Datasheet

LTC3129
19
3129fa
For more information www.linear.com/LTC3129
applicaTions inForMaTion
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 applications, an output capacitor between
10µF and 22µF will work well. To minimize output ripple
in Burst Mode operation, or transients incurred by large
step loads, values of 22µF 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.
Burst Mode operation and improve transient response. In
addition, due to the wide V
IN
and V
OUT
operating range
of the LTC3129 and its fixed internal loop compensation,
some applications may require the use of a feedforward
capacitor to assure light-load stability (less than ~15mA)
when operating in PWM mode (PWM pin pulled high).
Therefore, to reduce Burst Mode ripple and improve
phase margin at light load when PWM mode operation is
selected, a feedforward capacitor is recommended for all
applications. The recommended
feedforward capacitor
value can be calculated by:
C
FF
= 66/R1
Where R1 is the top feedback divider resistor value in MΩ
and C
FF
is the recommended feedforward capacitor value
in picofarads (use the nearest standard value). Refer to
the application circuits for examples.
Output Capacitor Selection
A low effective series resistance (ESR) output capacitor
of 4.7µF minimum should be connected at the output of
the buck-boost converter in order to minimize output volt
-
age ripple. Multilayer ceramic capacitors are an excellent
option
as they have low ESR and are available in small
footprints. The capacitor value should be chosen large
enough to reduce the output voltage ripple to acceptable
levels. Neglecting the capacitor's ESR and ESL (effec
-
tive series
inductance), the peak-to-peak output voltage
ripple
in PWM mode can be calculated by the following
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