LTC3331_15 Datasheet, PDF (17/34 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger

LTC3331
OPERATION
Overview
The LTC3331 combines a buck switching regulator and
a buck-boost switching regulator to produce an energy
harvesting solution with battery backup. The converters
are controlled by a prioritizer that selects which converter
to use based on the availability of a battery and/or har-
vestable energy. If harvested energy is available the buck
regulator is active and the buck-boost is OFF. An onboard
10mA shunt battery charger with low battery disconnect
enables charging of the backup battery to greatly extend
the life of the battery. An optional supercapacitor balancer
allows for significant energy storage at the output to handle
a variety of load requirements.
Energy Harvester
The energy harvester is an ultralow quiescent current power
supply designed to interface directly to a piezoelectric or
alternative A/C power source, rectify the input voltage,
and store harvested energy on an external capacitor while
maintaining a regulated output voltage. It can also bleed
off any excess input power via an internal protective shunt
regulator. It consists of an internal bridge rectifier, an un-
dervoltage lockout circuit, and a synchronous buck DC/DC.
Internal Bridge Rectifier
An internal full-wave bridge rectifier accessible via the dif-
ferential AC1 and AC2 inputs rectifies AC sources 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 bridge rectifier
has a total drop of about 800mV at typical piezo-generated
currents (~10μA), but is capable of carrying up to 50mA.
Either side of the bridge can be operated independently
as a single-ended AC or DC input.
Buck 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 capaci-
tor. When the input capacitor voltage is depleted below
the UVLO falling threshold the buck converter is disabled.
These thresholds can be set according to Table 4 which
offers UVLO rising thresholds from 4V to 18V with large
or small hysteresis windows. This allows for program-
ming of the UVLO window near the peak power point of
the input source. Extremely low quiescent current (450nA
typical) in UVLO allows energy to accumulate on the input
capacitor in situations where energy must be harvested
from low power sources.
Internal Rail Generation (CAP, V_IN2, V_IN3)
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 the UVLO threshold select
bits UV[3:0]. 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 capaci-
tors 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.
V_IN3 is an internal rail used by the buck and the buck-boost.
When the LTC3331 runs the buck V_IN3 will be a Schottky
diode drop below V_IN2. When it runs the buck-boost V_IN3
is equal to BB_IN.
18
16
14
12
V_IN
10
8
6
V_IN2
4
2
CAP
0
0
5
10
15
V_IN (V)
3331 F01
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
For more information www.linear.com/LTC3331
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