LT8331 datasheet(14/24 Pages) AD

14 / 24 page

LT8334

Rev. 0

For more information www.analog.com

should not allow peak inductor currents to exceed 5A

in steady state at maximum load. Due to tolerances, be

sure to account for minimum possible inductance value,

switching frequency and converter efficiency.

For inductor current operation in CCM and duty cycles

above 50%, the LT8334’s internal slope compensation

prevents subharmonic oscillations provided the inductor

value exceeds a minimum value given by Equation 13.

VIN

–35 •D2

+ 53 •D – 13

(

)• fOSC

( )

•

2 •D – 1

(

)

1– D

(

)

(13)

Lower L values are allowed if the inductor current oper-

ates in DCM or duty cycle operation is below 50%.

Table 2. Inductor Manufacturers

Sumida

www.sumida.com

TDK

www.tdk.com

Murata

www.murata.com

Coilcraft

www.coilcraft.com

Wurth

www.we-online.com

Boost Converter: Input Capacitor Selection

Bypass the input of the LT8334 circuit with a ceramic

capacitor 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 LT8334 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 LT8334.

A ceramic input capacitor combined with trace or cable

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

cuit. If the LT8334 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the LT8334’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 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 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 capac-

itance 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 output voltage ripple waveform for a typical boost

converter is illustrated in Figure 4.

VOUT

(AC)

tON

ΔVESR

RINGING DUE TO

TOTAL INDUCTANCE

(BOARD + CAP)

ΔVCOUT

8334 F04

tOFF

Figure 4. The Output Ripple Waveform of a Boost Converter

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/discharging

∆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 with Equation 14.

ESRCOUT ≤

0.01 • VOUT

ID(PEAK)

(14)

APPLICATIONS INFORMATION

LT8331 датащи(PDF) 14 Page - Analog Devices

LT8331 датащи(HTML) 14 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

датащи скачать

ссылки URL