LTC3522_15 Datasheet, PDF (15/20 Pages) Linear Technology – Synchronous 400mA Buck-Boost and 200mA Buck Converters

LTC3522
APPLICATIONS INFORMATION
response. If a large inductor or small output capacitor is
utilized the loop will be less stable and the phase margin
can be improved by increasing the value of R2.
Buck-Boost Inductor Selection
To achieve high efficiency, a low ESR inductor should be
utilized for the buck-boost converter. The inductor must
have a saturation rating greater than the worst case average
inductor current plus half the ripple current. The peak-to-
peak inductor current ripple will be larger in buck and boost
mode than in the buck-boost region. The peak-to-peak
inductor current ripple for each mode can be calculated
from the following formulas, where f is the frequency in
MHz and L is the inductance in μH:
( ) _{ΔIL,P-P,BUCK}
=
1•
fL
V_OUT
V_IN – V_OUT
V_IN
( ) _{ΔIL,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 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 voltage
ripple. Multi-layer 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:
( ) _{ΔVP-P(BOOST)}
=
I_LOAD V_OUT –
C_OUT • V_OUT
V_IN
•f
( ) _{ΔVP-P(BUCK)}
=
8
•L
1
• C_OUT
•
f²
•
V_IN – V_OUT
V_IN
V_OUT
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 4.7μF 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 inductors 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
3522fa
15