LTC3112_15 Datasheet, PDF (15/32 Pages) Linear Technology – 15V, 2.5A Synchronous Buck-Boost DC/DC Converter

LTC3112
Applications Information
Where f is the switching frequency in Hz and L is the
inductor value in Henries.
In addition to affecting output current ripple, the size of the
inductor can also impact 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 rec-
ommended that the inductor value be chosen less than
15μH if the converter is to be used in the boost region.
For 750kHz operation, a 4.7µH inductor is recommended
for 5V_OUT and a 10µH inductor for 12V_OUT.
The inductor DC resistance can impact the efficiency of
the buck-boost converter as well as the maximum output
current capability at low input voltage. In buck mode,
the output current is limited only by the inductor current
reaching the current limit value. However, in boost mode,
especially at large step-up ratios, the output current capa-
bility can also be limited by the total resistive losses in the
power stage. These include switch resistances, inductor
resistance, and PCB trace resistance. Use of an inductor
with high DC resistance can degrade the output current
capability from that shown in the graph in the Typical
Performance Characteristics section of this data sheet.
Different inductor core materials and styles have an impact
on the size and price of an inductor at any given current
rating. Shielded construction is generally preferred as it
minimizes the chances of interference with other circuitry.
The choice of inductor style depends upon the price, sizing,
and EMI requirements of a particular application. Table 1
provides a small sampling of inductors that are well suited
to many LTC3112 buck-boost converter applications. All
inductor specifications are listed at an inductance value
of 4.7µH for comparison purposes but other values within
these inductor families are generally well suited to this
application. Within each family (i.e. at a fixed size), the DC
resistance generally increases and the maximum current
generally decreases with increased inductance.
Table 1. Representative Buck-Boost Surface Mount Inductors
PART NUMBER
VALUE DCR MAX I SIZE (mm)
(µH) (mΩ) (A)
W×L×H
Coilcraft XPL7030-472ML
4.7 40.1 6.8
7×7×3
Coilcraft MSS1048-472NLB 4.7 12.3 6.46 10 × 10 × 4.8
Würth 744 311 470
4.7 24
6 7 × 6.9 × 3.8
Cooper Bussmann HC8-4R5-R 4.5 18.6 7.7 10.9 × 10.4 × 4
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.
The minimum output capacitor needed for a given output
voltage ripple (neglecting ESR and ESL) can be calculated
by the following formulas:
( ) _COUT
=
1
∆ V_{P−P, BUCK} 8 • L • f²
•
V_IN − V_OUT
V_IN
V_OUT
( ) _COUT
=
I_LOAD V_OUT
∆ V_{P−P, BOOST}
− V_IN
V_OUT
•f
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.
Given that 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. For
most applications a 47µF or greater output capacitor is
recommended.
INPUT CAPACITOR SELECTION
It is recommended that a low ESR ceramic capacitor with
a value of at least 10μF be located as close to the V_IN and
GND pins as possible. In addition, the return trace from
each pin to the ground plane should be made as short
as possible. For instances where the input source, such
as a bench supply, is far away from the converter, a bulk
capacitor of 100µF or greater is suggested to provide a
low ripple input voltage especially in buck mode.
3112fc
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