LTC3113 datasheet(12/24 Pages) LINER

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LTC3113

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 efﬁciency. 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 =

OUT

f• L

IN –VOUT

⎛

⎝⎜

⎞

⎠⎟

()

ΔI

L,P-P,BOOST =

f• L

OUT –VIN

OUT

⎛

⎝⎜

⎞

⎠⎟

()

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 speciﬁed 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 efﬁciency 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.

3113 F02

LTC3113

1.8V ≤ VOUT ≤ 5.5V

SGND

Figure 2. Setting the Output Voltage

The resistor divider values determine the buck-boost output

voltage according to the following formula:

OUT = 0.600 1+

⎛

⎝⎜

⎞

⎠⎟

()

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 efﬁciency, 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

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