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bridge rectifier with a high efficiency
buck converter that is optimized for
high output impedance piezoelectric
sources, and maintains a regulated
output voltage with a high efficiency
synchronous buck regulator.
An ultralow quiescent current
undervoltage lockout (UVLO) mode
with a wide hysteresis window allows
charge to accumulate on an input
capacitor until the buck converter can
efficiently transfer a portion of the
stored charge to the output, and the
buck converter turns on and off as
needed to maintain regulation.
Solar power
An example of the importance of
knowing the lower limit comes with
the buck converter, which starts when
the input voltage moves above the
UVLO rising threshold to transfer
charge from the input capacitor to
the output capacitor. The 1V UVLO
hysteresis window has a lower
threshold of around 300 mV above
the selected regulated output voltage,
and this prevents short cycling during
the buck power-up.
When the input capacitor voltage
is depleted below the UVLO falling
threshold, the buck converter is
disabled and the extremely low
quiescent current of 450nA allows
energy to accumulate on the input
capacitor from the piezoelectric
source. The hysteresis is determined
by an algorithm that controls the
output through internal feedback
from the voltage sense pin.
The four output voltages in the
LTC3588 show the lower voltages
that the energy harvesting source is
expected to power, from 1.8V, 2.5V
and 3.3V to 3.6V, and these are
pin selectable with up to 100mA of
continuous output current. An input
protective shunt set at 20V enables
greater energy storage for a given
amount of input capacitance.
The low-loss bridge rectifier has
a total drop of about 400mV, with
typical piezo generated currents
of around 10µA, and the bridge is
capable of carrying up to 50mA. All of
this allows the charge in the capacitor
to smooth out the intermittent nature
of the source of power and provide
the required voltage to the sensor or
controller.
Batteries are often used to collect the
charge from the energy source, but
these also need to be protected from
overcharging or undercharging. The
MAX17710 from Maxim Integrated can
manage the poorly regulated energy
harvesting sources with output levels
ranging from 1µW to 100mW. For a
0.8V harvest source and a 4.1V cell,
the device can deliver over 20 mA
(80mW) for as long as the harvest
source can support it.
To do this, the device includes a boost
regulator circuit for charging a lithium
battery from a source as low as 0.75V,
while an internal regulator to protect
the cell from overcharging and an
internal voltage protection prevents
the cell from over discharging. The
selectable output voltages from 1.8V
through 2.3V to 3.3V are regulated
using a low-dropout (LDO) linear
regulator.
An increasingly popular source for
energy harvesting is a solar cell, and
there are several different ways to
manage the power coming from such
cells. These different approaches
are freely available through digital
libraries, and can be tested out on a
range of evaluation boards.
The digital power software libraries
Figure 1: The quiescent current
undervoltage lockout (UVLO) uses
a hysteresis algorithm to protect
the power delivery in a piezoelectric
energy harvesting system.
Figure 2: The MAX17710 integrates
a low-dropout regulator to protect
a capacitor or lithium battery cell
from undercharging.
Figure 3a: The basic P&O algorithm
(Source: MATHWORKS)
New-Tech Magazine Europe l 68