Datasheet

LTC3129-1
18
31291fa
For more information www.linear.com/LTC3129-1
applicaTions inForMaTion
A standard application circuit for the LTC3129-1 is shown
on the front page of this data sheet. The appropriate selec-
tion 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, transient
response, required efficiency, thermal considerations and
cost. This section of the data sheet provides some basic
guidelines and considerations to aid in the selection of
external components and the design of the applications
circuit, as well as more application circuit examples.
V
CC
Capacitor Selection
The V
CC
output of the LTC3129-1 is generated from V
IN
by a low dropout linear regulator. The V
CC
regulator has
been designed for stable operation with a wide range
of output capacitors. For most applications, a low ESR
capacitor of at least 2.2µF should be used. The capacitor
should be located as close to the V
CC
pin as possible and
connected to the V
CC
pin and ground through the shortest
traces possible. V
CC
is the regulator output and is also the
internal supply pin for the LTC3129-1 control circuitry as
well as the gate
drivers and boost rail charging diodes.
The
V
CC
pin is not intended to supply current to other
external circuitry.
Inductor Selection
The choice of inductor used in LTC3129-1 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, when
compared to the internal switch resistance, or output
current capability and efficiency will be compromised.
Larger inductor values reduce inductor current ripple
but may not increase output current capability as is the
case with peak current mode control as described in the
Maximum Output Current section. 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 can compromise loop stability. Nearly all LTC3129-1
application circuits
deliver the best performance with
an inductor value between 3.3µH and 10µH. 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 large the step-up ratio is.
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 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 mini-
mum low time in µs. The switch pin minimum low time
is typically 0.09µs.
ΔI
L(P−P)(BUCK)
=
V
OUT
L
V
IN
– V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
1
f
– t
LOW
⎛
⎝
⎜
⎞
⎠
⎟
A
ΔI
L(P−P)(BOOST)
=
V
IN
L
V
OUT
– V
IN
V
OUT
⎛
⎝
⎜
⎞
⎠
⎟
1
f
– t
LOW
⎛
⎝
⎜
⎞
⎠
⎟
A
It should be noted that the worst-case peak-to-peak in-
ductor ripple
current occurs when the duty cycle in buck
mode is minimum (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) = 2.5V and V
IN
(maximum) = 15V, V
OUT
= 5V and L = 10µH, the peak-to-peak inductor ripples at
the voltage extremes (15V V
IN
for buck and 2.5V V
IN
for
boost) are:
BUCK = 248mA peak-to-peak
BOOST = 93mA 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.
To avoid the possibility of inductor saturation during load
transients, an inductor with a saturation current rating of
at least 600mA is recommended for all applications.
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
particularly 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 cur-
rent
limit threshold. However, in boost mode, especially