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LTC3109

3109fa

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 VAUX 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 VOUT 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 VOUT. This enables it to become

active as soon as VAUX has charged to 2.3V, while the

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