V25W Mide Technology Corporation, V25W Datasheet - Page 20

• Manufacturer Part Number: V25W
• Description: PIEZOELECTRIC ENERGY HARVESTER
• Manufacturer: Mide Technology Corporation
• Series: Volture™r

Specifications of V25W

• Sensor Type: Acceleration
• Sensing Range: 60Hz ~ 140Hz
• Output Type: Analog
• Operating Temperature: -40°C ~ 90°C
• Features: Piezoelectric Vibration Energy Harvesting, Hermetically Sealed
• Package / Case: Cantilever Piezo Film (Wafer)
• Mounting Type: Through Hole
• Termination: PC Pins
• Lead Free Status / RoHS Status: Lead free by exemption / RoHS compliant by exemption

APPLICATIONS INFORMATION -
Considerations for Conversion Efficiency
Converter efficiency is defined as the output power
divided by the input power times 100%.
The losses from the circuit’s front-end components
(leakage across the storage capacitor, comparator, and
R1-R3) will be small (uA) and largely proportional to
the C1 voltage setpoint. Minimizing losses here is
straightforward by appropriate component selection of
C1 and suitably large values of R1 – R3. These small
losses will occur for the entire time that voltage is
present on C1.
The remaining controllable losses will occur in the
step-down converter section and, apart from the
LT1474’s shutdown current, occur only during the ON-
time of the output. The main losses will come from
three sources: V
losses.
V
The V
components: the DC bias current and the gate charge
current of its internal P-channel switch. Both are
proportional to V
the DC bias current (from 9uA at no load to 100uA in
continuous mode) is negligible compared to the gate
charge losses. Each time the gate is switched on and
off, a packet of charge dQ moves from V
In continuous mode, I
charge of the internal switch and f is the switching
frequency. Since V
efficiency considerations, the best way to minimize this
loss is to keep the output duty cycle low.
Resistive (I
The resistances in the current path (The ON-resistance
IN
IN
current of the LT1474 is due to two
2
R) Losses
IN
IN
current, I
; however, at load currents > 1mA,
IN
GATE
is pre-set according to piezo
= fQp where Qp is the gate
2
R losses, and catch diode
REVISION N0. 001
IN
to ground.
LOAD ISOLATION EXAMPLE
REVISION DATE: 06-03-2010
of the internal switch, current sense resistor, and
inductor) contribute resistive losses. At low values of
V
by using a suitably large inductor and low I
higher supply voltages, these losses are proportional to
load.
Catch Diode Losses
The catch diode, D7, introduces a loss (V*I) as it
conducts during the switch off-time, proportional to its
forward voltage, and more pronounced at high supply
voltage where the switch on-time is shorter. Again, V
is fixed for piezo efficiency reasons, so this loss is best
minimized by minimizing the diode V
duty cycle. By lowering the duty cycle, a reduction in V
of the catch diode will improve efficiency even though
low-V
leakage currents, which will produce loss during the
switch ON-time. In any event, the catch diode must be
sized to safely handle I
(worst-case condition; output shorted).
The selection of 100uH inductor should be considered
a minimum, for use in conjunction with high I
values. Particularly in applications where space is not
constrained or I
large as 1000uH may be required for maximum
efficiency. The ideal range of inductor size at a given
I
higher-value inductors and the increased switching
rates required for lower-value inductors.
At light loads, the output duty cycle will rise and losses
in the conversion stage (leakage, switching and catch
diode losses) will accrue. This may or may not be a
problem; it could simply mean that more power is
available than the load can use.
MAX
IN
, switch losses will dominate, and can be minimized
is a trade-off between the increased resistance of
f
diodes tend to have much higher reverse
PEAK
must be reduced, inductors as
PEAK
at nearly 100% duty cycle
f
and converter
PEAK
PEAK
. At
20
IN
f