LTC3129_15 Datasheet, PDF (21/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129
APPLICATIONS INFORMATION
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 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 909kΩ 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 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.
For weak input sources with very high resistance (hun-
dreds of Ohms or more), the LTC3129 may still draw
more current than the source can provide, causing V_IN to
drop below the UVLO threshold. For these applications, it
is recommended that the programmable RUN feature be
used, as described in the previous section.
MPPC Compensation and Gain
When using MPPC, there are a number of variables that
affect the gain and phase of the input voltage control
loop. Primarily these are the input capacitance, the MPPC
divider ratio and the V_IN source resistance (or current). To
simplify the design of the application circuit, the MPPC
control loop in the LTC3129 is designed with a relatively
low gain, such that external MPPC loop compensation is
generally not required when using a V_IN capacitor value
of at least 22µF. The gain from the MPPC pin to the in-
ternal VC control voltage is about 12, so a drop of 50mV
on the MPPC pin (below the 1.175V MPPC threshold),
corresponds to a 600mV drop on the internal VC voltage,
which reduces the average inductor current all the way
to zero. Therefore, the programmed input MPPC voltage
will be maintained within about 4% over the load range.
Note that if large value V_IN capacitors are used (which may
have a relatively high ESR) a small ceramic capacitor of
at least 4.7µF should be placed in parallel across the V_IN
input, near the V_IN pin of the IC.
Bootstrapping the V_CC Regulator
The high and low side gate drivers are powered through
the V_CC rail, which is generated from the input voltage, V_IN,
through an internal linear regulator. In some applications,
especially at high input voltages, the power dissipation
in the linear regulator can become a major contributor to
thermal heating of the IC and overall efficiency. The Typical
Performance Characteristics section provides data on the
V_CC current and resulting power loss versus V_IN and V_OUT.
A significant performance advantage can be attained in high
V_IN applications where converter output voltage (V_OUT) is
programmed to 5V, if V_OUT is used to power the V_CC rail.
Powering V_CC in this manner is referred to as bootstrap-
ping. This can be done by connecting a Schottky diode
(such as a BAT54) from V_OUT to V_CC as shown in Figure 6.
With the bootstrap diode installed, the gate driver currents
are supplied by the buck-boost converter at high efficiency
rather than through the internal linear regulator. The in-
ternal linear regulator contains reverse blocking circuitry
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