Datasheet

LTC3129-1
20
31291fa
For more information www.linear.com/LTC3129-1
formula, where f is the frequency in MHz (1.2MHz), C
OUT
is the capacitance in µF, t
LOW
is the switch pin minimum
low time in µs (0.09µs typical) and I
LOAD
is the output
current in amperes.
ΔV
P−P(BUCK)
=
I
LOAD
t
LOW
C
OUT
V
ΔV
P−P(BOOST)
=
I
LOAD
fC
OUT
V
OUT
– V
IN
+ t
LOW
fV
IN
V
OUT
⎛
⎝
⎜
⎞
⎠
⎟
V
Examining the previous equations reveals that the output
voltage ripple increases with load current and is gener-
ally higher
in boost mode than in buck mode. Note that
these equations only take into account the voltage ripple
that
occurs from the inductor current to the output being
discontinuous. They provide a good approximation to the
ripple at any significant load current but underestimate the
output voltage ripple at very light loads where the output
voltage ripple is dominated by the inductor current ripple.
In addition to the output voltage ripple generated across
the output capacitance, there is also output voltage ripple
produced across the internal resistance of the output
capacitor. The ESR-generated output voltage ripple is
proportional to the series resistance of the output capacitor
and is given by the following expressions where R
ESR
is
the series resistance of the output capacitor and all other
terms as previously defined.
ΔV
P−P(BUCK)
=
I
LOAD
R
ESR
1– t
LOW
f
≅I
LOAD
R
ESR
V
ΔV
P−P(BOOST)
=
I
LOAD
R
ESR
V
OUT
V
IN
1– t
LOW
f
( )
≅I
LOAD
R
ESR
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
V
In most LTC3129-1 applications, an output capacitor be-
tween 10µF and 22µ
F will work well. To minimize output
ripple in Burst Mode operation, values of 22µF operation
or larger are recommended.
Input Capacitor Selection
The V
IN
pin carries the full inductor current and provides
power to internal control circuits in the IC. To minimize
input voltage ripple and ensure proper operation of the IC,
a low ESR bypass capacitor with a value of at least 4.7µF
should be located as close to the V
IN
pin as possible. The
traces connecting this capacitor to V
IN
and the ground
plane should be made as short as possible.
When powered through long leads or from a power source
with significant resistance, a larger value bulk input ca
-
pacitor may be required and is generally recommended.
In
such applications, a 47µF to 100µF low-ESR electrolytic
capacitor in parallel with a 1µF ceramic capacitor generally
yields a high performance, low cost solution.
Note that applications using the MPPC feature should
use a minimum C
IN
of 22µF. Larger values can be used
without limitation.
Recommended Input and Output Capacitor Types
The capacitors used to filter the input and output of
the
LTC3129-1 must have low ESR and must be rated to handle
the AC currents generated by the switching converter.
This is important to maintain proper functioning of the
IC and to reduce output voltage ripple. There are many
capacitor types that are well suited to these applications
including multilayer ceramic, low ESR tantalum, OS-CON
and POSCAP technologies. In addition, there are certain
types of electrolytic capacitors such as solid aluminum
organic polymer capacitors that are designed for low ESR
and high AC currents and these are also well suited to
some LTC3129-1 applications. The choice of capacitor
technology is primarily dictated by a trade-off between
size, leakage current and cost. In backup power applica
-
tions, the
input or output capacitor might be a super or
ultra
capacitor with a capacitance value measuring in the
farad range. The selection criteria in these applications
are generally similar except that voltage ripple is generally
not a concern. Some capacitors exhibit a high DC leak
-
age current
which may preclude their consideration for
applications
that require a very low quiescent current in
Burst Mode operation. Note that ultra capacitors may have
applicaTions inForMaTion