LTC3129-1_15 Datasheet, PDF (22/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129-1
Applications Information
For weak input sources with very high resistance (hun-
dreds of Ohms or more), the LTC3129-1 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 5.
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
that allows V_CC to be driven above its nominal regulation
level with only a very slight amount of reverse current.
Please note that the bootstrapping supply (either V_OUT or
a separate regulator) must be limited to less than 5.7V so
as not to exceed the maximum V_CC voltage of 5.5V after
the diode drop.
By maintaining V_CC above its UVLO threshold, bootstrap-
ping, even to a 3.3V output, also allows operation down
to the V_IN UVLO threshold of 1.8V (typical).
V_OUT
LTC3129-1
C_OUT
V_OUT
BAT54
V_CC
31291 F05
2.2µF
Figure 5. Example of V_CC Bootstrap
Sources of Small Photovoltaic Panels
A list of companies that manufacture small solar panels
(sometimes referred to as modules or solar cell arrays)
suitable for use with the LTC3129-1 is provided in Table 4.
Table 4. Small Photovoltaic Panel Manufacturers
Sanyo
http://panasonic.net/energy/amorton/en/
PowerFilm http://www.powerfilmsolar.com/
IXYS
http://www.ixys.com/ProductPortfolio/GreenEnergy.aspx
Corporation
G24
http://www.g24i.com/
Innovations
SolarPrint http://www.solarprint.ie/
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For more information www.linear.com/LTC3129-1
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