LTC3520
17
3520fa
In addition to affecting output current ripple, the size of
the inductor can also affect the stability of the feedback
loop. In boost mode, the converter transfer function has
a right half plane zero at a frequency that is inversely
proportional to the value of the inductor. As a result, a
large inductor can move this zero to a frequency that is
low enough to degrade the phase margin of the feedback
loop. It is recommended that the inductor value be chosen
less than 10μH if the buck-boost converter is to be used
in the boost region.
Buck-Boost Output Capacitor Selection
A low ESR output capacitor should be utilized at the buck-
boost converter output in order to minimize output volt-
age ripple. Multilayer ceramic capacitors are an excellent
choice as they have low ESR and are available in small
footprints. The capacitor should be chosen large enough
to reduce the output voltage ripple to acceptable levels.
Neglecting the capacitor ESR and ESL, the peak-to-peak
output voltage ripple can be calculated by the following
formulas, where f is the frequency in MHz, C
OUT
is the
capacitance in μF, L is the inductance in μH, and I
LOAD
is
the output current in amps.
Δ
Δ
V
IVV
CVf
V
P P BOOST
LOAD OUT IN
OUT OUT
PPB
−
−
=
−
,
,
()
UUCK
OUT
IN OUT OUT
IN
LC f
VV V
V
=
−
1
8
2
()
Since the output current is discontinuous in boost mode,
the ripple in this mode will generally be much larger than
the magnitude of the ripple in buck mode. In addition to
controlling the ripple magnitude, the value of the output
capacitor also affects the location of the resonant frequency
in the open loop converter transfer function. If the output
capacitor is too small, the bandwidth of the converter
will extend high enough to degrade the phase margin.
To prevent this from happening, it is recommended that
a minimum value of 22μF be used for the buck-boost
output capacitor.
Buck-Boost Input Capacitor Selection
The supply current to the buck-boost converter is provided
by the PV
IN1
and PV
IN3
pins. It is recommended that a
low ESR ceramic capacitor with a value of at least 22μF
be located as close to this pin as possible.
Inductor Style and Core Material
Different inductor core materials and styles have an
impact on the size and price of an inductor at any given
peak current rating. Toroid or shielded pot cores in ferrite
or permalloy materials are small and reduce emissions,
but generally cost more than powdered iron core induc-
tors with similar electrical characteristics. The choice of
inductor style depends upon the price, sizing, and EMI
requirements of a particular application. However, the
inductor must also have low ESR to provide acceptable
effi ciency and must be able to carry the highest current
required by the application without saturating. Table 3
provides a list of several manufacturers of inductors that
are well suited to LTC3520 applications.
Table 3. Inductor Vendor Information
MANUFACTURER PHONE WEB SITE
Coilcraft 847-639-6400 www.coilcraft.com
Murata 814-238-0490 www.murata.com
Sumida 847-956-0702 www.sumida.com
TDK 847-803-6296 www.component.tdk.com
TOKO 847-699-7864 www.tokoam.com
APPLICATIONS INFORMATION
