LTC3331_15 Datasheet, PDF (26/34 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger

LTC3331
APPLICATIONS INFORMATION
Supercapacitor Balancer
If supercapacitors are used at V_OUT the onboard superca-
pacitor balancer can be used to balance them with ±10mA
of balance current. A list of supercapacitor suppliers is
provided in Table 7.
Table 7. Supercapacitor Manufacturers
CAP-XX
NESS CAP
Maxwell
www.cap-xx.com
www.nesscap.com
www.maxwell.com
Bussman
AVX
Illinois Capacitor
Tecate Group
www.cooperbussman.com
www.avx.com
www.illcap.com
www.tecategroup.com
By seamlessly combining a battery source and an en-
ergy harvesting source, the LTC3331 enables the use of
supercapacitors in energy harvesting applications. The
battery provides the initial current required to overcome
the effects of the diffusion current when voltage is first
applied to the supercapacitors. The energy harvesting
source can then support the lower steady state leakage
current and average load current.
Summary of Digital inputs and outputs
There are 14 digital pin-strapped logic inputs to the
LTC3331 and two digital logic outputs. These and the rails
they are referenced to are summarized in Table 8.
Table 8. Digital Pin Summary
INPUT PIN
UV[3:0]
IPK[2:0]
OUT[2:0]
FLOAT[1:0], LBSEL
SHIP
LOGIC HIGH LEVEL
V_IN2
V_IN3
V_IN3
BAT_OUT = BB_IN
≥ 1.2V
OUTPUT PIN
PGVOUT
EH_ON
LOGIC HIGH LEVEL
MAX (BB_IN, V_IN2, V_OUT)
V_IN3
Battery Considerations
The shunt battery charger is designed to work with any
single Li-Ion, LiFeP0₄, or other chemistry with a termination
voltage compatible with the available levels. Table 9 lists
some batteries, their capacities and their equivalent series
resistance (ESR). The ESR causes BAT_OUT and BAT_IN
to droop by the product of the load current amplitude
multiplied by the ESR. This droop may trigger the low
battery disconnect while the battery itself may still have
ample capacity. An appropriate bypass capacitor placed
at BAT_OUT will help prevent large, low duty cycle load
transients from pulling down on BAT_OUT. The bypass
capacitor used at BB_IN, which is tied to BAT_OUT, to
bypass the buck-boost may be sufficient.
Table 9. Low Capacity Li-Ion and Thin-Film Batteries
MANUFACTURER
P/N
CAPACITY RESISTANCE
CYMBET
CBC012
12μAh
5k to 10k
CYMBET
CBC050
50μAh 1500Ω to 3k
GM Battery GMB031009 8mAh 10Ω to 20Ω
GS NanoTech
N/A
500μAh
40Ω
Power Stream LIR2032 40mAh
0.6Ω
V_MIN
3.0V
3.0V
2.75V
3.0V
3.0V
Charging the Battery
Charging the battery with the CHARGE pin allows the
battery to be charged only when the energy harvester is
sleeping, which prioritizes the V_OUT output over the battery.
The current that the CHARGE pin can supply is limited to
2mA and an appropriately chosen current limiting resistor
should be used. Use the following equation to calculate
the value of this resistor:
R_CHARGE
=
4.8V – V_LBD
I_CHARGE
– 60Ω
Here 4.8V is the output of the V_IN2 LDO which is the sup-
ply to the CHARGE pin, V_LBD is the selected low battery
disconnect threshold, 60Ω is the resistance of the CHARGE
pin PMOS, and I_CHARGE is the desired charge current. For
high charging currents approaching 2mA, a larger V_IN2
capacitor may improve transient behavior.
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