LT8331 datasheet(14/30 Pages) AD

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LT8331

Rev. C

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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 volt-

age. 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 out-

put 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

these three parameters (ESR, ESL and bulk C) on the out-

put 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 applica-

tion, and the following equations can easily be modi-

fied. 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 deter-

mined using the following equation:

IRMS(COUT) ≥ IO(MAX) •

DMAX

1 − DMAX

Multiple capacitors are often paralleled to meet ESR

requirements. Typically, once the ESR requirement is sat-

isfied, the capacitance is adequate for filtering and has the

required RMS current rating. Additional ceramic capaci-

tors in parallel are commonly used to reduce the effect of

parasitic inductance in the output capacitor, which reduces

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

when used with the LT8331 due to their piezoelectric

nature. 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 operation, the noise is typically very quiet to a

casual ear. If this is unacceptable, use a high performance

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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