SUPPLEMENT THE INTERNET OF THINGS
HARNESSING THE POWER OF WIRELESS SENSOR DEVICES
Dave Salerno, design section leader, power products at Linear Technology Corporation explores the advanced capabilities of the latest buck-boost converters built for IoT and micropowered applications
T
he proliferation of wireless sensors supporting the Internet of Things (IoT) has increased the need for small, efficient power converters tailored to untethered low power devices. The LTC3129 and LTC3129-1 from Linear Technology Corporation are designed to satisfy this need. Both 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 of the zero current of these two devices when disabled (on both VIN a quiescent current on VIN
and VOUT ), and of just 1.3µA
when power saving Burst Mode operation is selected; making them ideal for µPower 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. Other key features include a fixed
1.2MHz operating frequency, current mode control, internal loop compensation, automatic Burst Mode operation or low noise 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 optimising power transfer when operating from photovoltaic cells. The compact 3mm × 3mm QFN package
and the high level of integration eases this device’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. The circuit in Figure 1 demonstrates the 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
S10 JULY/AUGUST 2016 | ELECTRONICS
Linear Technology (UK) Ltd.
www.linear.com 01628 477 066
/ ELECTRONICS
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 1nA typical, high value resistors may be used to minimise current draw on VIN
.
In the application example shown in Figure 1, the energy stored on CIN
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 if the average output power demand exceeds the input power available, then VIN
drops until UVLO is reached, at which point the converter
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 micro amps of current (less in shutdown) until three conditions are satisfied: The RUN pin must exceed 1.22V (typical) The VIN VCC
VIN
pin must exceed 1.9V (typical). (which is internally generated from but can also be supplied externally)
must exceed 2.25V (typical). Until all three of these conditions are satisfied, the part remains in a “soft- shutdown” or standby state, drawing just 2µA. 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
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 optimise efficiency and
eliminate unnecessary loading of VOUT Figure 1:
3.3V solar powered converter operates from indoor light
, 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
. In 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
the case of the LTC3129-1, the VOUT current draw is sub-micro amp, due to
the high resistance internal feedback divider.
reenters soft-shutdown. At this point, VIN begins recharging, allowing the cycle to repeat. In this hiccup mode of operation, VIN
is used
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