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LT8331

8331fa

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inductance forms a high quality (under damped) tank cir-

cuit. If the LT8331 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the LT8331’s voltage rating. This situation is

easily avoided (see Application Note 88).

BOOST CONVERTER: OUTPUT CAPACITOR SELECTION

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple voltage.

Multilayer ceramic capacitors are an excellent choice, as

they are small and have extremely low ESR. Use X5R or

X7R types. This choice will provide low output ripple and

good transient response. A 10µF to 47µF output capacitor

is sufficient for most applications, but systems with very

low output currents may need only a 1µF or 2.2µF output

capacitor. Solid tantalum or OS-CON capacitor can be

used, but they will occupy more board area than a ceramic

and will have a higher ESR. Always use a capacitor with a

sufficient voltage rating.

Contributions of ESR (equivalent series resistance), ESL

(equivalent series inductance) and the bulk capacitance

must be considered when choosing the correct output

capacitors for a given output ripple voltage. The effect of

thesethreeparameters(ESR,ESLandbulkC)ontheoutput

voltage ripple waveform for a typical boost converter is

illustrated in Figure 4.

between ∆VESR and ∆VCOUT. This percentage ripple will

change, depending on the requirements of the application,

and the following equations can easily be modified. For a

1% contribution to the total ripple voltage, the ESR of the

output capacitor can be determined using the following

equation:

ESRCOUT ≤

0.01 • VOUT

ID(PEAK)

For the bulk C component, which also contributes 1% to

the total ripple:

COUT ≥

IO(MAX)

0.01 • VOUT • fOSC

The output capacitor in a boost regulator experiences high

RMS ripple currents, as shown in Figure 4. The RMS ripple

current rating of the output capacitor can be determined

using the following equation:

IRMS(COUT) ≥ IO(MAX) •

DMAX

− DMAX

Multiple capacitors are often paralleled to meet ESR

requirements. Typically, once the ESR requirement is

satisfied, the capacitance is adequate for filtering and has

therequiredRMScurrentrating.Additionalceramiccapaci-

tors in parallel are commonly used to reduce the effect of

parasiticinductanceintheoutputcapacitor,whichreduces

high frequency switching noise on the converter output.

CERAMIC CAPACITORS

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

whenusedwiththeLT8331duetotheirpiezoelectricnature.

When in Burst Mode operation, the LT8331’s switching

frequency depends on the load current, and at very light

loads the LT8331 can excite the ceramic capacitor at audio

frequencies, generating audible noise. Since the LT8331

operates at a lower current limit during Burst Mode op-

eration, the noise is typically very quiet to a casual ear.

If this is unacceptable, use a high performance tantalum

Figure 4. The Output Ripple Waveform of a Boost Converter

VOUT

(AC)

tON

∆VESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

∆VCOUT

8331 F04

tOFF

The choice of component(s) begins with the maximum

acceptable ripple voltage (expressed as a percentage of

the output voltage), and how this ripple should be divided

between the ESR step ∆VESR and the charging/discharg-

ing ∆VCOUT. For the purpose of simplicity, we will choose

2% for the maximum output ripple, to be divided equally

APPLICATIONS INFORMATION

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