LTC3114-1
19
31141fb
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applicaTions inForMaTion
A standard application circuit for the LTC3114-1 is shown
on the front page of this data sheet. The appropriate selec-
tion of external components is dependent upon the required
per
formance 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.
LDO Capacitor Selection
The LDO output of the LTC3114-1 is generated from V
IN
by a low dropout linear regulator. The LDO regulator has
been designed for stable operation with a wide range
of output capacitors. For most applications, a low ESR
capacitor of at least 4.7µF should be used. The capacitor
should be located as close to the PLDO pin as possible and
connected to the LDO pin and ground through the shortest
traces possible. PLDO is the regulator output and is also
the internal supply pin for the gate drivers and boost rail
charging diodes. The LDO pin is the supply connection for
the remainder of the control circuitry. The LDO and PLDO
pins must be connected together on the PCB. If the con
-
necting trace cannot be made short, an additional 0.1µF
bypass capacitor should be connected between the LDO
pin and ground as close to the package pins as possible.
Inductor Selection
The choice of inductor used in LTC3114-1 application cir
-
cuits 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 or output
current capability and 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 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 will also lower the
right half plane (RHP) zero frequency when operating in
boost mode, which requires the converter bandwidth to be
set lower in frequency, slowing the converter’s response
to load transients. Nearly all LTC3114-1 application cir
-
cuits deliver the best performance with an inductor value
between 4.7µH and
15µH. Buck mode-only applications
can use the larger inductor values as they are unaffected
by the RHP zero, while mostly boost applications generally
require inductance on the low end of this range depending
on how deep they will operate in boost mode.
Regardless of inductor value, the saturation current rating
should be selected such that it is greater than the worst-case
average inductor current plus half of 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 (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 0.05µs.
ΔI
L(P-P)(BUCK)
=
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
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) = 3.6V and V
IN
(maximum) =
40V, V
OUT
= 5V and L = 6.8µH, the peak-to-peak inductor
ripples at the voltage extremes (40V V
IN
for buck and 3.6V
V
IN
for boost) are:
Buck = 504mA peak-to-peak
Boost = 116mA 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.
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
