,
are required to complete the power
supply design. Internal loop compensation further simplifies the design process.
Other key features of the LTC3129 and
LTC3129-1 include a fixed 1.2MHz operating frequency, current mode control,
internal loop compensation, automatic
Burst Mode operation or low noise
Figure 1. 3.3V solar powered converter
operates from indoor light
22nF
VOC = 5V
UVLO = 3.5V
VIN
+
CIN
470µF
6.3V
BST1 SW1
4.7µF
SW2 BST2
VOUT
3.3V
VOUT
LTC3129-1
22µF
RUN
VCC
MPPC
VS2
2.26M
VCC
VS3
PWM
GND
PGND
•The RUN pin must exceed 1.22V (typical).
•The VIN pin must exceed 1.9V (typical).
•VCC (which is internally generated from
VIN but can also be supplied externally) must exceed 2.25V (typical).
PGOOD
VS1
10pF
The circuit in Figure 1 exploits the unique
ability of the LTC3129 and LTC3129‑1 to
start up and operate from an input power
source as weak as 7.5 microwatts—making them capable of operating from small
(less than 1in2), low cost solar cells with
indoor light levels less than 200-lux.
This enables such applications as indoor
light powered wireless sensors, where
the DC/DC converter must support an
extremely low average power requirement, due to a low duty cycle of operation, from very low available power, while
consuming as little power as possible.
To make this low current start-up possible,
the LTC3129 and LTC3129-1 draw a meager
two microamps of current (less in shutdown) until three conditions are satisfied:
22nF
VIN
4.22M
PV PANEL
SANYO
AM-1815
4.9cm × 5.8cm
L1
4.7µH
3.3V CONVERTER OPERATES FROM
INDOOR LIGHT USING A SMALL
SOLAR CELL
2.2µF
L1: Toko DEM2812C
Until all three of these conditions are satisfied, the part remains in a “soft-shutdown”
or standby state, drawing just 2µ A.
October 2013 : LT Journal of Analog Innovation | 15
The LTC3129 and LTC3129‑1 can start up and operate from an input power
source as weak as 7.5 microwatts—making them capable of operating from
small (less than 1in2), low cost solar cells with indoor light levels less than 200-lux.
This enables such applications as indoor light powered wireless sensors.
Figure 2. Solar powered converter
with coin cell backup
UVLO = 3.5V
VIN
L1
4.7µH
22nF
4.7µF
BST1 SW1
22nF
SW2 BST2
VOUT
VIN
LTC3129
4.22M
PV PANEL
SANYO AM-1815
VCC
6.3V
4.22M
S1
D1
2.43M
G1
D2
G2
VOUT
2.43M
MPPC
VOUT
3V TO 3.2V
S2
2.2µF
10pF
FB
CR2032
3V COIN CELL
PGOOD
2.26M
10pF
3.20V
22µF
RUN
+ 470µF
FDC6312P
DUAL PMOS
BAT54
VCC
PWM
GND
PGND
74LVC2G04
2.2µF
L1: Toko DEM2812C
This allows a weak input source to
charge the input storage capacitor until
the voltage is high enough to satisfy all
three previously mentioned conditions,
at which point the LTC3129/LTC3129-1
begins switching, and VOUT rises to regulation, provided the input capacitor has
sufficient stored energy. The input voltage at which the part exits UVLO can be
set anywhere from 2.4V to 15V using the
external resistive divider on the RUN pin.
With a RUN pin current of less than
1n A typical, high value resistors may be
used to minimize current draw on VIN .
In the application example shown in
Figure 1, the energy stored on CIN is
used to bring VOUT into regulation once
the converter starts. If the average
power demand on VOUT is less than the
power delivered by the solar cell, the
LTC3129/LTC3129-1 remains in Burst Mode
operation, and VOUT remains in regulation.
If the average output power demand
exceeds the input power available, then
VIN drops until UVLO is reached, at which
16 | October 2013 : LT Journal of Analog Innovation
point the converter reenters soft-shutdown. At this point, VIN begins recharging, allowing the cycle to repeat. In this
hiccup mode of operation, VIN is positioned hysteretically about the UVLO point,
with a VIN ripple of approximately
290mV in this example. This ripple is set
by the 100mV hysteresis at the RUN pin,
gained up by the UVLO divider ratio.
Note that by setting the converter’s
UVLO voltage to the MPP (maximum
power point) voltage for the chosen solar
cell (typically between 70% to 80% of
the open-circuit voltage), the cell always
operates near its maximum power transfer
voltage (unless the average load requirement is less than the power output of
the solar cell, in which case VIN climbs
and remains above the UVLO voltage).
To further optimize efficiency and eliminate unnecessary loading of VOUT, the
LTC3129/LTC3129-1 does not draw any
current from VOUT during soft-start or
at any time if Burst Mode operation is
selected. This prevents the converter from
discharging VOUT during soft-start, thereby
preserving charge on the output capacitor. In fact, when the LTC3129 is sleeping,
there is no current draw at all on VOUT. In
the case of the LTC3129-1, the VOUT current draw is sub-microamp, due to the
high resistance internal feedback divider.
ADDING A BATTERY BACKUP
In many solar powered applications, a
backup battery provides power when
solar power is insufficient. Figure 2 shows
an application where a primary lithium
coin cell and a few external components
have been added to the converter from
the previous example to provide backup
power to the output in the event that
the light source is unable to provide the
necessary power to maintain VOUT. The
LTC3129 is used in this case, allowing
VOUT to be programmed for 3.2V to better match the voltage of the coin cell.
In this example, the battery is used on
the output side of the converter, and the
LTC3129 is set to regulate VOUT slightly
design features
The LTC3129-1 can operate at high efficiency over a wide range of loads and input voltages,
with a minimal number of external components. The flexibility of running seamlessly from a
wide variety of power sources is an asset in critical field applications, such as military radios.
above the battery voltage. This assures
that there is no load on the battery whenever VOUT can be powered by the solar
input. In the event that VOUT droops due
to insufficient light to power the load,
the PGOOD output from the LTC3129 goes
low, switching the load from the converter output to the battery, thus holding VOUT at the battery voltage. During
this time, the converter’s input and
output capacitors are able to recharge
(if some light is available), enabling the
load to be periodically switched from
the battery back to the converter by the
PGOOD signal. In this manner the load is
powered by the solar input as much as
possible, and the battery is only used in
a time-shared manner, extending its life.
The diode connected from PGOOD to VCC is
used to hold PGOOD low during start-up,
before VCC (and therefore PGOOD) is valid.
CHOOSING WHERE TO PUT THE
BACKUP BATTERY
In the previous example, the backup
battery was placed on the output. For
light load applications, this has the
advantage of not exposing the battery—which may be a low capacity
battery with high internal resistance—to
relatively high converter start-up input
current bursts, causing significant battery droop and lossy internal power
dissipation, in turn reducing battery life.
The disadvantages of putting the backup
battery on the output of the converter
are that the battery voltage must be well
matched to the desired output voltage, and it must have a relatively flat
discharge curve so as to maintain reasonable regulation of VOUT. The 3V lithium
cell satisfies both of these requirements.
the VS1–VS3 pins, can be powered from a
5V USB input, a variety of battery options
or a 3V to 15V wall adapter. The flexibility
of running seamlessly from a wide variety
of power sources is an asset in critical
field applications, such as military radios.
Putting the backup battery on the input
side of the converter allows its voltage
to be different from the desired output
voltage, but it must be able to withstand
the higher currents that the converter
draws during start-up or load transients.
If used on the input side, a lithiumthionyl chloride battery is generally a
better choice for long life applications. It
can be diode-OR’d with the solar cell or
switched in and out with MOSFET switches,
in a similar manner to Figure 2.
The LTC3129-1’s low IQ of just 1.3µ A in
sleep mode, combined with a high resistance internal feedback divider, enables
it to maintain high efficiency over a wide
range of loads, as shown in Figure 4. At
a load current of just 100µ A, the efficiency is ~80% over nearly the entire
VIN range. This is an important feature
for extending battery life in applications that spend a large percentage
of the time in a low power state.
5V CONVERTER OPERATES
SEAMLESSLY FROM A VARIETY OF
INPUT SOURCES
The line step response (VIN is stepped
from 5V to 12V) is shown in Figure 5,
with VOUT measured under both heavy
and light load conditions. At a load of
200m A, the part is operating in PWM mode,
and VOUT overshoot is only 150mV (3%).
At a load of 10m A, the part is in Burst
Mode operation, with a burst ripple of
The ability of the LTC3129-1 to operate at
high efficiency over a wide range of loads
and input voltages with a minimal number
of external components is illustrated in
Figure 3. In this example, the output,
which has been programmed for 5V using
WALL ADAPTER
3V TO 15V
Figure 3. Multi-input 5V converter
L1
10µH
22nF
22nF
5V USB
VIN
1.8V TO 15V
BST1 SW1
SW2 BST2
VOUT
VIN
22µF
LTC3129-1
RUN
BATTERIES:
2–9 ALKALINE,
1–3 Li-ION,
OR Li-SOCl2
VCC
1M
PGOOD
MPPC
VOUT
5V
BAT54
OPTIONAL
PGOOD
VS1
10µF
VS2
VCC
VS3
PWM
GND
PGND
2.2µF
L1: Taiyo Yuden NR3015T
October 2013 : LT Journal of Analog Innovation | 17
The LTC3129 and LTC3129-1 include a maximum power point control (MPPC) feature
that allows the converter to servo VIN to a minimum voltage under load, as set by the
user. Regulating VIN maintains optimal power transfer in applications using higher current
solar cells or other sources with high internal resistance. This feature prevents the
converter from crashing the input voltage when operating from a current-limited source.
100
Burst Mode OPERATION
90
VOUT
200mV/DIV
(AC-COUPLED)
EFFICIENCY (%)
80
70
60
OUTDOOR SOLAR CONVERTER/
CHARGER WITH MPPC
ILOAD = 200mA (PWM MODE)
150mV
ILOAD = 10mA (Burst Mode OPERATION)
50
40
PWM
30
VIN = 2.5V
VIN = 3.6V
VIN = 5V
VIN = 10V
VIN = 15V
20
10
0
0.01
0.1
1
10
100
OUTPUT CURRENT (mA)
VOUT
200mV/DIV
(AC-COUPLED)
1000
Figure 4. Efficiency vs VIN and load of
the 5V converter in Figure 3
V
and less than 100mV of
VOUT overshoot due to the line step.
100m PK-PK (2%),
The VCC pin is the output of an internal
LDO that generates a nominal 3.9V from
VIN to power the IC. The LDO is designed
so that it can be externally back-driven up
to 5V. In this example, an optional bootstrap diode is shown from VOUT to VCC .
The addition of this external bootstrap
diode has two advantages. First, it
improves efficiency at low VIN and high
load current by providing a higher gate
drive voltage to the internal switches,
lowering their RDS(ON). Also, at high
VIN and light load, it improves efficiency by reducing the power lost in
the internal LDO used to generate VCC .
(Note that the VCC pin must not be
raised above 6V, so it cannot be diodeconnected to higher output voltages.)
100mV
VIN
5V/DIV
500µs/DIV
VIN = 5V TO 12V STEP
Figure 5. Line transient response of the 5V
converter in Figure 3
The MPPC control loop operates by
reducing the average inductor current commanded by the converter, thus
maintaining the minimum programmed
VIN voltage under load. This voltage is
set using an external resistor divider
connected to VIN and the MPPC pin, as
shown in the supercapacitor charging
example of Figure 6. The MPPC control
loop is designed to be stable with a
minimum input capacitance of 22µ F.
above its minimum value of 2.2V (by the
output voltage in this case), then the converter can operate at a lower input voltage,
down to 1.75V, where the fixed internal
VIN UVLO threshold is reached. This capability extends the usable voltage range
enough to make it possible to run from
two depleted alkaline batteries. Note that
if the battery voltage is below 2.4V and
the converter is shut down (or VOUT is
shorted), the IC is not be able to restart.
Figure 6. Outdoor solar powered supercapacitor
charger with maximum power point control
VMPPC = 6V
VIN
47µF
1M
The LTC3129 and LTC3129-1 include a
maximum power point control (MPPC)
feature that allows the converter to servo
VIN to a minimum voltage under load, as
set by the user. Regulating VIN maintains
optimal power transfer in applications
using higher current solar cells or other
sources with high internal resistance.
This feature prevents the converter from
crashing the input voltage when operating from a current-limited source.
L1
6.8µH
22nF
BST1 SW1
22nF
SW2 BST2
4.7µF
LTC3129
RUN
18 | October 2013 : LT Journal of Analog Innovation
2.8M
+
FB
MPPC
PowerFilm
MPT6-150
SOLAR
MODULE
1M
PGOOD
PWM
VCC
11.4cm × 15cm
The second advantage of adding a bootstrap diode is that it allows operation from
a lower VIN . After start-up, if VCC is held
VOUT
4.47V
VOUT
VIN
243k
GND
PGND
2.2µF
C1: Cooper Bussmann PB-5R0V105-R
L1: Coiltronics SD3118
C1
1F
5V
design features
The LTC3129 and LTC3129-1 monolithic buck-boost DC/DC converters offer
exceptional low power performance and power source flexibility demanded
by real-world wireless sensor and portable electronic instruments. The
ultralow 1.3µA quiescent current and high conversion efficiency can extend
battery lifetime indefinitely if used in concert with energy harvesting.
Note that reducing the inductor current under MPPC would cause the output
voltage to droop if it were driving a
conventional load. Therefore, most applications employing MPPC involve charging a large storage capacitor (or trickle
charging a battery) from a solar cell. The
MPPC feature assures that the capacitor or
battery is charged at the highest current
possible, while operating the solar cell
at its maximum power point voltage.
It is important to note that when the
LTC3129/LTC3129-1 is in MPPC control,
Burst Mode operation is inhibited, and
the VIN quiescent current is several milliamps, since the IC is switching continuously at 1.2MHz. Therefore, MPPC is not
appropriate for use with sources that
cannot supply a minimum of about
10m A. For applications requiring an
MPPC-like function with very weak input
sources, the accurate RUN pin should be
used to program a UVLO threshold, as
described in the example of Figure 1.
INTRINSIC SAFETY USING MPPC
INPUT CURRENT LIMIT USING MPPC
The MPPC feature can be used in other
applications, including those designed
for intrinsic safety, where the input
source has a series current limiting resistor between it and the DC/DC converter.
In this case, the MPPC loop prevents the
LTC3129/LTC3129-1 from drawing too
much current, especially during startup when the output capacitor is being
charged, and crashing the input voltage.
An example of this is shown in Figure 7,
where the input voltage is maintained at a
minimum of 3V, as set by the MPPC divider.
Note that the MPPC feature can be used to
set the maximum input current to a given
value. By choosing a series input resistor
value and setting the MPPC voltage to a
value below a fixed input source voltage,
the maximum input current is limited to:
In this case, because the input capacitor
value is limited to just 10µ F for safety
(less than the recommended minimum
value of 22µ F when using MPPC), an
additional RC compensation network
is added to the MPPC pin for improved
phase margin of the MPPC loop.
VMPPC = 3V
VIN
10Ω
L1
3.3µH
22nF
Figure 7. 3.3V Converter using MPPC
for intrinsic safety application
10µF
1.5M
BST1 SW1
1.5V
1.5V
RC
150k
CC
1nF
SW2 BST2
IOUT = 100mA
VOUT
VIN
10µF
LTC3129-1
VOUT
3.3V
VSOURCE − VMPPC
RSERIES
CONCLUSION
The LTC3129 and LTC3129-1 monolithic
buck-boost DC/DC converters offer exceptional low power performance and power
source flexibility demanded by real-world
wireless sensor and portable electronic
instruments. The ultralow 1.3µ A quiescent
current and high conversion efficiency
can extend battery lifetime indefinitely if
used in concert with energy harvesting.
A choice of maximum power point
control schemes allows optimization
of power performance over a wide
range of power sources. The expanding reach of wireless monitoring applications demands easy to use, efficient
and flexible DC/DC power converter
solutions. The LTC3129 and LTC3129-1
are ready to meet this challenge. n
RUN
MPPC
1.5V
22nF
IIN =
PGOOD
PWM
VCC
VS1
VCC
VS2
1M
VS3
GND
PGND
2.2µF
L1: Coilcraft EPL2014
NOTE: RC AND CC HAVE BEEN ADDED FOR IMPROVED MPPC LOOP STABILITY WHEN USING AN INPUT
CAPACITOR VALUE LESS THAN THE RECOMMENDED MINIMUM OF 22µF
October 2013 : LT Journal of Analog Innovation | 19
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