LTC3521
16
3521fb
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be calculated from the following formulas, where f is the
frequency in MHz and L is the inductance in μH:
ΔI
L,P-P,BUCK
=
1
fL
•
OUT
IN
OUT
V
IN
ΔI
L,P-P,BOOST
=
1
fL
•
V
IN
V
OUT
– V
IN
( )
V
OUT
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 chosen inductor value be
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:
ΔV
P-P,BOOST
=
LOAD
OUT
IN
C
OUT
• V
OUT
• f
ΔV
P-P,BUCK
=
1
8 • L • C
OUT
• f
2
•
V
IN
– V
OUT
( )
V
OUT
V
IN
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 10μF 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
pin. It is recommended that a low ESR ceramic
capacitor with a value of at least 4.7μF 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. Table 4 provides
a sampling of inductors that are well suited to many
LTC3521 application circuits.
Table 4. Representative Surface Mount Inductors
MANU-
FACTURER
PART NUMBER
V
ALUE
MAX
CURRENT
DCR
HEIGHT
Taiyo Yuden NP03SB4R7M 4.7μH 1.2A 0.047Ω 1.8mm
NP03SB6R8M 6.8μH 1A 0.084Ω 1.8mm
Coilcraft MSS7341-502NL 5μH 2.3A 0.024Ω 4.1mm
DT1608C-472ML 4.7µH 1.2A 0.085Ω 2.92mm
Cooper-
Bussmann
SD7030-5R0-R 5µH 2.4A 0.026Ω 3mm
SD20-6R2-R 6.2µH 1.12A 0.072Ω 2mm
Sumida CDR6D23MNNP-4R2 4.2µH 2.6A 0.052Ω 2.5mm
CDRH4D16FB/ND-
6R8N
6.8µH 1A 0.081Ω 1.8mm
applicaTions inForMaTion
