Buck-Boost Controller Simplifi

Buck-Boost Controller Simplifi es Design of DC/DC Converters for Handheld Products：A number of conventional solutions have been available for
the design of a DC/DC converter where the output voltage
is within the input voltage range—a common scenario
in Li-Ion battery-powered applications—but none were
very attractive until now. Conventional topologies, such
as SEPIC or boost followed by buck, have numerous
disadvantages, including low effi ciency, complex magnetics,
polarity inversion and/or circuit complexity/cost. The
LTC®3785 buck-boost controller yields a simple, effi cient,
low parts-count, single-converter solution that is easy
to implement, thus avoiding the drawbacks associated
with traditional solutions.
High Effi ciency Controller Capabilities
The LTC3785 serves applications requiring input and
output voltages in the range of 2.7 to 10V—ideal for
applications powered from one or two Li-Ion cells or
multiple cell NiMH, NiCad or Alkaline batteries. It supports
a single inductor, 4-switch, buck-boost topology, which is
ideal for an output voltage that is within the input voltage
range. The high level of integration yields a simple, low
parts-count solution. The LTC3785 provides for currentlimit
and shutdown in all modes of operation (which is
not normally available with boost converters).
Very high effi ciency is achieved by synchronous rectifi
cation, high side drive (allowing the use of N-channel
MOSFETs), RDS(ON) current sensing, and Burst Mode®
operation for effi cient light load operation. Protection
features include soft-start, overvoltage, undervoltage
and foldback current-limit with burp-mode or latch-off
for extended faults.
3.3V, 3A Converter Operates from 2.7V – 10V Source
The circuit shown in Figure 1 utilizes the LTC3785 controller
to produce a synchronous, 4-switch, buck-boost
design. It provides a fi xed 3.3V, 3A output from a 2.7V
– 10V input. It is short-circuit protected: the controller
offers a choice of recycling or latch-off protection for
severe overload faults.