LTC3109_15 Datasheet, PDF (9/24 Pages) Linear Technology – Auto-Polarity, Ultralow Voltage Step-Up Converter and Power Manager

LTC3109
Operation (Refer to the Block Diagram)
The LTC3109 is designed to use two small external
step-up transformers to create an ultralow input voltage
step-up DC/DC converter and power manager that can
operate from input voltages of either polarity. This unique
capability enables energy harvesting from thermoelectric
generators (TEGs) in applications where the temperature
differential across the TEG may be of either (or unknown)
polarity. It can also operate from low level AC sources. It
is ideally suited for low power wireless sensors and other
applications in which surplus energy harvesting is used to
generate system power because traditional battery power
is inconvenient or impractical.
The LTC3109 is designed to manage the charging and
regulation of multiple outputs in a system in which the
average power draw is very low, but where periodic pulses
of higher load current may be required. This is typical of
wireless sensor applications, where the quiescent power
draw is extremely low most of the time, except for transmit
pulses when circuitry is powered up to make measure-
ments and transmit data.
The LTC3109 can also be used to trickle charge a standard
capacitor, super capacitor or rechargeable battery, using
energy harvested from a TEG or low level AC source.
Resonant Oscillator
The LTC3109 utilizes MOSFET switches to form a reso-
nant step-up oscillator that can operate from an input of
either polarity using external step-up transformers and
small coupling capacitors. This allows it to boost input
voltages as low as 30mV high enough to provide multiple
regulated output voltages for powering other circuits. The
frequency of oscillation is determined by the inductance
of the transformer secondary winding, and is typically
in the range of 10kHz to 100kHz. For input voltages as
low as 30mV, transformers with a turns ratio of about
1:100 is recommended. For operation from higher input
voltages, this ratio can be lower. See the Applications
Information section for more information on selecting
the transformers.
Charge Pump and Rectifier
The AC voltage produced on the secondary winding of
the transformer is boosted and rectified using an external
charge pump capacitor (from the secondary winding to
pin C1A or C1B) and the rectifiers internal to the LTC3109.
The rectifier circuit feeds current into the V_AUX pin, provid-
ing charge to the external VAUX capacitor and the other
outputs.
VAUX
The active circuits within the LTC3109 are powered from
VAUX, which should be bypassed with a 1µF minimum
capacitor. Once VAUX exceeds 2.5V, the main V_OUT is al-
lowed to start charging.
An internal shunt regulator limits the maximum voltage
on VAUX to 5.25V typical. It shunts to ground any excess
current into VAUX when there is no load on the converter
or the input source is generating more power than is
required by the load. This current should be limited to
15mA max.
Voltage Reference
The LTC3109 includes a precision, micropower reference,
for accurate regulated output voltages. This reference
becomes active as soon as VAUX exceeds 2V.
Synchronous Rectifiers
Once VAUX exceeds 2V, synchronous rectifiers in paral-
lel with each of the internal rectifier diodes take over the
job of rectifying the input voltage at pins C1A and C1B,
improving efficiency.
Low Dropout Linear Regulator (LDO)
The LTC3109 includes a low current LDO to provide a
regulated 2.2V output for powering low power proces-
sors or other low power ICs. The LDO is powered by
the higher of VAUX or V_OUT . This enables it to become
active as soon as VAUX has charged to 2.3V, while the
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