LTC4071
14
4071fc
This system has two modes of operation, charging where
the batteries are being charged from energy harvested from
the piezoelectric generator while the load is negligible.
And discharging, where the load is pulling current from
the batteries, but insufficient energy is being harvested
to power the load.
This application allows the load to periodically draw more
current than would otherwise be available from the piezo-
electric generator by storing excess charge in a stack of
two Li-Ion cells. Each Li-Ion cell is protected from over-
charge and over discharge by a LTC4071 shunt regulator.
The two LTC4071s regulate V
IN
of the LTC3588-1 to 8.2V
(with both ADJ pins floating) shunting any excess current
that is not used by the load once the batteries achieve
their float voltages. When the load requires more current
than is available from the piezoelectric generator, the
voltage at V
IN
droops and current is supplied from the two
Li-Ion cells to power the step-down switching regulator.
If the load pulls enough current to discharge the batteries
below V
LBD
, the LTC4071s disconnect the batteries, and
V
IN
collapses until the piezoelectric generator resumes
supplying current.
The application in Figure 8 illustrates how to implement
“ship-mode,” where a battery is co-packaged with the
LTC4071 and then the entire device is latched-off, leaving
the battery fully charged but with the LTC4071 switched off.
The co-packaged battery and LTC4071 can then be stored
with a long shelf-life before being activated for normal use.
Ship-mode is triggered by pulling enough current through
the LTC4071 so as to drop V
CC
below the LBD threshold.
The current pulse amplitude should be less than 400mA
with a duration of less than 10ms. The peak current neces-
sary, I
PK
, depends on the equivalent series resistance of
the battery, B
ESR
, summed with the R
DSON
of the BAT-V
CC
FET, the battery voltage, V
BAT
and the selected disconnect
voltage, V
LBD
:
I
PK
=
V
BAT
V
LBD
R
DSON
+ BESR
Users may test that ship mode has been triggered by
simply checking if V
CC
is at GND and that there are no
longer any NTCBIAS pulses.
Re-activation of the LTC4071 and the battery requires
either applying power normally, or briefly shorting V
CC
to BAT to turn it on.
applicaTions inForMaTion
Figure 8. LTC4071 Ship-Mode Application for Extended Shelf Life
LTC4071
BAT
NTCBIAS
NTC
ADJ
LBSEL
1µF
CURRENT PULSE
TO TRIGGER
SHIP-MODE
GND
Li-Ion
V
CC
I
PK
0
+
4071 F08
*NTHS0402N02N1002F
10k
T*
