LTC3118_15 Datasheet, PDF (24/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

LTC3118
Applications Information
A standard application circuit for the LTC3118 is shown on
the front page of this data sheet. The appropriate selection
of external components is dependent upon the required
performance of the IC in each particular application, given
considerations and trade-offs such as PCB area, input
and output voltage range, output voltage ripple, required
efficiency, thermal considerations and cost. This section
of the data sheet provides some basic guidelines and con-
siderations to aid in the selection of external components
and the design of the applications circuit.
V_CC Capacitor Selection
V_CC is generated by a low dropout linear regulator from
either V_IN1 or V_IN2, whichever is selected. Both V_CC regula-
tors have been designed for stable operation with a wide
range of output capacitors. For most applications, a low
ESR capacitor of 4.7µF should be used. The capacitor
should be located as close to V_CC as possible and con-
nected to ground through the shortest trace possible. If
the connecting trace cannot be made short, an additional
0.1µF bypass capacitor should be connected between
V_CC and ground, as close to the package pins as possible.
Bootstrapping the V_CC Regulator with 5V V_OUT or
External Supply
The high and low side gate drivers are powered by V_CC,
which is generated from the selected 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 significant contributor to thermal
heating of the IC. The Typical Performance Characteristics
section of this data sheet provides data on V_CC current in
PWM operation, which is supplied by V_IN. A significant
performance advantage can be attained in applications
where V_OUT is programmed to 5V, if V_CC is powered by
V_OUT rather than the selected V_IN. This can be done by
connecting a Schottky diode from V_OUT to V_CC, as shown
in Figure 7. With the bootstrap diode installed, the gate
driver currents are supplied by the buck-boost converter
at high efficiency rather than through the less efficient
internal linear regulator. The internal linear regulator
contains reverse blocking circuitry that allows V_CC to be
driven slightly above their nominal regulation level with
only a slight amount of reverse current. Please note that the
bootstrapping supply (either V_OUT or a separate regulator)
must limit V_CC to less than 6V.
BST, Charge Pump and CM Capacitor Selection
Small ceramic capacitors are needed to provide a suf-
ficient amount of charge to the high side switches. As
shown in the applications circuits and the front page of
this data sheet, small capacitors are required from BST1
to SW1, BST2 to SW2, CN1 to CP1, CN2 to CP2, CM1 to
GND and CM2 to GND. Recommended initial values for
the BST to SW capacitors are 0.1µF with > 5V rating, CN
to CP capacitors are 10nF with > 20V rating, and CM to
GND capacitors are 47nF with > 20V rating.
Inductor Selection
The choice of inductor used in LTC3118 applications
influences the maximum deliverable output current, the
converter bandwidth, the magnitude of the inductor current
ripple and the overall converter efficiency. The inductor
must have a low DC series resistance and high output
V_OUT
V_OUT
LTC3118
V_CC
4.7µF
3118 F07
Figure 7. Bootstrapping V_CC
3118f
24
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