LTC3113
12
3113f
APPLICATIONS INFORMATION
The basic LTC3113 application circuit is shown as the
typical application on the front page of this data sheet.
The external component selection is dependent upon the
required performance of the IC in each particular appli-
cation given considerations and trade-offs such as PCB
area, output voltage, output current, output ripple voltage
and effi ciency. 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
application circuit.
OUTPUT VOLTAGE PROGRAMMING
The buck-boost output voltage is set via an external resistor
divider connected to the FB pin as shown in Figure 2.
formulas, where f is the frequency in MHz and L is the
inductance in μH:
ΔI
L,P-P,BUCK
=
V
OUT
f•L
V
IN
–V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
A
()
ΔI
L,P-P,BOOST
=
V
IN
f•L
V
OUT
–V
IN
V
OUT
⎛
⎝
⎜
⎞
⎠
⎟
A
()
To ensure operation without triggering the reverse current
comparator under no load conditions it is recommended
that the peak-to-peak inductor ripple not exceed 800mA
taking into account the maximum reverse current limit of
–0.4A specifi ed in the Electrical Characteristics section.
Utilizing this recommendation for applications operating
at a switching frequency of 300kHz requires a minimum
inductance of 6.8μH, similarly an application operation at
a frequency of 2MHz would require a minimum of 1μH.
In addition to affecting output current ripple, the value of
the inductor can also impact the stability of the feedback
loop. In boost and buck-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.
In addition to affecting the effi ciency of the buck-boost
converter, the inductor DC resistance can also impact the
maximum output capability of the buck-boost converter
at low input voltage. In buck mode, the buck-boost 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 capability 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.
FB
R1
3113 F02
R2
LTC3113
1.8V ≤ V
OUT
≤ 5.5V
SGND
Figure 2. Setting the Output Voltage
The resistor divider values determine the buck-boost output
voltage according to the following formula:
V
OUT
= 0.600 1+
R2
R1
⎛
⎝
⎜
⎞
⎠
⎟
V
()
As noted in the Current Limit Operation section: “for the
current limit feature to be most effected, the Thevenin re-
sistance (R1||R2) from FB to ground should exceed 100k.”
INDUCTOR SELECTION
To achieve high effi ciency, a low ESR inductor should be
selected for the buck-boost converter. In addition, the
inductor must have a saturation current rating that is
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
