LTC3588-2_10 Datasheet, PDF (11/18 Pages) Linear Technology – Piezoelectric Energy Harvesting Power Supply with 14V Minimum VIN

applications information
LTC3588-2
1µF
6V
10µF
25V
4.7µF
6V
PZ1
PZ2
V_IN
PGOOD
CAP ^LTC3588-2 SW
V_IN2
V_OUT
D1
D0
GND
22µH
EN
T_X
5V
MICROPROCESSOR
CORE
GND
47µF
6V
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OUTPUT
VOLTAGE
50mV/DIV
AC-COUPLED
LOAD
CURRENT
25mA/DIV
5mA
V_IN = 18V
250µs/DIV
L = 22µH, C_OUT = 47µF
LOAD STEP BETWEEN 5mA and 55mA
Figure 5. 5V Piezoelectric Energy Harvester Powering a Microprocessor
with a Wireless Transmitter and 50mA Load Step Response
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The LTC3588-2 will gather energy and convert it to a use-
able output voltage to power microprocessors, wireless
sensors, and wireless transmission components. Such a
wireless sensor application may require much more peak
power than a piezoelectric element can produce. However,
the LTC3588-2 accumulates energy over a long period of
time to enable efficient use for short power bursts. For
continuous operation, these bursts must occur with a low
duty cycle such that the total output energy during the burst
does not exceed the average source power integrated over
an energy accumulation cycle. For piezoelectric inputs the
time between cycles could be minutes, hours, or longer
depending on the selected capacitor values and the nature
of the vibration source.
PGOOD Signal
The PGOOD signal can be used to enable a sleeping
microprocessor or other circuitry when V_OUT reaches
regulation, as shown in Figure 5. Typically V_IN will be
somewhere between the UVLO thresholds at this time
and a load could only be supported by the output capaci-
tor. Alternatively, waiting a period of time after PGOOD
goes high would let the input capacitor accumulate more
energy allowing load current to be maintained longer as
the buck efficiently transfers that energy to the output.
While active, a microprocessor may draw a small load
when operating sensors, and then draw a large load to
transmit data. Figure 5 shows the LTC3588-2 responding
smoothly to such a load step.
Input and Output Capacitor Selection
The input and output capacitors should be selected based
on the energy needs and load requirements of the ap-
plication. In every case the V_IN capacitor should be rated
to withstand the highest voltage ever present at V_IN.
For 100mA or smaller loads, storing energy at the input
takes advantage of the high voltage input since the buck
can deliver 100mA average load current efficiently to the
output. The input capacitor should then be sized to store
enough energy to provide output power for the length of
time required. This may involve using a large capacitor,
letting V_IN charge to a high voltage, or both. Enough energy
should be stored on the input so that the buck does not
reach the UVLO falling threshold which would halt energy
transfer to the output. In general:
( ) ^PLOAD^tLOAD
=
1
2
ηC_IN
V_IN2
−
V_U
2
VLO(FALLING)
V_{UVLO(FALLING)} ≤ V_IN ≤ V_SHUNT
The above equation can be used to size the input capaci-
tor to meet the power requirements of the output for an
application with continuous input energy. Here η is the
average efficiency of the buck converter over the input
range and V_IN is the input voltage when the buck begins to
switch. This equation may overestimate the input capaci-
tor necessary since load current can deplete the output
capacitor all the way to the lower PGOOD threshold. It also
assumes that the input source charging has a negligible
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