LTC3331_15 Datasheet, PDF (23/34 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger

LTC3331
APPLICATIONS INFORMATION
The input capacitor to the buck on V_IN and the V_OUT capaci-
tor can vary widely and should be selected to optimize the
use of an energy harvesting source depending on whether
storage of the harvested energy is needed at the input or
the output. Storing energy at the input takes advantage of
the high input voltage as the energy stored in a capacitor
increases with the square of its voltage. Storage at the
output may be necessary to handle load transients greater
than the 100mA the buck can provide.
The input or output capacitor should be sized to store
enough energy to provide output power for the length
of time required. If enough energy is stored so that the
buck does not reach the UVLO falling threshold during
a load transient then the battery current will always be
zero. Spacing load transients so that the average power
required to service the application is less than or equal
to the power available from the energy harvesting source
will then greatly extend the life of the battery. The V_IN
capacitor should be rated to withstand the highest voltage
ever present at V_IN.
The following equation can be used to size the input ca-
pacitor to meet the power requirements of the output for
the desired duration:
P_LOAD
t_LOAD
=
1
2
ηC_IN( V_IN2
–
V_UVLOFALLING2)
V_UVLOFALLING ≤ V_IN ≤ V_SHUNT
Here η is the average efficiency of the buck converter over
the input voltage range and V_IN is the input voltage when
the buck begins to switch. Typically V_IN will be the UVLO
rising threshold. This equation may overestimate the input
capacitor necessary as it may be acceptable to allow the
load current to deplete the output capacitor all the way to
the lower PGVOUT threshold. It also assumes that the input
source charging has a negligible effect during this time.
The duration for which the buck or buck-boost regulator
sleeps depends on the load current and the size of the V_OUT
capacitor. The sleep time decreases as the load current
increases and/or as the output capacitor decreases. The
DC sleep hysteresis window is ±6mV for the 1.8V output
and scales linearly with the output voltage setting (±12mV
for the 3.6V setting, etc.). Ideally this means that the sleep
time is determined by the following equation:
12mV • ^VOUT
t_SLEEP = C_OUT
1.8V
I_LOAD
This is true for output capacitors on the order of 100μF
or larger, but as the output capacitor decreases towards
10μF, delays in the internal sleep comparator along with
the load current itself may result in the V_OUT voltage slew-
ing past the DC thresholds. This will lengthen the sleep
time and increase V_OUT ripple. A capacitor less than 10μF
is not recommended as V_OUT ripple could increase to an
undesirable level. If transient load currents above 100mA
are required then a larger capacitor should be used at the
output. This capacitor will be continuously discharged
during a load condition and the capacitor can be sized for
an acceptable drop in V_OUT:
( ) C_OUT =
V_OUT ⁺ –V_OUT ^–
I_LOAD –I_DC/DC
t _LOAD
Here V_OUT+ is the value of V_OUT when PGVOUT goes
high and V_OUT– is the acceptable lower limit of V_OUT.
I_DC/DC is the average current being delivered from either
the buck converter or the buck-boost converter. The
buck converter typically delivers 125mA on average
to the output as the inductor current is ramped up to
250mA and down to zero. The current the buck-boost
delivers depends on the mode of operation and the
I_{PEAK_BB} setting. In buck mode the deliverable current
is I_{PEAK_BB/}2. In buck-boost and boost modes the de-
liverable current also depends on the V_IN to V_OUT ratio:
Buck-boost mode:
I_DC/DC
=
3
4
I_{PEAK_BB}
V_IN
V_OUT
Boost mode:
I_DC/DC
=
21I^PEAK_BB
V_IN
V_OUT
A standard surface mount ceramic capacitor can be used for
C_OUT, though some applications may be better suited to a low
leakage aluminum electrolytic capacitor or a supercapacitor.
These capacitors can be obtained from manufacturers such
as Vishay, Illinois Capacitor, AVX, or CAP-XX.
For more information www.linear.com/LTC3331
3331fb
23