LTC3115-2_15 Datasheet, PDF (18/42 Pages) Linear Technology – 40V, 2A Synchronous Buck-Boost DC/DC Converter

LTC3115-2
Applications Information
The standard LTC3115-2 application circuit is shown as the
typical application 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, cost, output and input voltage, allowable ripple
voltage, efficiency and thermal considerations. 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 application circuit.
V_CC Capacitor Selection
The V_CC output on the LTC3115-2 is generated from the
input voltage by an internal low dropout regulator. The V_CC
regulator has been designed for stable operation with a
wide range of output capacitors. For most applications,
a low ESR ceramic capacitor of at least 4.7µF should be
utilized. The capacitor should be placed as close to the
pin as possible and should connect to the PV_CC pin and
ground through the shortest traces possible. The PV_CC
pin is the regulator output and is also the internal supply
pin for the gate drivers and boost rail charging diodes.
The V_CC pin is the supply connection for the remainder
of the control circuitry. The PV_CC and V_CC pins must be
connected together on the application PCB. If the trace
connecting V_CC to PV_CC cannot be made via a short con-
nection, an additional 0.1µF bypass capacitor should be
placed between the V_CC pin and ground using the shortest
connections possible.
Inductor Selection
The choice of inductor used in LTC3115-2 application
circuits influences the maximum deliverable output cur-
rent, the magnitude of the inductor current ripple, and
the power conversion efficiency. The inductor must have
low DC series resistance or output current capability and
efficiency will be compromised. Larger inductance values
reduce inductor current ripple and will therefore gener-
ally yield greater output current capability. For a fixed DC
resistance, a larger value of inductance will yield higher
efficiency by reducing the peak current to be closer to the
average output current and therefore minimize resistive
losses due to high RMS currents. However, a larger induc-
tor value within any given inductor family will generally
have a greater series resistance, thereby counteracting
this efficiency advantage. In general, inductors with larger
inductance values and lower DC resistance will increase
the deliverable output current and improve the efficiency
of LTC3115-2 applications.
An inductor used in LTC3115-2 applications should have a
saturation current rating that is greater than the worst-case
average inductor current plus half the ripple current. The
peak-to-peak inductor current ripple for each operational
mode can be calculated from the following formula, where
f is the switching frequency, L is the inductance, and
t_LOW is the switch pin minimum low time. The switch pin
minimum low time can be determined from curves given
in the Typical Performance Characteristics section of this
data sheet.
∆I_{L( P-P) (BUCK)} =
V_OUT
L
⎛
⎝⎜
V_IN
– V_OUT
V_IN
⎞
⎠⎟
⎛⎝⎜
1–
f
t_LOW
⎞⎠⎟
∆I_{L( P-P) (BOOST )} =
V_IN
L
⎛
⎝⎜
V_OUT – V_IN
V_OUT
⎞
⎠⎟
⎛⎝⎜
1–
f
t_LOW
⎞⎠⎟
In addition to its influence on power conversion efficiency,
the inductor DC resistance can also impact the maximum
output current capability of the buck-boost converter par-
ticularly at low input voltages. In buck mode, the output
current of the buck-boost converter is generally limited
only by the inductor current reaching the current limit
threshold. However, in boost mode, especially at large
step-up ratios, the output current capability can also be
limited by the total resistive losses in the power stage.
These include switch resistances, inductor resistance,
and PCB trace resistance. Use of an inductor with high DC
resistance can degrade the output current capability from
that shown in the Typical Performance Characteristics sec-
tion of this data sheet. As a guideline, in most applications
the inductor DC resistance should be significantly smaller
than the typical power switch resistance of 150mΩ.
Different inductor core materials and styles have an impact
on the size and price of an inductor at any given current
rating. Shielded construction is generally preferred as it
minimizes the chances of interference with other circuitry.
The choice of inductor style depends upon the price, sizing,
and EMI requirements of a particular application. Table 1
31152fa
18
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