7
dc2048af
DEMO MANUAL DC2048A
Figure 5. MIDE V25W Open Circuit AC Voltage
with 1g
rms
, 60Hz Acceleration Applied
connection to a Dust mote (Dc9003a-B)
Figure 7. Mide V25W Charging the 18µF Input
Capacitance from 4.48V to 5.92V in 208ms
Figure 6 is a plot of the output power and load voltage
of the V25W piezoelectric transducer into a 42.2kΩ load
for various rms acceleration levels. The output power
compares well with the input power that is charging C
IN
during the sleep cycle between VIN_UVLO_FALLING and
VIN_UVLO_RISING thresholds at an acceleration force of
1grms, shown in Figure 7.
In Figure 7, the input capacitor is being recharged from
the V25W piezoelectric transducer. The input capacitor is
charging from 4.48V to 5.92V in 208 milli-seconds. The
power delivered from the V25W is 648µW.
Assuming that the circuit is configured as shown in
Figure 8, it will take a significant amount of time for the
piezo transducer to charge the 0.09F supercapacitor on
the output of the LTC3330. As used above, the 22µF input
capacitor is only 18µF at an applied voltage of 5V, so
every VIN_UVLO_RISING and FALLING event produces
26 micro-coulombs [(5.92V – 4.48V
) • 18µ
F)] that may be
transferred from the input capacitor to the output capacitor,
minus the losses of the buck regulator in the LTC3330.
The buck regulator efficiency is approximately 90% at V
IN
equal to 5V and V
OUT
between 2.5V and 3.6V. Thus, for
every UVLO event, 23.3 microcoulombs are added to the
output supercapacitor. Given a 0.09F output supercapacitor
charging to 3.6V, 324 millicoulombs are required to fully
charge the supercapacitor. Assuming no additional load on
the output, it takes 13,906 (.324/23.3e-6) UVLO events to
charge the output supercapacitor to 3.6V. From Figure 7, it
can be observed that each VIN_UVLO event takes 208ms
so the total time to charge the output capacitor from 0V
to 3.6V will be greater than 2900s. Figure 9 shows the no
load charging of the output supercapacitor, which takes
approximately 3300s. The above calculation neglects the
lower efficiency at low output voltages and the time it
takes to transfer the energy from the input capacitor to
the output supercapacitor so predicting the actual value
within –12% is to be expected.
Figure 6. Mide V25W Output Power Into a 42.2kΩ Load
with 1g
rms
, 60Hz Acceleration Applied to the Mide V25W
Piezoelectric Transducer, [√
2
• sin(2π • 60Hz • t)]
1ms/DIV
DC2048A F05
VAC
OC 10
FORGE (g)
POWER (µW)
LOAD VOL
TAGE (V
RMS
)
DC2048A F02
700
600
400
500
200
100
300
0
6
4
5
2
1
3
0
0.25 0.375
0.75 0.875
1
0.625
0.5
POWER (µW)
LOAD VOLTAGE (V
RMS
)
42.2k LOAD 60Hz
455µs/DIV
DC2048A F07
BH_ON
5V/DIV
V
OUT
1V/DIV
V
IN
2V/DIV
