LTC3129-1_15 Datasheet, PDF (20/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129-1
Applications Information
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.
∆V_{P−P(BUCK )}
=
I_LOAD t _LOW
C_OUT
V
∆V_{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.
∆V_{P−P(BUCK )}
= ^ILOADRESR
1– t_LOW f
≅ I_LOADR_ESR
V
( ) ^{∆VP−P(BOOST}
)
=
I_LOADR_ESR V_OUT
V_IN 1– t_LOW f
≅
I_LOADR_ESR



V_OUT
V_IN



V
In most LTC3129-1 applications, an output capacitor be-
tween 10µF and 22µF 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
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