LTC3113 datasheet(20/32 Pages) AD

20 / 32 page

LT3154

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Examining the previous equations reveals that the output

voltage ripple increases with load current and is gener‑

ally higher in boost mode than in buck mode. Note that

these equations only take into account the voltage ripple

that occurs from the inductor current to the output 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 the output

voltage ripple is dominated by the inductor current ripple.

In addition to the 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 as previously defined.

ΔVP−P(BUCK) = ΔIL RESRVolts

ΔVP−P(BOOST) =ILOADRESR

VOUT

VIN

⎛

⎝

⎜

⎞

⎠

⎟Volts

In most LT3154 applications, an output capacitor between

68µF and 220µF from PVOUT to PGND will work well. An

additional low value ceramic capacitor such as 4.7µF can

be placed between VOUT and GND to reduce switching

noise to the control circuitry.

Input Capacitor Selection

The PVIN pin carries the full inductor current and provides

power to internal switches and drivers where VIN powers

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 22µF should be located

as close to PVIN as possible. The traces connecting this

capacitor to PVIN and the ground plane (PGND) should be

madeasshortaspossible.Anadditionallowvalueceramic

capacitor such as 2.2µF can be placed between VIN and

GND to reduce switching noise to the control circuitry.

Recommended Input and Output Capacitors

The capacitors used to filter the input and output of the

LT3154 must have low ESR and must be rated to handle

the large AC currents generated by the switching con‑

verters. While there are many capacitor types for these

applications (including low ESR tantalum, OS‑CON and

POSCAP),ceramiccapacitorsareoftenutilizedinswitching

converter applications due to their small size, low ESR and

low leakage currents. Table 6 provides a partial listing of

multilayer ceramic capacitors. Many ceramic capacitors

intended for power applications experience a significant

loss in capacitance from their rated value as the DC bias

voltage on the capacitor increases. It is not uncommon for

a small surface mount capacitor to lose more than 50%

of its rated capacitance when operated nears it maximum

rated voltage. This effect is generally reduced as the case

size is increased for the same nominal value capacitor. As a

result,itisoftennecessarytousealargervaluecapacitance

or a higher voltage rated capacitor than would ordinarily

be required to actually realize the intended capacitance at

the operating voltage of the application. X5R, X6S, X7R

or X8R dielectric types are recommended as they exhibit

the best performance over the wide operating range and

temperature. To verify that the intended capacitance is

achieved in the application circuit, be sure to consult the

capacitorvendor’scurveofcapacitancevsDCbiasvoltage.

Table 6. Representative Multilayer Ceramic Capacitors

Part Series

Values

(µF)

Voltage

(V)

Capacitor

Types

AVX

www.avx.com

0805, 1206, 1210

Up to 100

6.3, 10

X5R, X6S, X7R

Kemet

www.kemet.com

C0805–C1210

Up to 100

6.3, 10

X5R, X7R

Murata

www.murata.com

GRM

Up to 100

6.3, 10

X5R, X7R

Taiyo Yuden

www.t‑yuden.com

EMK, JMK, LMK

Up to 47

6.3, 10

X5R, X6S, X7R

TDK

www.tdk.com

CGA4 – CGA9

Up to 47

6.3

X5R, X7R

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