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dc2042af
DEMO MANUAL DC2042A
APPLICATION INFORMATION
LTC3105: SUPPLIED BY DIODE VOLTAGE DROP IN
4mA TO 20mA LOOP
The LTC3105 4mA to 20mA loop, diode voltage drop-
powered energy harvester is selected by installing the
power selection jumper, JP3. The PGOOD signal, PGOOD_
LTC3105, can be routed to the Dust Header by installing
jumper JP7. The PGOOD_LTC3105 signal is an open-drain
output. The pull-down is disabled at the beginning of the
first sleep event after the output voltage has risen above
90% of its regulation value. PGOOD_LTC3105 remains
asserted until VOUT drops below 90% of its regulation
value at which point PGOOD_LTC3105 will pull low. The
pull-down is also disabled while the IC is in shutdown or
start-up mode. The Dust Eterna board will switch from
battery power to energy harvester power whenever the
PGOOD signal is high (>1.4V). Again a resistor divider is
generated by the pull-up resistor R23 on the DC2042A
and R4, the pull-down resistor on the DC9003A-A or
DC9003A-B. Provided that R4 is changed as described
above to 5.1MΩ, the voltage seen by the analog switch
will be sufficient to register as a “Logic 1” and switch the
Mote or Manager from the battery to the energy harvested
source.
If the application would benefit from a wider PGOOD
hyteresis window than the LTC3105 provides (sleep to
V
OUT
minus 10%), please refer to the above section for a
complete operational description of and how to use the
independent PGOOD signal (PGOOD_LTC3459), shown
in Figure 12, generated by the LTC2935-2 and available
on JP8.
The PGOOD_LTC3459 signal is always used to switch the
output voltage on the Header.
Some loads do not like to
see a slowly rising input voltage. Switch Q3 ensures that
VSUPPLY and VMCU on the headers are off until the energy
harvested output voltage is high enough to power the load.
The PGOOD_LTC3459 signal can be used in place of any
of the PGOOD signals generated by the harvester circuits.
The optional components shown on the schematic are not
populated for a standard assembly. The function of R22 and
Q2 is to generate a short PGOOD pulse that will indicate
when the output capacitor is charged to its maximum value.
The short pulse occurs every time the output capacitor
charges up to the “output sleep threshold”, which for a
3.3V output is 3.312V. By populating these components
the application can use this short pulse as a sequence timer
to step through the program sequence or as an indication
of when it can perform energy intensive functions, such as
a sensor read or a wireless transmission and/or receive,
knowing precisely how much charge is available in the
output capacitors. When this optional circuit is not used,
the amount of charge in the output capacitors is anywhere
between the maximum (C
OUT
• V
OUT_SLEEP
) to ten percent
low. In the case where the energy harvesting source can
support the average load continuously, this optional circuit
is not needed.
Figure 10. Detailed Schematic of LTC3105 4mA to 20mA Loop, Diode Voltage Drop Energy Harvester
