LT8331 datasheet(15/30 Pages) AD

15 / 30 page

LT8365

Rev. A

For more information www.analog.com

Table 2. Inductor Manufacturers

Sumida

(847) 956-0666

www.sumida.com

TDK

(847) 803-6100

www.tdk.com

Murata

(714) 852-2001

www.murata.com

Coilcraft

(847) 639-6400

www.coilcraft.com

Wurth

(605) 886-4385

www.we-online.com

BOOST CONVERTER: INPUT CAPACITOR SELECTION

Bypass the input of the LT8365 circuit with a ceramic ca-

pacitor of X7R or X5R type placed as close as possible to

the VIN and GND pins. Y5V types have poor performance

over temperature and applied voltage, and should not be

used. A 4.7µF to 10µF ceramic capacitor is adequate to

bypass the LT8365 and will easily handle the ripple cur-

rent. If the input power source has high impedance, or

there is significant inductance due to long wires or cables,

additional bulk capacitance may be necessary. This can

be provided with a low performance electrolytic capacitor.

A precaution regarding the ceramic input capacitor con-

cerns the maximum input voltage rating of the LT8365.

A ceramic input capacitor combined with trace or cable

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

cuit. If the LT8365 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the LT8365’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 4.7µF 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. For High-Voltage DCM applica-

tions, smaller output capacitor values may be sufficient.

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.

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

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

1 − DMAX

Figure 4. The Output Ripple Waveform of a Boost Converter

VOUT

(AC)

tON

ΔVESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

ΔVCOUT

8365 F04

tOFF

APPLICATIONS INFORMATION

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