LTC3107
9
3107f
For more information www.linear.com/LTC3107
operaTion
The LTC3107 is an ultra-low input voltage step-up DC/DC
converter and power manager for extending the battery
life of low power wireless sensors and other low power
applications that utilize a primary battery. The LTC3107
intelligently manages harvested energy from sources
such as TEGs (Thermo-Electric Generators) to service the
output while minimizing battery drain, thereby maximizing
battery life.
To simplify the adoption of energy harvesting by applica
-
tions presently powered by a primary battery, the LTC3107
is designed to use the voltage on the V
BAT
pin not only as
an energy source to power the outputs in the absence of
harvested input energy, but also as a voltage reference
to regulate V
OUT
. In this way, the LTC3107 automatically
adapts V
OUT
to track whatever battery voltage the applica-
tion is already designed for, in the range of 2V to 4.0V.
The LTC3107 is
suitable for extending the battery life in
applications where 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 a high
percentage of the time, except
during transmit bursts
when circuitry is powered up to make measurements and
transmit data.
The LTC3107 can also be used to trickle charge a standard
capacitor or supercapacitor to store excess harvested
energy when it is available. This further extends the life
of the primary battery, by allowing the converter to ride-
through periods of heavier load, or times when no harvested
energy is coming in by drawing from this reservoir before
switching over to the battery.
V
BAT
Input
The V
BAT
input should be connected to a primary battery
with a voltage between 2V and 4.0V. Typical examples are
2 Alkaline cells, a single 3V Lithium coin cell, or a 3.6V
Li-SOCl
2
battery. These are representative batteries that
would normally power the application without the benefit
of energy harvesting.
The LTC3107 is designed to use the battery to start-up
the IC and power V
OUT
and VLDO with or without any
harvested energy available. If there is no harvester input,
or insufficient harvester input to power the load, then V
OUT
will be provided by the battery through a current-limited
switch internal to the LTC3107, and will be hysteretically
regulated to a voltage 230mV below
the battery voltage.
The VLDO output will be fixed at 2.2V unless V
BAT
is below
2.2V, in which case it will track V
BAT
. When no harvested
energy is available, the LTC3107 average quiescent current
draw from the battery is typically 6µA.
If there is sufficient harvested energy available, then V
OUT
will be regulated to a voltage approximately 30mV below
the V
BAT
voltage, and the battery will not be used to power
V
OUT
. In this case, the battery current draw will drop to
just 80nA typical.
Oscillator
The LTC3107 utilizes a MOSFET switch to form a resonant
step-up oscillator using an external step-up transformer
and a small coupling capacitor. This allows it to boost
input voltages as low as 20mV up to values 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-100kHz. For input
voltages as low as 20mV, a primary-secondary turns ratio
of about 1:100 is recommended. For higher input voltages,
this ratio can be lower. See the Applications section for
more information on selecting the transformer.
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
C1) and the rectifiers internal to the LTC3107. The rectifier
circuit feeds current into the VAUX pin, providing charge
to the external VAUX capacitor. Once VAUX exceeds 2V,
synchronous rectifiers in parallel with each of the diodes
take over the job of rectifying the input voltage, improv
-
ing efficiency.
