LTC3129_15 Datasheet, PDF (17/30 Pages) Linear Technology – 15V, 200mA Synchronous Buck-Boost DC/DC Converter with 1.3A Quiescent Current

LTC3129
APPLICATIONS INFORMATION
A standard application circuit for the LTC3129 is shown on
the front page of this data sheet. The appropriate selection
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.
Programming V_OUT
The output voltage of the LTC3129 is set by connecting
the FB pin to an external resistor divider from V_OUT to
ground, as shown in Figure 5, according to the equation:
V_OUT = 1.175V • (1+ R1/R2)
V_OUT
LTC3129
FB
C_OUT
V_OUT
R1
C_FF
R2
3129 F05
Figure 5. V_OUT Feedback Divider
A small feedforward capacitor can be added in parallel with
R1 (in Figure 5) to reduce Burst Mode ripple and improve
transient response. Details on selecting a feedforward
capacitor are provided later in this data sheet.
V_CC Capacitor Selection
The V_CC output of the LTC3129 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 pos-
sible. V_CC is the regulator output and is also the internal
supply pin for the LTC3129 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 application circuits
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 capabil-
ity 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 Cur-
rent 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 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 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 0.09µs.
∆I_{L(P−P )(BUCK )}
=
V_OUT
L



V_IN
– V_OUT
V_IN


1
f
–
t_LOW


A
∆I_{L(P−P)(BOOST)}
=
V_IN
L


V_OUT
–
V_IN
 ^V_OUT



1
f
–
t_LOW



A
For more information www.linear.com/LTC3129
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