LTC3588-2_10 Datasheet, PDF (8/18 Pages) Linear Technology – Piezoelectric Energy Harvesting Power Supply with 14V Minimum VIN

LTC3588-2
Operation
The LTC3588-2 is an ultralow quiescent current power
supply designed specifically for energy harvesting and/or
low current step-down applications. The part is designed to
interface directly to a piezoelectric or alternative A/C power
source, rectify a voltage waveform and store harvested
energy on an external capacitor, bleed off any excess power
via an internal shunt regulator, and maintain a regulated
output voltage by means of a nanopower high efficiency
synchronous buck regulator.
Internal Bridge Rectifier
The LTC3588-2 has an internal full-wave bridge rectifier
accessible via the differential PZ1 and PZ2 inputs that
rectifies AC inputs such as those from a piezoelectric
element. The rectified output is stored on a capacitor at
the V_IN pin and can be used as an energy reservoir for the
buck converter. The low-loss bridge rectifier has a total
drop of about 400mV with typical piezo generated currents
(~10µA). The bridge is capable of carrying up to 50mA.
One side of the bridge can be operated as a single-ended
DC input. PZ1 and PZ2 should never be shorted together
when the bridge is in use.
Undervoltage Lockout (UVLO)
When the voltage on V_IN rises above the UVLO rising
threshold the buck converter is enabled and charge is
transferred from the input capacitor to the output capacitor.
A wide (~2V) UVLO hysteresis window allows a portion of
the energy stored on the input capacitor to be transferred
to the output capacitor by the buck. When the input capaci-
tor voltage is depleted below the UVLO falling threshold
the buck converter is disabled. Extremely low quiescent
current (830nA typical, V_IN = 12V) in UVLO allows energy
to accumulate on the input capacitor in situations where
energy must be harvested from low power sources.
Internal Rail Generation
Two internal rails, CAP and V_IN2, are generated from V_IN and
are used to drive the high side PMOS and low side NMOS
of the buck converter, respectively. Additionally the V_IN2
rail serves as logic high for output voltage select bits D0
and D1. The V_IN2 rail is regulated at 4.8V above GND while
the CAP rail is regulated at 4.8V below V_IN. These are not
intended to be used as external rails. Bypass capacitors
8
are connected to the CAP and V_IN2 pins to serve as energy
reservoirs for driving the buck switches. When V_IN is below
4.8V, V_IN2 is equal to V_IN and CAP is held at GND. Figure 1
shows the ideal V_IN, V_IN2 and CAP relationship.
18
16
14
12
V_IN
10
8
6
V_IN2
4
2
CAP
0
0
5
10
15
V_IN (V)
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Figure 1. Ideal V_IN, V_IN2 and CAP Relationship
Buck Operation
The buck regulator uses a hysteretic voltage algorithm
to control the output through internal feedback from the
V_OUT sense pin. The buck converter charges an output
capacitor through an inductor to a value slightly higher
than the regulation point. It does this by ramping the
inductor current up to 260mA through an internal PMOS
switch and then ramping it down to 0mA through an
internal NMOS switch. This efficiently delivers energy
to the output capacitor. The ramp rate is determined by
V_IN, V_OUT, and the inductor value. If the input voltage
falls below the UVLO falling threshold before the output
voltage reaches regulation, the buck converter will shut
off and will not be turned on until the input voltage again
rises above the UVLO rising threshold. During this time
the output voltage will be loaded by approximately 100nA.
When the buck brings the output voltage into regulation
the converter enters a low quiescent current sleep state
that monitors the output voltage with a sleep comparator.
During this operating mode load current is provided by
the buck output capacitor. When the output voltage falls
below the regulation point the buck regulator wakes up
and the cycle repeats. This hysteretic method of providing
a regulated output reduces losses associated with FET
switching and maintains an output at light loads. The buck
delivers a minimum of 100mA of average current to the
output when it is switching.
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