LTC3118_15 Datasheet, PDF (25/38 Pages) Linear Technology – 18V, 2A Buck-Boost DC/DC Converter with Low-Loss Dual Input PowerPath

LTC3118
Applications Information
current capability or efficiency will be compromised. Larger
inductor values reduce inductor current ripple, but will
not increase output current capability as is the case with
peak current mode control, as described in the Inductor
Current Sense and Maximum Output Current section of
this data sheet. Larger value inductors also tend to have
a higher DC series resistance for a given case size, which
will have a negative impact on efficiency. Larger values
of inductance also lower the right half plane (RHP) zero
frequency when operating in boost mode, requiring the
converter bandwidth to be set lower in frequency, thereby
slowing the converter’s load transient response. Most
LTC3118 application circuits deliver the best performance
with an inductor value between 3.3µH and 10µH. In gen-
eral, a 3.3µH inductor is recommended for V_OUT up to 5V,
6.8µH for V_OUT = 12V and 10µH for V_OUT = 18V. Inductor
values for other output voltages can be scaled accordingly.
Regardless of inductor value, the saturation current rat-
ing should be such that it is greater than the worst-case
average inductor current plus half of the ripple cur-
rent. The peak-to-peak inductor current ripple for each
operational mode can be calculated from the following
formula, where f is the switching frequency (1.2MHz),
L is the inductance in µH, and t_LOW is the switch pin
minimum low time in µs. The switch pin minimum low
time is typically 0.1µs.
ΔI_L(P-P)BUCK =
ripples at the voltage extremes (18V V_IN for buck and 2.7V
V_IN for boost) are:
Buck = 600mA peak-to-peak
Boost = 200mA peak-to-peak
One-half of this inductor ripple current must be added to
the highest expected average inductor current in order to
select the proper saturation current rating for the inductor
(about 4A).
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 primarily limited
by the inductor current reaching the average current limit
threshold defined by VC. However, in boost mode, espe-
cially at large step-up ratios, the output current capability
can also be limited by the total resistive losses in the power
stage. These losses include switch resistances, inductor
DC resistance and PCB trace resistance. Avoid inductors
with a high DC resistance (DCR), as they can degrade the
maximum output current capability from what is shown
in the Typical Performance Characteristics section. As a
guideline, the inductor DCR should be significantly less
than the typical power switch resistance of 100mΩ. The
only exceptions are applications that have a maximum
output current much less than what the LTC3118 is ca-
pable of delivering.
V_OUT
L
⎛
⎜
⎝
V_IN
− V_OUT
V_IN
⎞⎛
⎠⎟⎝⎜
1
f
−
t_LOW
⎞
⎠⎟
Amps
ΔI_L(P-P)BOOST =
V_IN
L
⎛
⎜
⎝
V_OUT − V_IN
V_OUT
⎞⎛
⎠⎟⎝⎜
1
f
−
t_LOW
⎞
⎠⎟
Amps
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
provides a small sampling of inductors that are well suited
to many LTC3118 applications.
It should be noted that the worst-case inductor peak-to-
peak inductor ripple current occurs when the duty cycle
in buck mode is maximum (highest V_IN), and in boost
mode when the duty cycle is 50% (V_OUT = 2 • V_IN). As an
example, if V_IN (minimum) = 2.7V and V_IN (maximum) =
18V, V_OUT = 5V and L = 3.3µH, the peak-to-peak inductor
Output Capacitor Selection
A low effective series resistance (ESR) output capacitor
should be connected at the output of the buck-boost con-
verter in order to minimize output voltage ripple. Multilayer
ceramic capacitors are an excellent option as they have low
ESR and are available in small footprints. The capacitor
3118f
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