V20W Datasheet, PDF (19/24 Pages) List of Unclassifed Manufacturers – PIEZOELECTRIC ENERGY HARVESTERS

APPLICATIONS INFORMATION - LOAD ISOLATION EXAMPLE
Piezo Bimorph
X1
X2
VCC
D1
BAV199
BAV199
D2
+ ^C1
100uF
V REF
R4
100K
R3
15M (Optional)
1 _-IN
3 _+IN
5 _HYS
6 ^REF
OUT ⁸
7
V+
2
V-
LTC154 0CMS8
C2 ^R5
2.2M
10nF
BAT43W
VCC
R6
1
RUN
U2 ^D6
7
1
VIN VFB
6
2
3 ^SENSE
LBI
LBO
SW
5 SW
8
4
RUN GND
LTC1474CMS8
VFB
VOUT
+ C3
1uF
D8
V4 ^BAT43W
12pF
VOUT
GND
GND
Figure 2: Simple high-efficiency voltage conversion and load-isolation circuit for piezo energy harvesting applications. The C1
voltage setpoint in this example is fixed, but could be made adaptive using additional circuitry.
deplete C1 until its voltage drops below the hysteresis
band, at which point the converter is disabled and the
cycle repeats. Thus C1 is maintained at approximately
V_oc/2. The typical output is a train of voltage-regulated
pulses ideally suited to charging a battery or capacitor,
or directly powering an intermittently-operating sensor.
Since the voltage at C1 is held approximately constant,
the effective load seen by the piezo is not significantly
affected by activation of U2 or changes to the actual
load. Additionally, the voltage maintained on C1
ensures charge is drawn from the piezo beam only
when its voltage exceeds C1’s voltage by one diode
drop, which occurs only when the beam approaches its
maximum deflection. Thus a limited amount of charge
is drawn on each cycle of the piezo beam, timed to
coincide with the maximum deflection of the beam.
This combination of factors helps prevent excessive
mechanical damping of the beam, allowing usable
output voltages to be output at lower vibration
amplitudes while the piezo beam is driven near its
mechanical resonance.
U2 provides a regulated output voltage determined by
the ratio of R7 and R8. The power output per G of
vibration will remain relatively constant across G-levels
and loads, provided the load is capable of drawing at
least as much power as is being supplied (Figure 5).
The converter duty cycle is approximated by the ratio
of input power (G-level) vs. output power (voltage *
current * time), less any small conversion losses and
leakage. When the input power exceeds the output and
losses, duty cycle will be 100%, and excess voltage at
C1 is safely disposed of through D5.
R6 sets the peak current through inductor L1, and can
be specified according to the equation:
R_SENSE
0.067 _0.25
I _MAX
where I_MAX is the maximum desired output current to
the load. Generally, the peak inductor current should be
set to maximum to improve efficiency with smaller
(<300uH) inductors in space-limited applications;
refer to the LT1474 datasheet for more thoughtful
trade-offs between peak current and inductor size.
Battery-charging applications requiring a (pulsed)
constant-current charge phase, or where both the
charge circuit and the load are directly connected
across the battery pack, may require a lower I_PEAK
setting to reduce voltage ripple caused by the charge
pulses.
REVISION N0. 002 REVISION DATE: 01-23-2013
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