V20W Datasheet, PDF (21/24 Pages) List of Unclassifed Manufacturers – PIEZOELECTRIC ENERGY HARVESTERS

APPLICATIONS INFORMATION - LOAD ISOLATION EXAMPLE
Application Example
The steps to configure this circuit optimally for a given
load are presented below. This example assumes the
following are known:
Required output voltage
Required output current
Nominal piezo open-circuit voltage
1) Set output voltage
The output voltage is determined by
Vout = 1.23 1+ ^R7 V
R8
Where V_f is the forward voltage drop of D8 under load.
In direct sensor-power applications where reverse
leakage into the circuit during its off-time is not a
concern, D8 may be omitted (but see important
warnings below). To minimize no-load supply current,
resistor values in the megaohm range should be used.
2) Choose inductor size and maximum inductor
current, following the guidelines of the LT1474
datasheet. For greatest efficiency where space is not a
concern, first choose the inductor current sense
resistor R6 according to desired output current
(R_SENSE Equation above), then size the inductor
accordingly. For space-limited applications, size L1 as
dictated by the available space and set I_PEAK as high
as the inductor will tolerate without saturating.
3) Choose C1 bias voltage to be approximately ½ the
open-circuit piezo voltage in its natural vibration
environment. In applications with highly variable
amplitudes, a trade-off must be made between
efficiency at higher amplitudes and the ability to
harvest power from lower amplitudes: no charge will
be drawn by this circuit if the piezo open-circuit voltage
after rectification never reaches the set turn-on voltage.
(Circuit adaptations for self-adjusting bias voltage may
be explored if the power gains exceed the consumption
of the added circuitry.)
The LTC1540 features a programmable hysteresis
band of up to 100mV. However, a larger hysteresis is
generally preferable to allow for smaller values of C1
and/or longer output ON-times, particularly for directly-
powered sensors. This can be accomplished by
bypassing the onboard hysteresis adjustment (R4=0
ohms; R5=open) and adding a small amount of
positive feedback via R3.
First select the desired center voltage (Vc) for C1 and
the desired hysteresis band (V_hb). Choose a value for
R1 such that the current across it at the trip point is
substantially greater than the comparator’s input
leakage (1nA typ.), for example 100nA. The 5.7M value
shown is perfectly reasonable, but the following will
more formally dictate its upper limit. The LTC1540’s
internal reference (Vref) is 1.182V. The current through
R1 at the lower trip point is (Vc-(Vhb/2)-Vref) / R1.
Thus the upper limit for R1 is
Vc
R1
V_{hb 1.182}
2
I _R1
Choose R2 to set the center voltage, Vc. Neglecting the
small influence of R3,
R2 ^{1.182 R1}
Vc 1.182
Choose R3 to set the hysteresis. At the lower
comparator trip point, the threshold voltage will be
equal to
V_L
R2
R2 R1 R3
REVISION N0. 002 REVISION DATE: 01-23-2013
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