Datasheet

LTC3129-1
22
31291fa
For more information www.linear.com/LTC3129-1
Figure 5. Example of V
CC
Bootstrap
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).
applicaTions inForMaTion
31291 F05
LTC3129-1
V
OUT
V
OUT
BAT54
C
OUT
V
CC
2.2µF
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
Corporation
http://www.ixys.com/ProductPortfolio/GreenEnergy.aspx
G24
Innovations
http://www.g24i.com/
SolarPrint http://www.solarprint.ie/