LTC3129_15 Datasheet, PDF (20/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129
APPLICATIONS INFORMATION
When powered through long leads or from a power source
with significant resistance, a larger value bulk input ca-
pacitor may be required and is generally recommended.
In such applications, a 47µF to 100µF low-ESR electrolytic
capacitor in parallel with a 1µF ceramic capacitor generally
yields a high performance, low cost solution.
Note that applications using the MPPC feature should
use a minimum C_IN of 22µF. Larger values can be used
without limitation.
Recommended Input and Output Capacitor Types
The capacitors used to filter the input and output of the
LTC3129 must have low ESR and must be rated to handle
the AC currents generated by the switching converter.
This is important to maintain proper functioning of the
IC and to reduce output voltage ripple. There are many
capacitor types that are well suited to these applications
including multilayer ceramic, low ESR tantalum, OS-CON
and POSCAP technologies. In addition, there are certain
types of electrolytic capacitors such as solid aluminum
organic polymer capacitors that are designed for low
ESR and high AC currents and these are also well suited
to some LTC3129 applications. The choice of capacitor
technology is primarily dictated by a trade-off between
size, leakage current and cost. In backup power applica-
tions, the input or output capacitor might be a super or
ultra capacitor with a capacitance value measuring in the
Farad range. The selection criteria in these applications
are generally similar except that voltage ripple is generally
not a concern. Some capacitors exhibit a high DC leak-
age current which may preclude their consideration for
applications that require a very low quiescent current in
Burst Mode operation. Note that ultra capacitors may have
a rather high ESR, therefore a 4.7µF (minimum) ceramic
capacitor is recommended in parallel, close to the IC pins.
Ceramic capacitors are often utilized in switching con-
verter applications due to their small size, low ESR and
low leakage currents. However, many ceramic capacitors
intended for power applications experience a significant
loss in capacitance from their rated value as the DC bias
voltage on the capacitor increases. It is not uncommon for
a small surface mount capacitor to lose more than 50%
of its rated capacitance when operated at even half of its
maximum rated voltage. This effect is generally reduced
as the case size is increased for the same nominal value
capacitor. As a result, it is often necessary to use a larger
value capacitance or a higher voltage rated capacitor than
would ordinarily be required to actually realize the intended
capacitance at the operating voltage of the application. X5R
and X7R dielectric types are recommended as they exhibit
the best performance over the wide operating range and
temperature of the LTC3129. To verify that the intended
capacitance is achieved in the application circuit, be sure
to consult the capacitor vendor's curve of capacitance
versus DC bias voltage.
Using the Programmable RUN Function to Operate
from Extremely Weak Input Sources
Another application of the programmable RUN pin is that
it can be used to operate the converter in a hiccup mode
from extremely low current sources. This allows operation
from sources that can only generate microamps of output
current, and would be far too weak to sustain normal steady-
state operation, even with the use of the MPPC pin. Because
the LTC3129 draws only 1.9µA typical from V_IN until it is
enabled, the RUN pin can be programmed to keep the IC
disabled until V_IN reaches the programmed voltage level.
In this manner, the input source can trickle-charge an input
storage capacitor, even if it can only supply microamps of
current, until V_IN reaches the turn-on threshold set by the
RUN pin divider. The converter will then be enabled using
the stored charge in the input capacitor, until Vin drops
below the turn-off threshold, at which point the converter
will turn off and the process will repeat.
This approach allows the converter to run from weak
sources such as thin-film solar cells using indoor lighting.
Although the converter will be operating in bursts, it is
enough to charge an output capacitor to power low duty
cycle loads, such as wireless sensor applications, or to
trickle charge a battery. In addition, note that the input
voltage will be cycling (with a small ripple as set by the
RUN hysteresis) about a fixed voltage, as determined by
the divider. This allows the high impedance source to
operate at the programmed optimal voltage for maximum
power transfer.
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