LTC3129-1_15 Datasheet, PDF (21/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129-1
Applications Information
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 LTC3129-1. 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 opera-
tion 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 LTC3129-1 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 V_IN 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.
When using high value divider resistors (in the MΩ range)
to minimize current draw on V_IN, a small noise filter ca-
pacitor may be necessary across the lower divider resis-
tor to prevent noise from erroneously tripping the RUN
comparator. The capacitor value should be minimized
so as not to introduce a time delay long enough for the
input voltage to drop significantly below the desired V_IN
threshold before the converter is turned off. Note that
larger V_IN decoupling capacitor values will minimize this
effect by providing more holdup time on V_IN.
Programming the MPPC Voltage
As discussed in the previous section, the LTC3129-1 in-
cludes an MPPC function to optimize performance when
operating from voltage sources with relatively high source
resistance. Using an external voltage divider from V_IN, the
MPPC function takes control of the average inductor current
when necessary to maintain a minimum input voltage, as
programmed by the user. Referring to Figure 3:
V_IN(MPPC) = 1.175V • (1 + R5/R6)
This is useful for such applications as photovoltaic pow-
ered converters, since the maximum power transfer point
occurs when the photovoltaic panel is operated at about
75% of its open-circuit voltage. For example, when operat-
ing from a photovoltaic panel with an open-circuit voltage
of 5V, the maximum power transfer point will be when
the panel is loaded such that its output voltage is about
3.75V. Choosing values of 2MΩ for R5 and 909k for R6
will program the MPPC function to regulate the maximum
input current so as to maintain V_IN at a minimum of 3.74V
(typical). Note that if the panel can provide more power
than the LTC3129-1 can draw, the input voltage will rise
above the programmed MPPC point. This is fine as long
as the input voltage doesn't exceed 15V.
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