LTC3119 datasheet(20/36 Pages) LINER

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LTC3119

3119fb

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applicaTions inForMaTion

Higher switching frequencies facilitate the use of smaller

inductors as well as smaller input and output filter capaci-

tors which results in a smaller solution size and reduced

componentheight.However,higherswitchingfrequencies

also generally reduce conversion efficiency due to the

increased switching losses.

Output Capacitor Selection

A low ESR output capacitor should be utilized at the buck-

boost converter output in order to minimize output voltage

ripple.Multilayerceramiccapacitorsareanexcellentoption

as they have low ESR and are available in small footprints.

The capacitor value 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 fSW is the switching frequency, COUT is

the output capacitance, tLOW is the switch pin minimum

low time, and ILOAD is the output current. Curves for the

value of tLOW as a function of VCC voltage and temperature

can be found in Typical Performance Characteristics sec-

tion of this data sheet.

∆VP-P BUCK

(

) @

ILOADtLOW

COUT

∆VP-P BOOST

(

) @

ILOAD

fSW COUT

VOUT −VIN +tLOWfSWVIN

VOUT













The output voltage ripple increases with load current and

is generally higher in boost mode than in buck mode.

These expressions only take into account the output

voltage ripple that results from the output current being

discontinuous. They provide a good approximation to the

ripple at any significant load current but underestimate

the output voltage ripple at very light loads where output

voltage ripple is dominated by the inductor current ripple.

In addition to output voltage ripple generated across the

output capacitance, there is also output voltage ripple

produced across the internal resistance of the output

capacitor. The ESR-generated output voltage ripple is

proportionaltotheseriesresistanceoftheoutputcapacitor

and is given by the following expressions where RESR is

the series resistance of the output capacitor and all other

terms are as previously defined.

∆VP-P BUCK

(

) @

ILOADRESR

− tLOWfSW

@ILOADRESR

∆VP-P BOOST

(

) @

ILOADRESRVOUT

VIN(1− tLOWfSW)

@ILOADRESR

VOUT

VIN









Input Capacitor Selection

The PVIN pin carries the full inductor current and provides

power to internal control circuits in the IC. To minimize

input voltage ripple and ensure proper operation of the IC,

a low ESR bypass capacitor with a value of at least 10μF

should be located as close to this pin as possible. The

traces connecting this capacitor to PVIN and the ground

plane should be made as short as possible. The VIN pin

provides power to the VCC regulator and other internal

circuitry. If the PCB trace connecting VIN to PVIN is long, it

may be necessary to add an additional small value bypass

capacitor near the VIN pin.

When powered through long leads or from a high ESR

power source, a larger value bulk input capacitor may be

required.Insuchapplications,a47μFto100μFelectrolytic

capacitor in parallel with a 1μF ceramic capacitor generally

yields a high performance, low cost solution.

When powered through an inductive connection such as

a long cable, the inductance of the power source and the

input bypass capacitor form a high-Q resonant LC filter. In

such applications, hot-plugging into a powered source can

lead to a significant voltage overshoot, even up to twice the

nominal input source voltage. Care must be taken in such

situationstoensurethattheabsolutemaximuminputvoltage

ratingoftheLTC3119isnotviolated.SeeLinearTechnology

Application Note 88 for solutions to increase damping in

the input filter and minimize this voltage overshoot.

Inductor Selection

The choice of inductor used in LTC3119 application circuits

influences the maximum deliverable output current, the

converter bandwidth, the magnitude of the inductor current

rippleandtheoverallconverterefficiency.Theinductormust

have a low DC series resistance, when compared to the

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